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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="users_guide.html">User's Guide</a><br><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/stdlib-1.18.3.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>STDLIB</strong><br><strong>Reference Manual</strong><br><small>Version 1.18.3</small></p>
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<li title="crypto_key_fun-1"><a href="beam_lib.html#crypto_key_fun-1">crypto_key_fun/1</a></li>
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<li title="referenced_byte_size-1"><a href="binary.html#referenced_byte_size-1">referenced_byte_size/1</a></li>
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<!-- refpage --><center><h1>qlc</h1></center>
<h3>MODULE</h3>
<div class="REFBODY">qlc</div>
<h3>MODULE SUMMARY</h3>
<div class="REFBODY">Query Interface to Mnesia, ETS, Dets, etc</div>
<h3>DESCRIPTION</h3>
<div class="REFBODY"><p>
<p>The <span class="code">qlc</span> module provides a query interface to Mnesia, ETS,
Dets and other data structures that implement an iterator style
traversal of objects. </p>
</p></div>
<h3><a name="id200435">Overview</a></h3>
<div class="REFBODY">
<p>The <span class="code">qlc</span> module implements a query interface to <strong>QLC
tables</strong>. Typical QLC tables are ETS, Dets, and Mnesia
tables. There is also support for user defined tables, see the
<span class="bold_code"><a href="#implementing_a_qlc_table">Implementing a QLC
table</a></span> section. <a name="query_list_comprehension"></a>
A <strong>query</strong> is stated using
<strong>Query List Comprehensions</strong> (QLCs). The answers to a
query are determined by data in QLC tables that fulfill the
constraints expressed by the QLCs of the query. QLCs are similar
to ordinary list comprehensions as described in the Erlang
Reference Manual and Programming Examples except that variables
introduced in patterns cannot be used in list expressions. In
fact, in the absence of optimizations and options such as
<span class="code">cache</span> and <span class="code">unique</span> (see below), every QLC free of
QLC tables evaluates to the same list of answers as the
identical ordinary list comprehension.</p>
<p>While ordinary list comprehensions evaluate to lists, calling
<span class="bold_code"><a href="#q">qlc:q/1,2</a></span> returns a <a name="query_handle"></a><strong> Query
Handle</strong>. To obtain all the answers to a query, <span class="bold_code"><a href="#eval">qlc:eval/1,2</a></span> should be called with the
query handle as first argument. Query handles are essentially
functional objects ("funs") created in the module calling <span class="code">q/1,2</span>.
As the funs refer to the module's code, one should
be careful not to keep query handles too long if the module's
code is to be replaced.
Code replacement is described in the <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','code_loading.html');">Erlang Reference
Manual</a></span>. The list of answers can also be traversed in
chunks by use of a <a name="query_cursor"></a><strong>Query Cursor</strong>. Query cursors are
created by calling <span class="bold_code"><a href="#cursor">qlc:cursor/1,2</a></span> with a query handle as
first argument. Query cursors are essentially Erlang processes.
One answer at a time is sent from the query cursor process to
the process that created the cursor.</p>
</div>
<h3><a name="id200536">Syntax</a></h3>
<div class="REFBODY">
<p>Syntactically QLCs have the same parts as ordinary list
comprehensions:</p>
<div class="example"><pre>[Expression || Qualifier1, Qualifier2, ...]</pre></div>
<p><span class="code">Expression</span> (the <strong>template</strong>) is an arbitrary
Erlang expression. Qualifiers are either <strong>filters</strong> or
<strong>generators</strong>. Filters are Erlang expressions returning
<span class="code">bool()</span>. Generators have the form
<span class="code">Pattern <- ListExpression</span>, where
<span class="code">ListExpression</span> is an expression evaluating to a query
handle or a list. Query handles are returned from
<span class="code">qlc:table/2</span>, <span class="code">qlc:append/1,2</span>, <span class="code">qlc:sort/1,2</span>,
<span class="code">qlc:keysort/2,3</span>, <span class="code">qlc:q/1,2</span>, and
<span class="code">qlc:string_to_handle/1,2,3</span>.</p>
</div>
<h3><a name="id200606">Evaluation</a></h3>
<div class="REFBODY">
<p>The evaluation of a query handle begins by the inspection of
options and the collection of information about tables. As a
result qualifiers are modified during the optimization phase.
Next all list expressions are evaluated. If a cursor has been
created evaluation takes place in the cursor process. For those
list expressions that are QLCs, the list expressions of the
QLCs' generators are evaluated as well. One has to be careful if
list expressions have side effects since the order in which list
expressions are evaluated is unspecified. Finally the answers
are found by evaluating the qualifiers from left to right,
backtracking when some filter returns <span class="code">false</span>, or
collecting the template when all filters return <span class="code">true</span>.</p>
<p>Filters that do not return <span class="code">bool()</span> but fail are handled
differently depending on their syntax: if the filter is a guard
it returns <span class="code">false</span>, otherwise the query evaluation fails.
This behavior makes it possible for the <span class="code">qlc</span> module to do
some optimizations without affecting the meaning of a query. For
example, when testing some position of a table and one or more
constants for equality, only
the objects with equal values are candidates for further
evaluation. The other objects are guaranteed to make the filter
return <span class="code">false</span>, but never fail. The (small) set of
candidate objects can often be found by looking up some key
values of the table or by traversing the table using a match
specification. It is necessary to place the guard filters
immediately after the table's generator, otherwise the candidate
objects will not be restricted to a small set. The reason is
that objects that could make the query evaluation fail must not
be excluded by looking up a key or running a match
specification.</p>
</div>
<h3><a name="id200656">Join</a></h3>
<div class="REFBODY">
<p>The <span class="code">qlc</span> module supports fast join of two query handles.
Fast join is possible if some position <span class="code">P1</span> of one query
handler and some position <span class="code">P2</span> of another query handler are
tested for equality. Two fast join methods have been
implemented:</p>
<ul>
<li>Lookup join traverses all objects of one query handle and
finds objects of the other handle (a QLC table) such that the
values at <span class="code">P1</span> and <span class="code">P2</span> match or compare equal.
The <span class="code">qlc</span> module does not create
any indices but looks up values using the key position and
the indexed positions of the QLC table.
</li>
<li>Merge join sorts the objects of each query handle if
necessary and filters out objects where the values at
<span class="code">P1</span> and <span class="code">P2</span> do not compare equal. If there are
many objects with the same value of <span class="code">P2</span> a temporary
file will be used for the equivalence classes.
</li>
</ul>
<p>The <span class="code">qlc</span> module warns at compile time if a QLC
combines query handles in such a way that more than one join is
possible. In other words, there is no query planner that can
choose a good order between possible join operations. It is up
to the user to order the joins by introducing query handles.</p>
<p>The join is to be expressed as a guard filter. The filter must
be placed immediately after the two joined generators, possibly
after guard filters that use variables from no other generators
but the two joined generators. The <span class="code">qlc</span> module inspects
the operands of
<span class="code">=:=/2</span>, <span class="code">==/2</span>, <span class="code">is_record/2</span>, <span class="code">element/2</span>,
and logical operators (<span class="code">and/2</span>, <span class="code">or/2</span>,
<span class="code">andalso/2</span>, <span class="code">orelse/2</span>, <span class="code">xor/2</span>) when
determining which joins to consider.</p>
</div>
<h3><a name="id200772">Common options</a></h3>
<div class="REFBODY">
<p>The following options are accepted by <span class="code">cursor/2</span>,
<span class="code">eval/2</span>, <span class="code">fold/4</span>, and <span class="code">info/2</span>:</p>
<ul>
<li>
<span class="code">{cache_all, Cache}</span> where <span class="code">Cache</span> is
equal to <span class="code">ets</span> or <span class="code">list</span> adds a
<span class="code">{cache, Cache}</span> option to every list expression
of the query except tables and lists. Default is
<span class="code">{cache_all, no}</span>. The option <span class="code">cache_all</span> is
equivalent to <span class="code">{cache_all, ets}</span>.
</li>
<li>
<span class="code">{max_list_size, MaxListSize}</span> <a name="max_list_size"></a> where <span class="code">MaxListSize</span> is the
size in bytes of terms on the external format. If the
accumulated size of collected objects exceeds
<span class="code">MaxListSize</span> the objects are written onto a temporary
file. This option is used by the <span class="code">{cache, list}</span>
option as well as by the merge join method. Default is
512*1024 bytes.
</li>
<li>
<span class="code">{tmpdir_usage, TmpFileUsage}</span> determines the
action taken when <span class="code">qlc</span> is about to create temporary
files on the directory set by the <span class="code">tmpdir</span> option. If the
value is <span class="code">not_allowed</span> an error tuple is returned,
otherwise temporary files are created as needed. Default is
<span class="code">allowed</span> which means that no further action is taken.
The values <span class="code">info_msg</span>, <span class="code">warning_msg</span>, and
<span class="code">error_msg</span> mean that the function with the corresponding
name in the module <span class="code">error_logger</span> is called for printing
some information (currently the stacktrace).
</li>
<li>
<span class="code">{tmpdir, TempDirectory}</span> sets the directory used by
merge join for temporary files and by the
<span class="code">{cache, list}</span> option. The option also overrides
the <span class="code">tmpdir</span> option of <span class="code">keysort/3</span> and
<span class="code">sort/2</span>. The default value is <span class="code">""</span> which means that
the directory returned by <span class="code">file:get_cwd()</span> is used.
</li>
<li>
<span class="code">{unique_all, true}</span> adds a
<span class="code">{unique, true}</span> option to every list expression of
the query. Default is <span class="code">{unique_all, false}</span>. The
option <span class="code">unique_all</span> is equivalent to
<span class="code">{unique_all, true}</span>.
</li>
</ul>
</div>
<h3><a name="id200951">Getting started</a></h3>
<div class="REFBODY">
<p><a name="getting_started"></a> As already mentioned
queries are stated in the list comprehension syntax as described
in the <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','expressions.html');">Erlang
Reference Manual</a></span>. In the following some familiarity
with list comprehensions is assumed. There are examples in
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/programming_examples','list_comprehensions.html');">Programming
Examples</a></span> that can get you started. It should be
stressed that list comprehensions do not add any computational
power to the language; anything that can be done with list
comprehensions can also be done without them. But they add a
syntax for expressing simple search problems which is compact
and clear once you get used to it.</p>
<p>Many list comprehension expressions can be evaluated by the
<span class="code">qlc</span> module. Exceptions are expressions such that
variables introduced in patterns (or filters) are used in some
generator later in the list comprehension. As an example
consider an implementation of lists:append(L):
<span class="code">[X ||Y <- L, X <- Y]</span>.
Y is introduced in the first generator and used in the second.
The ordinary list comprehension is normally to be preferred when
there is a choice as to which to use. One difference is that
<span class="code">qlc:eval/1,2</span> collects answers in a list which is finally
reversed, while list comprehensions collect answers on the stack
which is finally unwound.</p>
<p>What the <span class="code">qlc</span> module primarily adds to list
comprehensions is that data can be read from QLC tables in small
chunks. A QLC table is created by calling <span class="code">qlc:table/2</span>.
Usually <span class="code">qlc:table/2</span> is not called directly from the query
but via an interface function of some data structure. There are
a few examples of such functions in Erlang/OTP:
<span class="code">mnesia:table/1,2</span>, <span class="code">ets:table/1,2</span>, and
<span class="code">dets:table/1,2</span>. For a given data structure there can be
several functions that create QLC tables, but common for all
these functions is that they return a query handle created by
<span class="code">qlc:table/2</span>. Using the QLC tables provided by OTP is
probably sufficient in most cases, but for the more advanced
user the section <span class="bold_code"><a href="#implementing_a_qlc_table">Implementing a QLC
table</a></span> describes the implementation of a function
calling <span class="code">qlc:table/2</span>.</p>
<p>Besides <span class="code">qlc:table/2</span> there are other functions that
return query handles. They might not be used as often as tables,
but are useful from time to time. <span class="code">qlc:append</span> traverses
objects from several tables or lists after each other. If, for
instance, you want to traverse all answers to a query QH and
then finish off by a term <span class="code">{finished}</span>, you can do that by
calling <span class="code">qlc:append(QH, [{finished}])</span>. <span class="code">append</span> first
returns all objects of QH, then <span class="code">{finished}</span>. If there is
one tuple <span class="code">{finished}</span> among the answers to QH it will be
returned twice from <span class="code">append</span>.</p>
<p>As another example, consider concatenating the answers to two
queries QH1 and QH2 while removing all duplicates. The means to
accomplish this is to use the <span class="code">unique</span> option:</p>
<div class="example"><pre>
qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})</pre></div>
<p>The cost is substantial: every returned answer will be stored
in an ETS table. Before returning an answer it is looked up in
the ETS table to check if it has already been returned. Without
the <span class="code">unique</span> options all answers to QH1 would be returned
followed by all answers to QH2. The <span class="code">unique</span> options keeps
the order between the remaining answers.</p>
<p>If the order of the answers is not important there is the
alternative to sort the answers uniquely:</p>
<div class="example"><pre>
qlc:sort(qlc:q([X || X <- qlc:append(QH1, QH2)], {unique, true})).</pre></div>
<p>This query also removes duplicates but the answers will be
sorted. If there are many answers temporary files will be used.
Note that in order to get the first unique answer all answers
have to be found and sorted. Both alternatives find duplicates
by comparing answers, that is, if A1 and A2 are answers found in
that order, then A2 is a removed if A1 == A2.</p>
<p>To return just a few answers cursors can be used. The following
code returns no more than five answers using an ETS table for
storing the unique answers:</p>
<div class="example"><pre>
C = qlc:cursor(qlc:q([X || X <- qlc:append(QH1, QH2)],{unique,true})),
R = qlc:next_answers(C, 5),
ok = qlc:delete_cursor(C),
R.</pre></div>
<p>Query list comprehensions are convenient for stating
constraints on data from two or more tables. An example that
does a natural join on two query handles on position 2:</p>
<div class="example"><pre>
qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2])</pre></div>
<p>The <span class="code">qlc</span> module will evaluate this differently depending on
the query
handles <span class="code">QH1</span> and <span class="code">QH2</span>. If, for example, <span class="code">X2</span> is
matched against the key of a QLC table the lookup join method
will traverse the objects of <span class="code">QH2</span> while looking up key
values in the table. On the other hand, if neither <span class="code">X2</span> nor
<span class="code">Y2</span> is matched against the key or an indexed position of a
QLC table, the merge join method will make sure that <span class="code">QH1</span>
and <span class="code">QH2</span> are both sorted on position 2 and next do the
join by traversing the objects one by one.</p>
<p>The <span class="code">join</span> option can be used to force the <span class="code">qlc</span> module
to use a
certain join method. For the rest of this section it is assumed
that the excessively slow join method called "nested loop" has
been chosen:</p>
<div class="example"><pre>
qlc:q([{X1,X2,X3,Y1} ||
{X1,X2,X3} <- QH1,
{Y1,Y2} <- QH2,
X2 =:= Y2],
{join, nested_loop})</pre></div>
<p>In this case the filter will be applied to every possible pair
of answers to QH1 and QH2, one at a time. If there are M answers
to QH1 and N answers to QH2 the filter will be run M*N
times.</p>
<p>If QH2 is a call to the function for <span class="code">gb_trees</span> as defined
in the <span class="bold_code"><a href="#implementing_a_qlc_table">Implementing
a QLC table</a></span> section, <span class="code">gb_table:table/1</span>, the
iterator for the gb-tree will be initiated for each answer to
QH1 after which the objects of the gb-tree will be returned one
by one. This is probably the most efficient way of traversing
the table in that case since it takes minimal computational
power to get the following object. But if QH2 is not a table but
a more complicated QLC, it can be more efficient use some RAM
memory for collecting the answers in a cache, particularly if
there are only a few answers. It must then be assumed that
evaluating QH2 has no side effects so that the meaning of the
query does not change if QH2 is evaluated only once. One way of
caching the answers is to evaluate QH2 first of all and
substitute the list of answers for QH2 in the query. Another way
is to use the <span class="code">cache</span> option. It is stated like this:</p>
<div class="example"><pre>
QH2' = qlc:q([X || X <- QH2], {cache, ets})</pre></div>
<p>or just</p>
<div class="example"><pre>
QH2' = qlc:q([X || X <- QH2], cache)</pre></div>
<p>The effect of the <span class="code">cache</span> option is that when the
generator QH2' is run the first time every answer is stored in
an ETS table. When next answer of QH1 is tried, answers to QH2'
are copied from the ETS table which is very fast. As for the
<span class="code">unique</span> option the cost is a possibly substantial amount
of RAM memory. The <span class="code">{cache, list}</span> option offers the
possibility to store the answers in a list on the process heap.
While this has the potential of being faster than ETS tables
since there is no need to copy answers from the table it can
often result in slower evaluation due to more garbage
collections of the process' heap as well as increased RAM memory
consumption due to larger heaps. Another drawback with cache
lists is that if the size of the list exceeds a limit a
temporary file will be used. Reading the answers from a file is
very much slower than copying them from an ETS table. But if the
available RAM memory is scarce setting the <span class="bold_code"><a href="#max_list_size">limit</a></span> to some low value is an
alternative.</p>
<p>There is an option <span class="code">cache_all</span> that can be set to
<span class="code">ets</span> or <span class="code">list</span> when evaluating a query. It adds a
<span class="code">cache</span> or <span class="code">{cache, list}</span> option to every list
expression except QLC tables and lists on all levels of the
query. This can be used for testing if caching would improve
efficiency at all. If the answer is yes further testing is
needed to pinpoint the generators that should be cached.</p>
</div>
<h3><a name="id201318">Implementing a QLC table</a></h3>
<div class="REFBODY">
<p><a name="implementing_a_qlc_table"></a>As an example of
how to use the <span class="bold_code"><a href="#q">qlc:table/2</a></span>
function the implementation of a QLC table for the <span class="bold_code"><a href="gb_trees.html">gb_trees</a></span> module is given:</p>
<div class="example"><pre>
-module(gb_table).
-export([table/1]).
table(T) ->
TF = fun() -> qlc_next(gb_trees:next(gb_trees:iterator(T))) end,
InfoFun = fun(num_of_objects) -> gb_trees:size(T);
(keypos) -> 1;
(is_sorted_key) -> true;
(is_unique_objects) -> true;
(_) -> undefined
end,
LookupFun =
fun(1, Ks) ->
lists:flatmap(fun(K) ->
case gb_trees:lookup(K, T) of
{value, V} -> [{K,V}];
none -> []
end
end, Ks)
end,
FormatFun =
fun({all, NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, NElements, ElementFun)]),
io_lib:format("gb_table:table(~s)", [ValsS]);
({lookup, 1, KeyValues, _NElements, ElementFun}) ->
ValsS = io_lib:format("gb_trees:from_orddict(~w)",
[gb_nodes(T, infinity, ElementFun)]),
io_lib:format("lists:flatmap(fun(K) -> "
"case gb_trees:lookup(K, ~s) of "
"{value, V} -> [{K,V}];none -> [] end "
"end, ~w)",
[ValsS, [ElementFun(KV) || KV <- KeyValues]])
end,
qlc:table(TF, [{info_fun, InfoFun}, {format_fun, FormatFun},
{lookup_fun, LookupFun},{key_equality,'=='}]).
qlc_next({X, V, S}) ->
[{X,V} | fun() -> qlc_next(gb_trees:next(S)) end];
qlc_next(none) ->
[].
gb_nodes(T, infinity, ElementFun) ->
gb_nodes(T, -1, ElementFun);
gb_nodes(T, NElements, ElementFun) ->
gb_iter(gb_trees:iterator(T), NElements, ElementFun).
gb_iter(_I, 0, _EFun) ->
'...';
gb_iter(I0, N, EFun) ->
case gb_trees:next(I0) of
{X, V, I} ->
[EFun({X,V}) | gb_iter(I, N-1, EFun)];
none ->
[]
end.</pre></div>
<p><span class="code">TF</span> is the traversal function. The <span class="code">qlc</span> module
requires that there is a way of traversing all objects of the
data structure; in <span class="code">gb_trees</span> there is an iterator function
suitable for that purpose. Note that for each object returned a
new fun is created. As long as the list is not terminated by
<span class="code">[]</span> it is assumed that the tail of the list is a nullary
function and that calling the function returns further objects
(and functions).</p>
<p>The lookup function is optional. It is assumed that the lookup
function always finds values much faster than it would take to
traverse the table. The first argument is the position of the
key. Since <span class="code">qlc_next</span> returns the objects as
{Key, Value} pairs the position is 1. Note that the lookup
function should return {Key, Value} pairs, just as the
traversal function does.</p>
<p>The format function is also optional. It is called by
<span class="code">qlc:info</span> to give feedback at runtime of how the query
will be evaluated. One should try to give as good feedback as
possible without showing too much details. In the example at
most 7 objects of the table are shown. The format function
handles two cases: <span class="code">all</span> means that all objects of the
table will be traversed; <span class="code">{lookup, 1, KeyValues}</span>
means that the lookup function will be used for looking up key
values.</p>
<p>Whether the whole table will be traversed or just some keys
looked up depends on how the query is stated. If the query has
the form</p>
<div class="example"><pre>
qlc:q([T || P <- LE, F])</pre></div>
<p>and P is a tuple, the <span class="code">qlc</span> module analyzes P and F in
compile time to find positions of the tuple P that are tested
for equality to constants. If such a position at runtime turns
out to be the key position, the lookup function can be used,
otherwise all objects of the table have to be traversed. It is
the info function <span class="code">InfoFun</span> that returns the key position.
There can be indexed positions as well, also returned by the
info function. An index is an extra table that makes lookup on
some position fast. Mnesia maintains indices upon request,
thereby introducing so called secondary keys. The <span class="code">qlc</span>
module prefers to look up objects using the key before secondary
keys regardless of the number of constants to look up.</p>
</div>
<h3><a name="id201449">Key equality</a></h3>
<div class="REFBODY">
<p>In Erlang there are two operators for testing term equality,
namely <span class="code">==/2</span> and <span class="code">=:=/2</span>. The difference between them
is all about the integers that can be represented by floats. For
instance, <span class="code">2 == 2.0</span> evaluates to
<span class="code">true</span> while <span class="code">2 =:= 2.0</span> evaluates to <span class="code">false</span>.
Normally this is a minor issue, but the <span class="code">qlc</span> module cannot
ignore the difference, which affects the user's choice of
operators in QLCs.</p>
<p>If the <span class="code">qlc</span> module can find out at compile time that some
constant is free of integers, it does not matter which one of
<span class="code">==/2</span> or <span class="code">=:=/2</span> is used:</p>
<div class="example"><pre>
1> <span class="bold_code">E1 = ets:new(t, [set]), % uses =:=/2 for key equality</span>
<span class="bold_code">Q1 = qlc:q([K ||</span>
<span class="bold_code">{K} <- ets:table(E1),</span>
<span class="bold_code">K == 2.71 orelse K == a]),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(Q1)]).</span>
ets:match_spec_run(lists:flatmap(fun(V) ->
ets:lookup(20493, V)
end,
[a,2.71]),
ets:match_spec_compile([{{'$1'},[],['$1']}]))</pre></div>
<p>In the example the <span class="code">==/2</span> operator has been handled
exactly as <span class="code">=:=/2</span> would have been handled. On the other
hand, if it cannot be determined at compile time that some
constant is free of integers and the table uses <span class="code">=:=/2</span>
when comparing keys for equality (see the option <span class="bold_code"><a href="#key_equality">key_equality</a></span>), the
<span class="code">qlc</span> module will not try to look up the constant. The
reason is that there is in the general case no upper limit on
the number of key values that can compare equal to such a
constant; every combination of integers and floats has to be
looked up:</p>
<div class="example"><pre>
2> <span class="bold_code">E2 = ets:new(t, [set]),</span>
<span class="bold_code">true = ets:insert(E2, [{{2,2},a},{{2,2.0},b},{{2.0,2},c}]),</span>
<span class="bold_code">F2 = fun(I) -></span>
<span class="bold_code">qlc:q([V || {K,V} <- ets:table(E2), K == I])</span>
<span class="bold_code">end,</span>
<span class="bold_code">Q2 = F2({2,2}),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(Q2)]).</span>
ets:table(53264,
[{traverse,
{select,[{{'$1','$2'},[{'==','$1',{const,{2,2}}}],['$2']}]}}])
3> <span class="bold_code">lists:sort(qlc:e(Q2)).</span>
[a,b,c]</pre></div>
<p>Looking up just <span class="code">{2,2}</span> would not return <span class="code">b</span> and
<span class="code">c</span>.</p>
<p>If the table uses <span class="code">==/2</span> when comparing keys for equality,
the <span class="code">qlc</span> module will look up the constant regardless of
which operator is used in the QLC. However, <span class="code">==/2</span> is to
be preferred:</p>
<div class="example"><pre>
4> <span class="bold_code">E3 = ets:new(t, [ordered_set]), % uses ==/2 for key equality</span>
<span class="bold_code">true = ets:insert(E3, [{{2,2.0},b}]),</span>
<span class="bold_code">F3 = fun(I) -></span>
<span class="bold_code">qlc:q([V || {K,V} <- ets:table(E3), K == I])</span>
<span class="bold_code">end,</span>
<span class="bold_code">Q3 = F3({2,2}),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(Q3)]).</span>
ets:match_spec_run(ets:lookup(86033, {2,2}),
ets:match_spec_compile([{{'$1','$2'},[],['$2']}]))
5> <span class="bold_code">qlc:e(Q3).</span>
[b]</pre></div>
<p>Lookup join is handled analogously to lookup of constants in a
table: if the join operator is <span class="code">==/2</span> and the table where
constants are to be looked up uses <span class="code">=:=/2</span> when testing
keys for equality, the <span class="code">qlc</span> module will not consider
lookup join for that table.</p>
</div>
<h3>DATA TYPES</h3>
<p>
<span class="bold_code"><a name="type-abstract_expr">abstract_expr()</a> = <span class="bold_code"><a href="erl_parse.html#type-abstract_expr">erl_parse:abstract_expr()</a></span></span><br></p>
<div class="REFBODY"><p><p>Parse trees for Erlang expression, see the <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','erts','absform.html');">abstract format</a></span>
documentation in the ERTS User's Guide.</p></p></div>
<p>
<span class="bold_code"><a name="type-answer">answer()</a> = term()</span><br></p>
<p>
<span class="bold_code"><a name="type-answers">answers()</a> = [<span class="bold_code"><a href="#type-answer">answer()</a></span>]</span><br></p>
<p>
<span class="bold_code"><a name="type-cache">cache()</a> = ets | list | no</span><br></p>
<p>
<span class="bold_code"><a name="type-match_expression">match_expression()</a> = <span class="bold_code"><a href="ets.html#type-match_spec">ets:match_spec()</a></span></span><br></p>
<div class="REFBODY"><p><p>Match specification, see the <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','erts','match_spec.html');">match specification</a></span>
documentation in the ERTS User's Guide and <span class="bold_code"><a href="ms_transform.html">ms_transform(3).</a></span></p></p></div>
<p>
<span class="bold_code"><a name="type-no_files">no_files()</a> = integer() >= 1</span><br></p>
<div class="REFBODY"><p><p>Actually an integer > 1.</p></p></div>
<p>
<span class="bold_code"><a name="type-key_pos">key_pos()</a> = integer() >= 1 | [integer() >= 1]</span><br></p>
<p>
<span class="bold_code"><a name="type-max_list_size">max_list_size()</a> = integer() >= 0</span><br></p>
<p>
<span class="bold_code"><a name="type-order">order()</a> = ascending | descending | <span class="bold_code"><a href="#type-order_fun">order_fun()</a></span></span><br></p>
<p>
<span class="bold_code"><a name="type-order_fun">order_fun()</a> = fun((term(), term()) -> boolean())</span><br></p>
<p>
<span class="bold_code"><a name="type-query_cursor">query_cursor()</a></span><br></p>
<div class="REFBODY"><p><p>A <span class="bold_code"><a href="#query_cursor">query cursor</a></span>.</p>
</p></div>
<p>
<span class="bold_code"><a name="type-query_handle">query_handle()</a></span><br></p>
<div class="REFBODY"><p><p>A <span class="bold_code"><a href="#query_handle">query handle</a></span>.</p>
</p></div>
<p>
<span class="bold_code"><a name="type-query_handle_or_list">query_handle_or_list()</a> = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span> | list()</span><br></p>
<p>
<span class="bold_code"><a name="type-query_list_comprehension">query_list_comprehension()</a> = term()</span><br></p>
<div class="REFBODY"><p><p>A literal
<span class="bold_code"><a href="#query_list_comprehension">query
list comprehension</a></span>.</p></p></div>
<p>
<span class="bold_code"><a name="type-spawn_options">spawn_options()</a> = default | [<span class="bold_code"><a href="proc_lib.html#type-spawn_option">proc_lib:spawn_option()</a></span>]</span><br></p>
<p>
<span class="bold_code"><a name="type-sort_options">sort_options()</a> = [<span class="bold_code"><a href="#type-sort_option">sort_option()</a></span>] | <span class="bold_code"><a href="#type-sort_option">sort_option()</a></span></span><br></p>
<p>
<span class="bold_code"><a name="type-sort_option">sort_option()</a> = {compressed, boolean()}<br> | {no_files, <span class="bold_code"><a href="#type-no_files">no_files()</a></span>}<br> | {order, <span class="bold_code"><a href="#type-order">order()</a></span>}<br> | {size, integer() >= 1}<br> | {tmpdir, <span class="bold_code"><a href="#type-tmp_directory">tmp_directory()</a></span>}<br> | {unique, boolean()}</span><br></p>
<div class="REFBODY"><p><p>See <span class="bold_code"><a href="file_sorter.html">file_sorter(3)</a></span>.</p></p></div>
<p>
<span class="bold_code"><a name="type-tmp_directory">tmp_directory()</a> = [] | <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','kernel','file.html#type-name');">file:name()</a></span></span><br></p>
<p>
<span class="bold_code"><a name="type-tmp_file_usage">tmp_file_usage()</a> = allowed<br> | not_allowed<br> | info_msg<br> | warning_msg<br> | error_msg</span><br></p>
<h3>EXPORTS</h3>
<p><a name="append-1"></a><span class="bold_code">append(QHL) -> QH</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QHL = [<span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span>]</span></div>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a query handle. When evaluating the query handle
<span class="code">QH</span> all answers to the first query handle in
<span class="code">QHL</span> are returned followed by all answers
to the rest of the query handles in <span class="code">QHL</span>.</p>
</p></div>
<p><a name="append-2"></a><span class="bold_code">append(QH1, QH2) -> QH3</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH1 = QH2 = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">QH3 = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a query handle. When evaluating the query handle
<span class="code">QH3</span> all answers to
<span class="code">QH1</span> are returned followed by all answers
to <span class="code">QH2</span>.</p>
<p><span class="code">append(QH1, QH2)</span> is equivalent to
<span class="code">append([QH1, QH2])</span>.</p>
</p></div>
<p><a name="cursor-1"></a><span class="bold_code">cursor(QH) -> Cursor</span><br><a name="cursor-2"></a><span class="bold_code">cursor(QH, Options) -> Cursor</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {cache_all, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache_all<br> | {max_list_size, <span class="bold_code"><a href="#type-max_list_size">max_list_size()</a></span>}<br> | {spawn_options, <span class="bold_code"><a href="#type-spawn_options">spawn_options()</a></span>}<br> | {tmpdir_usage, <span class="bold_code"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>}<br> | {tmpdir, <span class="bold_code"><a href="#type-tmp_directory">tmp_directory()</a></span>}<br> | {unique_all, boolean()}<br> | unique_all</span></div>
<div class="REFTYPES"><span class="bold_code">Cursor = <span class="bold_code"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p><a name="cursor"></a>Creates a query cursor and
makes the calling process the owner of the cursor. The
cursor is to be used as argument to <span class="code">next_answers/1,2</span>
and (eventually) <span class="code">delete_cursor/1</span>. Calls
<span class="code">erlang:spawn_opt</span> to spawn and link a process which
will evaluate the query handle. The value of the option
<span class="code">spawn_options</span> is used as last argument when calling
<span class="code">spawn_opt</span>. The default value is <span class="code">[link]</span>.</p>
<div class="example"><pre>
1> <span class="bold_code">QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),</span>
<span class="bold_code">QC = qlc:cursor(QH),</span>
<span class="bold_code">qlc:next_answers(QC, 1).</span>
[{a,1}]
2> <span class="bold_code">qlc:next_answers(QC, 1).</span>
[{a,2}]
3> <span class="bold_code">qlc:next_answers(QC, all_remaining).</span>
[{b,1},{b,2}]
4> <span class="bold_code">qlc:delete_cursor(QC).</span>
ok</pre></div>
<p><span class="code">cursor(QH)</span> is equivalent to
<span class="code">cursor(QH, [])</span>.</p>
</p></div>
<p><a name="delete_cursor-1"></a><span class="bold_code">delete_cursor(QueryCursor) -> ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QueryCursor = <span class="bold_code"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Deletes a query cursor. Only the owner of the cursor can
delete the cursor.</p>
</p></div>
<p><a name="eval-1"></a><span class="bold_code">eval(QH) -> Answers | Error</span><br><a name="eval-2"></a><span class="bold_code">eval(QH, Options) -> Answers | Error</span><br><a name="e-1"></a><span class="bold_code">e(QH) -> Answers | Error</span><br><a name="e-2"></a><span class="bold_code">e(QH, Options) -> Answers | Error</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {cache_all, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache_all<br> | {max_list_size, <span class="bold_code"><a href="#type-max_list_size">max_list_size()</a></span>}<br> | {tmpdir_usage, <span class="bold_code"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>}<br> | {tmpdir, <span class="bold_code"><a href="#type-tmp_directory">tmp_directory()</a></span>}<br> | {unique_all, boolean()}<br> | unique_all</span></div>
<div class="REFTYPES"><span class="bold_code">Answers = <span class="bold_code"><a href="#type-answers">answers()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = <span class="bold_code"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p><a name="eval"></a>Evaluates a query handle in the
calling process and collects all answers in a list.</p>
<div class="example"><pre>
1> <span class="bold_code">QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),</span>
<span class="bold_code">qlc:eval(QH).</span>
[{a,1},{a,2},{b,1},{b,2}]</pre></div>
<p><span class="code">eval(QH)</span> is equivalent to
<span class="code">eval(QH, [])</span>.</p>
</p></div>
<p><a name="fold-3"></a><span class="bold_code">fold(Function, Acc0, QH) -> Acc1 | Error</span><br><a name="fold-4"></a><span class="bold_code">fold(Function, Acc0, QH, Options) -> Acc1 | Error</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Function = fun((<span class="bold_code"><a href="#type-answer">answer()</a></span>, AccIn) -> AccOut)</span></div>
<div class="REFTYPES"><span class="bold_code">Acc0 = Acc1 = AccIn = AccOut = term()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {cache_all, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache_all<br> | {max_list_size, <span class="bold_code"><a href="#type-max_list_size">max_list_size()</a></span>}<br> | {tmpdir_usage, <span class="bold_code"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>}<br> | {tmpdir, <span class="bold_code"><a href="#type-tmp_directory">tmp_directory()</a></span>}<br> | {unique_all, boolean()}<br> | unique_all</span></div>
<div class="REFTYPES"><span class="bold_code">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = <span class="bold_code"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Calls <span class="code">Function</span> on successive answers to
the query handle together with an extra argument
<span class="code">AccIn</span>. The query handle and the function
are evaluated in the calling process.
<span class="code">Function</span> must return a new accumulator
which is passed to the next call.
<span class="code">Acc0</span> is returned if there are no answers
to the query handle.</p>
<div class="example"><pre>
1> <span class="bold_code">QH = [1,2,3,4,5,6],</span>
<span class="bold_code">qlc:fold(fun(X, Sum) -> X + Sum end, 0, QH).</span>
21</pre></div>
<p><span class="code">fold(Function, Acc0,
QH)</span> is equivalent to
<span class="code">fold(Function, Acc0,
QH, [])</span>.</p>
</p></div>
<p><a name="format_error-1"></a><span class="bold_code">format_error(Error) -> Chars</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Error = {error, module(), term()}</span></div>
<div class="REFTYPES"><span class="bold_code">Chars = <span class="bold_code"><a href="io_lib.html#type-chars">io_lib:chars()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a descriptive string in English of an error tuple
returned by some of the functions of the <span class="code">qlc</span> module
or the parse transform. This function is mainly used by the
compiler invoking the parse transform.</p>
</p></div>
<p><a name="info-1"></a><span class="bold_code">info(QH) -> Info</span><br><a name="info-2"></a><span class="bold_code">info(QH, Options) -> Info</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = EvalOption | ReturnOption</span></div>
<div class="REFTYPES"><span class="bold_code">EvalOption = {cache_all, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache_all<br> | {max_list_size, <span class="bold_code"><a href="#type-max_list_size">max_list_size()</a></span>}<br> | {tmpdir_usage, <span class="bold_code"><a href="#type-tmp_file_usage">tmp_file_usage()</a></span>}<br> | {tmpdir, <span class="bold_code"><a href="#type-tmp_directory">tmp_directory()</a></span>}<br> | {unique_all, boolean()}<br> | unique_all</span></div>
<div class="REFTYPES"><span class="bold_code">ReturnOption = {depth, Depth}<br> | {flat, boolean()}<br> | {format, Format}<br> | {n_elements, NElements}</span></div>
<div class="REFTYPES"><span class="bold_code">Depth = infinity | integer() >= 0</span></div>
<div class="REFTYPES"><span class="bold_code">Format = abstract_code | string</span></div>
<div class="REFTYPES"><span class="bold_code">NElements = infinity | integer() >= 1</span></div>
<div class="REFTYPES"><span class="bold_code">Info = <span class="bold_code"><a href="#type-abstract_expr">abstract_expr()</a></span> | string()</span></div>
</div></p>
<div class="REFBODY"><p>
<p><a name="info"></a>Returns information about a
query handle. The information describes the simplifications
and optimizations that are the results of preparing the
query for evaluation. This function is probably useful
mostly during debugging.</p>
<p>The information has the form of an Erlang expression where
QLCs most likely occur. Depending on the format functions of
mentioned QLC tables it may not be absolutely accurate.</p>
<p>The default is to return a sequence of QLCs in a block, but
if the option <span class="code">{flat, false}</span> is given, one single
QLC is returned. The default is to return a string, but if
the option <span class="code">{format, abstract_code}</span> is given,
abstract code is returned instead. In the abstract code
port identifiers, references, and pids are represented by
strings. The default is to return
all elements in lists, but if the
<span class="code">{n_elements, NElements}</span> option is given, only a
limited number of elements are returned. The default is to
show all of objects and match specifications, but if the
<span class="code">{depth, Depth}</span> option is given, parts of terms
below a certain depth are replaced by <span class="code">'...'</span>.</p>
<div class="example"><pre>
1> <span class="bold_code">QH = qlc:q([{X,Y} || X <- [x,y], Y <- [a,b]]),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(QH, unique_all)]).</span>
begin
V1 =
qlc:q([
SQV ||
SQV <- [x,y]
],
[{unique,true}]),
V2 =
qlc:q([
SQV ||
SQV <- [a,b]
],
[{unique,true}]),
qlc:q([
{X,Y} ||
X <- V1,
Y <- V2
],
[{unique,true}])
end</pre></div>
<p>In this example two simple QLCs have been inserted just to
hold the <span class="code">{unique, true}</span> option.</p>
<div class="example"><pre>
1> <span class="bold_code">E1 = ets:new(e1, []),</span>
<span class="bold_code">E2 = ets:new(e2, []),</span>
<span class="bold_code">true = ets:insert(E1, [{1,a},{2,b}]),</span>
<span class="bold_code">true = ets:insert(E2, [{a,1},{b,2}]),</span>
<span class="bold_code">Q = qlc:q([{X,Z,W} ||</span>
<span class="bold_code">{X, Z} <- ets:table(E1),</span>
<span class="bold_code">{W, Y} <- ets:table(E2),</span>
<span class="bold_code">X =:= Y]),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(Q)]).</span>
begin
V1 =
qlc:q([
P0 ||
P0 = {W,Y} <- ets:table(17)
]),
V2 =
qlc:q([
[G1|G2] ||
G2 <- V1,
G1 <- ets:table(16),
element(2, G1) =:= element(1, G2)
],
[{join,lookup}]),
qlc:q([
{X,Z,W} ||
[{X,Z}|{W,Y}] <- V2
])
end</pre></div>
<p>In this example the query list comprehension <span class="code">V2</span> has
been inserted to show the joined generators and the join
method chosen. A convention is used for lookup join: the
first generator (<span class="code">G2</span>) is the one traversed, the second
one (<span class="code">G1</span>) is the table where constants are looked up.</p>
<p><span class="code">info(QH)</span> is equivalent to
<span class="code">info(QH, [])</span>.</p>
</p></div>
<p><a name="keysort-2"></a><span class="bold_code">keysort(KeyPos, QH1) -> QH2</span><br><a name="keysort-3"></a><span class="bold_code">keysort(KeyPos, QH1, SortOptions) -> QH2</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">KeyPos = <span class="bold_code"><a href="#type-key_pos">key_pos()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">SortOptions = <span class="bold_code"><a href="#type-sort_options">sort_options()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">QH1 = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">QH2 = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a query handle. When evaluating the query handle
<span class="code">QH2</span> the answers to the query handle
<span class="code">QH1</span> are sorted by <span class="bold_code"><a href="file_sorter.html">file_sorter:keysort/4</a></span>
according to the options.</p>
<p>The sorter will use temporary files only if
<span class="code">QH1</span> does not evaluate to a list and the
size of the binary representation of the answers exceeds
<span class="code">Size</span> bytes, where <span class="code">Size</span> is the value of the
<span class="code">size</span> option.</p>
<p><span class="code">keysort(KeyPos, QH1)</span>
is equivalent to
<span class="code">keysort(KeyPos, QH1, [])</span>.</p>
</p></div>
<p><a name="next_answers-1"></a><span class="bold_code">next_answers(QueryCursor) -> Answers | Error</span><br><a name="next_answers-2"></a><span class="bold_code">next_answers(QueryCursor, NumberOfAnswers) -> Answers | Error</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QueryCursor = <span class="bold_code"><a href="#type-query_cursor">query_cursor()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Answers = <span class="bold_code"><a href="#type-answers">answers()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">NumberOfAnswers = all_remaining | integer() >= 1</span></div>
<div class="REFTYPES"><span class="bold_code">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = <span class="bold_code"><a href="file_sorter.html#type-reason">file_sorter:reason()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns some or all of the remaining answers to a query
cursor. Only the owner of <span class="code">QueryCursor</span> can
retrieve answers.</p>
<p>The optional argument <span class="code">NumberOfAnswers</span>determines the
maximum number of answers returned. The default value is
<span class="code">10</span>. If less than the requested number of answers is
returned, subsequent calls to <span class="code">next_answers</span> will
return <span class="code">[]</span>.</p>
</p></div>
<p><a name="q-1"></a><span class="bold_code">q(QLC) -> QH</span><br><a name="q-2"></a><span class="bold_code">q(QLC, Options) -> QH</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {max_lookup, MaxLookup}<br> | {cache, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache<br> | {join, Join}<br> | {lookup, Lookup}<br> | {unique, boolean()}<br> | unique</span></div>
<div class="REFTYPES"><span class="bold_code">MaxLookup = integer() >= 0 | infinity</span></div>
<div class="REFTYPES"><span class="bold_code">Join = any | lookup | merge | nested_loop</span></div>
<div class="REFTYPES"><span class="bold_code">Lookup = boolean() | any</span></div>
<div class="REFTYPES"><span class="bold_code">QLC = <span class="bold_code"><a href="#type-query_list_comprehension">query_list_comprehension()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p><a name="q"></a>Returns a query handle for a query
list comprehension. The query list comprehension must be the
first argument to <span class="code">qlc:q/1,2</span> or it will be evaluated
as an ordinary list comprehension. It is also necessary to
add the line</p>
<div class="example"><pre>
-include_lib("stdlib/include/qlc.hrl").</pre></div>
<p>to the source file. This causes a parse transform to
substitute a fun for the query list comprehension. The
(compiled) fun will be called when the query handle is
evaluated.</p>
<p>When calling <span class="code">qlc:q/1,2</span> from the Erlang shell the
parse transform is automatically called. When this happens
the fun substituted for the query list comprehension is not
compiled but will be evaluated by <span class="code">erl_eval(3)</span>. This
is also true when expressions are evaluated by means of
<span class="code">file:eval/1,2</span> or in the debugger.</p>
<p>To be very explicit, this will not work:</p>
<div class="example"><pre>
...
A = [X || {X} <- [{1},{2}]],
QH = qlc:q(A),
...</pre></div>
<p>The variable <span class="code">A</span> will be bound to the evaluated value
of the list comprehension (<span class="code">[1,2]</span>). The compiler
complains with an error message ("argument is not a query
list comprehension"); the shell process stops with a
<span class="code">badarg</span> reason.</p>
<p><span class="code">q(QLC)</span> is equivalent to
<span class="code">q(QLC, [])</span>.</p>
<p>The <span class="code">{cache, ets}</span> option can be used to cache
the answers to a query list comprehension. The answers are
stored in one ETS table for each cached query list
comprehension. When a cached query list comprehension is
evaluated again, answers are fetched from the table without
any further computations. As a consequence, when all answers
to a cached query list comprehension have been found, the
ETS tables used for caching answers to the query list
comprehension's qualifiers can be emptied. The option
<span class="code">cache</span> is equivalent to <span class="code">{cache, ets}</span>.</p>
<p>The <span class="code">{cache, list}</span> option can be used to cache
the answers to a query list comprehension just like
<span class="code">{cache, ets}</span>. The difference is that the answers
are kept in a list (on the process heap). If the answers
would occupy more than a certain amount of RAM memory a
temporary file is used for storing the answers. The option
<span class="code">max_list_size</span> sets the limit in bytes and the temporary
file is put on the directory set by the <span class="code">tmpdir</span> option.</p>
<p>The <span class="code">cache</span> option has no effect if it is known that
the query list comprehension will be evaluated at most once.
This is always true for the top-most query list
comprehension and also for the list expression of the first
generator in a list of qualifiers. Note that in the presence
of side effects in filters or callback functions the answers
to query list comprehensions can be affected by the
<span class="code">cache</span> option.</p>
<p>The <span class="code">{unique, true}</span> option can be used to remove
duplicate answers to a query list comprehension. The unique
answers are stored in one ETS table for each query list
comprehension. The table is emptied every time it is known
that there are no more answers to the query list
comprehension. The option <span class="code">unique</span> is equivalent to
<span class="code">{unique, true}</span>. If the <span class="code">unique</span> option is
combined with the <span class="code">{cache, ets}</span> option, two ETS
tables are used, but the full answers are stored in one
table only. If the <span class="code">unique</span> option is combined with the
<span class="code">{cache, list}</span> option the answers are sorted
twice using <span class="code">keysort/3</span>; once to remove duplicates, and
once to restore the order.</p>
<p>The <span class="code">cache</span> and <span class="code">unique</span> options apply not only
to the query list comprehension itself but also to the
results of looking up constants, running match
specifications, and joining handles. </p>
<div class="example"><pre>
1> <span class="bold_code">Q = qlc:q([{A,X,Z,W} ||</span>
<span class="bold_code">A <- [a,b,c],</span>
<span class="bold_code">{X,Z} <- [{a,1},{b,4},{c,6}],</span>
<span class="bold_code">{W,Y} <- [{2,a},{3,b},{4,c}],</span>
<span class="bold_code">X =:= Y],</span>
<span class="bold_code">{cache, list}),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(Q)]).</span>
begin
V1 =
qlc:q([
P0 ||
P0 = {X,Z} <-
qlc:keysort(1, [{a,1},{b,4},{c,6}], [])
]),
V2 =
qlc:q([
P0 ||
P0 = {W,Y} <-
qlc:keysort(2, [{2,a},{3,b},{4,c}], [])
]),
V3 =
qlc:q([
[G1|G2] ||
G1 <- V1,
G2 <- V2,
element(1, G1) == element(2, G2)
],
[{join,merge},{cache,list}]),
qlc:q([
{A,X,Z,W} ||
A <- [a,b,c],
[{X,Z}|{W,Y}] <- V3,
X =:= Y
])
end</pre></div>
<p>In this example the cached results of the merge join are
traversed for each value of <span class="code">A</span>. Note that without the
<span class="code">cache</span> option the join would have been carried out
three times, once for each value of <span class="code">A</span></p>
<p><span class="code">sort/1,2</span> and <span class="code">keysort/2,3</span> can also be used for
caching answers and for removing duplicates. When sorting
answers are cached in a list, possibly stored on a temporary
file, and no ETS tables are used.</p>
<p>Sometimes (see <span class="bold_code"><a href="#lookup_fun">qlc:table/2</a></span> below) traversal
of tables can be done by looking up key values, which is
assumed to be fast. Under certain (rare) circumstances it
could happen that there are too many key values to look up.
<a name="max_lookup"></a> The
<span class="code">{max_lookup, MaxLookup}</span> option can then be used
to limit the number of lookups: if more than
<span class="code">MaxLookup</span> lookups would be required no lookups are
done but the table traversed instead. The default value is
<span class="code">infinity</span> which means that there is no limit on the
number of keys to look up.</p>
<div class="example"><pre>
1> <span class="bold_code">T = gb_trees:empty(),</span>
<span class="bold_code">QH = qlc:q([X || {{X,Y},_} <- gb_table:table(T),</span>
<span class="bold_code">((X == 1) or (X == 2)) andalso</span>
<span class="bold_code">((Y == a) or (Y == b) or (Y == c))]),</span>
<span class="bold_code">io:format("~s~n", [qlc:info(QH)]).</span>
ets:match_spec_run(
lists:flatmap(fun(K) ->
case
gb_trees:lookup(K,
gb_trees:from_orddict([]))
of
{value,V} ->
[{K,V}];
none ->
[]
end
end,
[{1,a},{1,b},{1,c},{2,a},{2,b},{2,c}]),
ets:match_spec_compile([{{{'$1','$2'},'_'},[],['$1']}]))</pre></div>
<p>In this example using the <span class="code">gb_table</span> module from the
<span class="bold_code"><a href="#implementing_a_qlc_table">Implementing a
QLC table</a></span> section there are six keys to look up:
<span class="code">{1,a}</span>, <span class="code">{1,b}</span>, <span class="code">{1,c}</span>, <span class="code">{2,a}</span>,
<span class="code">{2,b}</span>, and <span class="code">{2,c}</span>. The reason is that the two
elements of the key {X, Y} are compared separately.</p>
<p>The <span class="code">{lookup, true}</span> option can be used to ensure
that the <span class="code">qlc</span> module will look up constants in some
QLC table. If there
are more than one QLC table among the generators' list
expressions, constants have to be looked up in at least one
of the tables. The evaluation of the query fails if there
are no constants to look up. This option is useful in
situations when it would be unacceptable to traverse all
objects in some table. Setting the <span class="code">lookup</span> option to
<span class="code">false</span> ensures that no constants will be looked up
(<span class="code">{max_lookup, 0}</span> has the same effect). The
default value is <span class="code">any</span> which means that constants will
be looked up whenever possible.</p>
<p>The <span class="code">{join, Join}</span> option can be used to ensure
that a certain join method will be used:
<span class="code">{join, lookup}</span> invokes the lookup join method;
<span class="code">{join, merge}</span> invokes the merge join method; and
<span class="code">{join, nested_loop}</span> invokes the method of
matching every pair of objects from two handles. The last
method is mostly very slow. The evaluation of the query
fails if the <span class="code">qlc</span> module cannot carry out the chosen
join method. The
default value is <span class="code">any</span> which means that some fast join
method will be used if possible.</p>
</p></div>
<p><a name="sort-1"></a><span class="bold_code">sort(QH1) -> QH2</span><br><a name="sort-2"></a><span class="bold_code">sort(QH1, SortOptions) -> QH2</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">SortOptions = <span class="bold_code"><a href="#type-sort_options">sort_options()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">QH1 = <span class="bold_code"><a href="#type-query_handle_or_list">query_handle_or_list()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">QH2 = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a query handle. When evaluating the query handle
<span class="code">QH2</span> the answers to the query handle
<span class="code">QH1</span> are sorted by <span class="bold_code"><a href="file_sorter.html">file_sorter:sort/3</a></span> according
to the options.</p>
<p>The sorter will use temporary files only if
<span class="code">QH1</span> does not evaluate to a list and the
size of the binary representation of the answers exceeds
<span class="code">Size</span> bytes, where <span class="code">Size</span> is the value of the
<span class="code">size</span> option.</p>
<p><span class="code">sort(QH1)</span> is equivalent to
<span class="code">sort(QH1, [])</span>.</p>
</p></div>
<p><a name="string_to_handle-1"></a><span class="bold_code">string_to_handle(QueryString) -> QH | Error</span><br><a name="string_to_handle-2"></a><span class="bold_code">string_to_handle(QueryString, Options) -> QH | Error</span><br><a name="string_to_handle-3"></a><span class="bold_code">string_to_handle(QueryString, Options, Bindings) -> QH | Error</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">QueryString = string()</span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {max_lookup, MaxLookup}<br> | {cache, <span class="bold_code"><a href="#type-cache">cache()</a></span>}<br> | cache<br> | {join, Join}<br> | {lookup, Lookup}<br> | {unique, boolean()}<br> | unique</span></div>
<div class="REFTYPES"><span class="bold_code">MaxLookup = integer() >= 0 | infinity</span></div>
<div class="REFTYPES"><span class="bold_code">Join = any | lookup | merge | nested_loop</span></div>
<div class="REFTYPES"><span class="bold_code">Lookup = boolean() | any</span></div>
<div class="REFTYPES"><span class="bold_code">Bindings = <span class="bold_code"><a href="erl_eval.html#type-binding_struct">erl_eval:binding_struct()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Error = {error, module(), Reason}</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = <span class="bold_code"><a href="erl_parse.html#type-error_info">erl_parse:error_info()</a></span> | <span class="bold_code"><a href="erl_scan.html#type-error_info">erl_scan:error_info()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>A string version of <span class="code">qlc:q/1,2</span>. When the query handle
is evaluated the fun created by the parse transform is
interpreted by <span class="code">erl_eval(3)</span>. The query string is to be
one single query list comprehension terminated by a
period.</p>
<div class="example"><pre>
1> <span class="bold_code">L = [1,2,3],</span>
<span class="bold_code">Bs = erl_eval:add_binding('L', L, erl_eval:new_bindings()),</span>
<span class="bold_code">QH = qlc:string_to_handle("[X+1 || X <- L].", [], Bs),</span>
<span class="bold_code">qlc:eval(QH).</span>
[2,3,4]</pre></div>
<p><span class="code">string_to_handle(QueryString)</span>
is equivalent to
<span class="code">string_to_handle(QueryString, [])</span>.</p>
<p><span class="code">string_to_handle(QueryString,
Options)</span>
is equivalent to
<span class="code">string_to_handle(QueryString,
Options, erl_eval:new_bindings())</span>.</p>
<p>This function is probably useful mostly when called from
outside of Erlang, for instance from a driver written in C.</p>
</p></div>
<p><a name="table-2"></a><span class="bold_code">table(TraverseFun, Options) -> QH</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">TraverseFun = TraverseFun0 | TraverseFun1</span></div>
<div class="REFTYPES"><span class="bold_code">TraverseFun0 = fun(() -> TraverseResult)</span></div>
<div class="REFTYPES"><span class="bold_code">TraverseFun1 = fun((<span class="bold_code"><a href="#type-match_expression">match_expression()</a></span>) -> TraverseResult)</span></div>
<div class="REFTYPES"><span class="bold_code">TraverseResult = Objects | term()</span></div>
<div class="REFTYPES"><span class="bold_code">Objects = [] | [term() | ObjectList]</span></div>
<div class="REFTYPES"><span class="bold_code">ObjectList = TraverseFun0 | Objects</span></div>
<div class="REFTYPES"><span class="bold_code">Options = [Option] | Option</span></div>
<div class="REFTYPES"><span class="bold_code">Option = {format_fun, FormatFun}<br> | {info_fun, InfoFun}<br> | {lookup_fun, LookupFun}<br> | {parent_fun, ParentFun}<br> | {post_fun, PostFun}<br> | {pre_fun, PreFun}<br> | {key_equality, KeyComparison}</span></div>
<div class="REFTYPES"><span class="bold_code">FormatFun = undefined | fun((SelectedObjects) -> FormatedTable)</span></div>
<div class="REFTYPES"><span class="bold_code">SelectedObjects = all<br> | {all, NElements, DepthFun}<br> | {match_spec, <span class="bold_code"><a href="#type-match_expression">match_expression()</a></span>}<br> | {lookup, Position, Keys}<br> | {lookup, Position, Keys, NElements, DepthFun}</span></div>
<div class="REFTYPES"><span class="bold_code">NElements = infinity | integer() >= 1</span></div>
<div class="REFTYPES"><span class="bold_code">DepthFun = fun((term()) -> term())</span></div>
<div class="REFTYPES"><span class="bold_code">FormatedTable = {Mod, Fun, Args} | <span class="bold_code"><a href="#type-abstract_expr">abstract_expr()</a></span> | string()</span></div>
<div class="REFTYPES"><span class="bold_code">InfoFun = undefined | fun((InfoTag) -> InfoValue)</span></div>
<div class="REFTYPES"><span class="bold_code">InfoTag = indices | is_unique_objects | keypos | num_of_objects</span></div>
<div class="REFTYPES"><span class="bold_code">InfoValue = undefined | term()</span></div>
<div class="REFTYPES"><span class="bold_code">LookupFun = undefined | fun((Position, Keys) -> LookupResult)</span></div>
<div class="REFTYPES"><span class="bold_code">LookupResult = [term()] | term()</span></div>
<div class="REFTYPES"><span class="bold_code">ParentFun = undefined | fun(() -> ParentFunValue)</span></div>
<div class="REFTYPES"><span class="bold_code">PostFun = undefined | fun(() -> term())</span></div>
<div class="REFTYPES"><span class="bold_code">PreFun = undefined | fun((PreArgs) -> term())</span></div>
<div class="REFTYPES"><span class="bold_code">PreArgs = [PreArg]</span></div>
<div class="REFTYPES"><span class="bold_code">PreArg = {parent_value, ParentFunValue} | {stop_fun, StopFun}</span></div>
<div class="REFTYPES"><span class="bold_code">ParentFunValue = undefined | term()</span></div>
<div class="REFTYPES"><span class="bold_code">StopFun = undefined | fun(() -> term())</span></div>
<div class="REFTYPES"><span class="bold_code">KeyComparison = '=:=' | '=='</span></div>
<div class="REFTYPES"><span class="bold_code">Position = integer() >= 1</span></div>
<div class="REFTYPES"><span class="bold_code">Keys = [term()]</span></div>
<div class="REFTYPES"><span class="bold_code">Mod = Fun = atom()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Args = [term()]</span></div>
<div class="REFTYPES"><span class="bold_code">QH = <span class="bold_code"><a href="#type-query_handle">query_handle()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p><a name="table"></a>Returns a query handle for a
QLC table. In Erlang/OTP there is support for ETS, Dets and
Mnesia tables, but it is also possible to turn many other
data structures into QLC tables. The way to accomplish this
is to let function(s) in the module implementing the data
structure create a query handle by calling
<span class="code">qlc:table/2</span>. The different ways to traverse the table
as well as properties of the table are handled by callback
functions provided as options to <span class="code">qlc:table/2</span>.</p>
<p>The callback function <span class="code">TraverseFun</span> is
used for traversing the table. It is to return a list of
objects terminated by either <span class="code">[]</span> or a nullary fun to
be used for traversing the not yet traversed objects of the
table. Any other return value is immediately returned as
value of the query evaluation. Unary
<span class="code">TraverseFun</span>s are to accept a match
specification as argument. The match specification is
created by the parse transform by analyzing the pattern of
the generator calling <span class="code">qlc:table/2</span> and filters using
variables introduced in the pattern. If the parse transform
cannot find a match specification equivalent to the pattern
and filters, <span class="code">TraverseFun</span> will be called
with a match specification returning every object. Modules
that can utilize match specifications for optimized
traversal of tables should call <span class="code">qlc:table/2</span> with a
unary
<span class="code">TraverseFun</span> while other modules can
provide a nullary
<span class="code">TraverseFun</span>. <span class="code">ets:table/2</span> is an
example of the former; <span class="code">gb_table:table/1</span> in the
<span class="bold_code"><a href="#implementing_a_qlc_table">Implementing a
QLC table</a></span> section is an example of the latter.</p>
<p><span class="code">PreFun</span> is a unary callback function
that is called once before the table is read for the first
time. If the call fails, the query evaluation fails.
Similarly, the nullary callback function
<span class="code">PostFun</span> is called once after the table
was last read. The return value, which is caught, is
ignored. If <span class="code">PreFun</span> has been called for a
table,
<span class="code">PostFun</span> is guaranteed to be called for
that table, even if the evaluation of the query fails for
some reason. The order in which pre (post) functions for
different tables are evaluated is not specified. Other table
access than reading, such as calling
<span class="code">InfoFun</span>, is assumed to be OK at any
time. The argument <span class="code">PreArgs</span> is a list of
tagged values. Currently there are two tags,
<span class="code">parent_value</span> and <span class="code">stop_fun</span>, used by Mnesia for
managing transactions. The value of <span class="code">parent_value</span> is
the value returned by <span class="code">ParentFun</span>, or
<span class="code">undefined</span> if there is no <span class="code">ParentFun</span>.
<span class="code">ParentFun</span> is called once just before the
call of
<span class="code">PreFun</span> in the context of the process
calling
<span class="code">eval</span>, <span class="code">fold</span>, or
<span class="code">cursor</span>. The value of <span class="code">stop_fun</span> is a nullary fun
that deletes the cursor if called from the parent, or
<span class="code">undefined</span> if there is no cursor.</p>
<p><a name="lookup_fun"></a>The binary callback
function <span class="code">LookupFun</span> is used for looking
up objects in the table. The first argument
<span class="code">Position</span> is the key position or an
indexed position and the second argument
<span class="code">Keys</span> is a sorted list of unique values.
The return value is to be a list of all objects (tuples)
such that the element at <span class="code">Position</span> is a member of
<span class="code">Keys</span>. Any other return value is
immediately returned as value of the query evaluation.
<span class="code">LookupFun</span> is called instead of
traversing the table if the parse transform at compile time
can find out that the filters match and compare the element
at <span class="code">Position</span> in such a way that only
<span class="code">Keys</span> need to be looked up in order to
find all potential answers. The key position is obtained by
calling
<span class="code">InfoFun(keypos)</span> and the indexed
positions by calling
<span class="code">InfoFun(indices)</span>. If the key position
can be used for lookup it is always chosen, otherwise the
indexed position requiring the least number of lookups is
chosen. If there is a tie between two indexed positions the
one occurring first in the list returned by
<span class="code">InfoFun</span> is chosen. Positions requiring
more than <span class="bold_code"><a href="#max_lookup">max_lookup</a></span>
lookups are ignored.</p>
<p>The unary callback function <span class="code">InfoFun</span> is
to return information about the table. <span class="code">undefined</span>
should be returned if the value of some tag is unknown:</p>
<ul>
<li>
<span class="code">indices</span>. Returns a list of indexed
positions, a list of positive integers.
</li>
<li>
<span class="code">is_unique_objects</span>. Returns <span class="code">true</span> if
the objects returned by <span class="code">TraverseFun</span> are unique.
</li>
<li>
<span class="code">keypos</span>. Returns the position of the table's
key, a positive integer.
</li>
<li>
<span class="code">is_sorted_key</span>. Returns <span class="code">true</span> if
the objects returned by <span class="code">TraverseFun</span> are sorted
on the key.
</li>
<li>
<span class="code">num_of_objects</span>. Returns the number of
objects in the table, a non-negative integer.
</li>
</ul>
<p>The unary callback function <span class="code">FormatFun</span>
is used by <span class="bold_code"><a href="#info">qlc:info/1,2</a></span>
for displaying the call that created the table's query
handle. The default value, <span class="code">undefined</span>, means that
<span class="code">info/1,2</span> displays a call to <span class="code">'$MOD':'$FUN'/0</span>.
It is up to <span class="code">FormatFun</span> to present the
selected objects of the table in a suitable way. However, if
a character list is chosen for presentation it must be an
Erlang expression that can be scanned and parsed (a trailing
dot will be added by <span class="code">qlc:info</span> though).
<span class="code">FormatFun</span> is called with an argument
that describes the selected objects based on optimizations
done as a result of analyzing the filters of the QLC where
the call to
<span class="code">qlc:table/2</span> occurs. The possible values of the
argument are:</p>
<ul>
<li>
<span class="code">{lookup, Position, Keys, NElements, DepthFun}</span>.
<span class="code">LookupFun</span> is used for looking up objects in the
table.
</li>
<li>
<span class="code">{match_spec, MatchExpression}</span>. No way of
finding all possible answers by looking up keys was
found, but the filters could be transformed into a
match specification. All answers are found by calling
<span class="code">TraverseFun(MatchExpression)</span>.
</li>
<li>
<span class="code">{all, NElements, DepthFun}</span>. No optimization was
found. A match specification matching all objects will be
used if <span class="code">TraverseFun</span> is unary.
</li>
</ul>
<p><span class="code">NElements</span> is the value of the <span class="code">info/1,2</span> option
<span class="code">n_elements</span>, and <span class="code">DepthFun</span> is a function that
can be used for limiting the size of terms; calling
<span class="code">DepthFun(Term)</span> substitutes <span class="code">'...'</span> for parts of
<span class="code">Term</span> below the depth specified by the <span class="code">info/1,2</span>
option <span class="code">depth</span>. If calling
<span class="code">FormatFun</span> with an argument including
<span class="code">NElements</span> and <span class="code">DepthFun</span> fails,
<span class="code">FormatFun</span> is called once again with an
argument excluding
<span class="code">NElements</span> and <span class="code">DepthFun</span>
(<span class="code">{lookup, Position, Keys}</span> or
<span class="code">all</span>).</p>
<p><a name="key_equality"></a>The value of
<span class="code">key_equality</span> is to be <span class="code">'=:='</span> if the table
considers two keys equal if they match, and to be
<span class="code">'=='</span> if two keys are equal if they compare equal. The
default is <span class="code">'=:='</span>.</p>
<p>See <span class="bold_code"><a href="ets.html#qlc_table">ets(3)</a></span>,
<span class="bold_code"><a href="dets.html#qlc_table">dets(3)</a></span> and
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','mnesia','mnesia.html#qlc_table');">mnesia(3)</a></span>
for the various options recognized by <span class="code">table/1,2</span> in
respective module.</p>
</p></div>
<h3><a name="id203805">See Also</a></h3>
<div class="REFBODY">
<p><span class="bold_code"><a href="dets.html">dets(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/reference_manual','users_guide.html');">
Erlang Reference Manual</a></span>,
<span class="bold_code"><a href="erl_eval.html">erl_eval(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','erts','erlang.html');">erlang(3)</a></span>,
<span class="bold_code"><a href="ets.html">ets(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','kernel','file.html');">file(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','error_logger','file.html');">error_logger(3)</a></span>,
<span class="bold_code"><a href="file_sorter.html">file_sorter(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','mnesia','mnesia.html');">mnesia(3)</a></span>,
<span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/programming_examples','users_guide.html');">
Programming Examples</a></span>,
<span class="bold_code"><a href="shell.html">shell(3)</a></span></p>
</div>
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