<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>61.2. Index Access Method Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="index-api.html" title="61.1. Basic API Structure for Indexes" /><link rel="next" href="index-scanning.html" title="61.3. Index Scanning" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">61.2. Index Access Method Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-api.html" title="61.1. Basic API Structure for Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 61. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 61. Index Access Method Interface Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="index-scanning.html" title="61.3. Index Scanning">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="INDEX-FUNCTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">61.2. Index Access Method Functions</h2></div></div></div><p>
The index construction and maintenance functions that an index access
method must provide in <code class="structname">IndexAmRoutine</code> are:
</p><p>
</p><pre class="programlisting">
IndexBuildResult *
ambuild (Relation heapRelation,
Relation indexRelation,
IndexInfo *indexInfo);
</pre><p>
Build a new index. The index relation has been physically created,
but is empty. It must be filled in with whatever fixed data the
access method requires, plus entries for all tuples already existing
in the table. Ordinarily the <code class="function">ambuild</code> function will call
<code class="function">IndexBuildHeapScan()</code> to scan the table for existing tuples
and compute the keys that need to be inserted into the index.
The function must return a palloc'd struct containing statistics about
the new index.
</p><p>
</p><pre class="programlisting">
void
ambuildempty (Relation indexRelation);
</pre><p>
Build an empty index, and write it to the initialization fork (<code class="symbol">INIT_FORKNUM</code>)
of the given relation. This method is called only for unlogged indexes; the
empty index written to the initialization fork will be copied over the main
relation fork on each server restart.
</p><p>
</p><pre class="programlisting">
bool
aminsert (Relation indexRelation,
Datum *values,
bool *isnull,
ItemPointer heap_tid,
Relation heapRelation,
IndexUniqueCheck checkUnique,
IndexInfo *indexInfo);
</pre><p>
Insert a new tuple into an existing index. The <code class="literal">values</code> and
<code class="literal">isnull</code> arrays give the key values to be indexed, and
<code class="literal">heap_tid</code> is the TID to be indexed.
If the access method supports unique indexes (its
<code class="structfield">amcanunique</code> flag is true) then
<code class="literal">checkUnique</code> indicates the type of uniqueness check to
perform. This varies depending on whether the unique constraint is
deferrable; see <a class="xref" href="index-unique-checks.html" title="61.5. Index Uniqueness Checks">Section 61.5</a> for details.
Normally the access method only needs the <code class="literal">heapRelation</code>
parameter when performing uniqueness checking (since then it will have to
look into the heap to verify tuple liveness).
</p><p>
The function's Boolean result value is significant only when
<code class="literal">checkUnique</code> is <code class="literal">UNIQUE_CHECK_PARTIAL</code>.
In this case a true result means the new entry is known unique, whereas
false means it might be non-unique (and a deferred uniqueness check must
be scheduled). For other cases a constant false result is recommended.
</p><p>
Some indexes might not index all tuples. If the tuple is not to be
indexed, <code class="function">aminsert</code> should just return without doing anything.
</p><p>
If the index AM wishes to cache data across successive index insertions
within a SQL statement, it can allocate space
in <code class="literal">indexInfo->ii_Context</code> and store a pointer to the
data in <code class="literal">indexInfo->ii_AmCache</code> (which will be NULL
initially).
</p><p>
</p><pre class="programlisting">
IndexBulkDeleteResult *
ambulkdelete (IndexVacuumInfo *info,
IndexBulkDeleteResult *stats,
IndexBulkDeleteCallback callback,
void *callback_state);
</pre><p>
Delete tuple(s) from the index. This is a <span class="quote">“<span class="quote">bulk delete</span>”</span> operation
that is intended to be implemented by scanning the whole index and checking
each entry to see if it should be deleted.
The passed-in <code class="literal">callback</code> function must be called, in the style
<code class="literal">callback(<em class="replaceable"><code>TID</code></em>, callback_state) returns bool</code>,
to determine whether any particular index entry, as identified by its
referenced TID, is to be deleted. Must return either NULL or a palloc'd
struct containing statistics about the effects of the deletion operation.
It is OK to return NULL if no information needs to be passed on to
<code class="function">amvacuumcleanup</code>.
</p><p>
Because of limited <code class="varname">maintenance_work_mem</code>,
<code class="function">ambulkdelete</code> might need to be called more than once when many
tuples are to be deleted. The <code class="literal">stats</code> argument is the result
of the previous call for this index (it is NULL for the first call within a
<code class="command">VACUUM</code> operation). This allows the AM to accumulate statistics
across the whole operation. Typically, <code class="function">ambulkdelete</code> will
modify and return the same struct if the passed <code class="literal">stats</code> is not
null.
</p><p>
</p><pre class="programlisting">
IndexBulkDeleteResult *
amvacuumcleanup (IndexVacuumInfo *info,
IndexBulkDeleteResult *stats);
</pre><p>
Clean up after a <code class="command">VACUUM</code> operation (zero or more
<code class="function">ambulkdelete</code> calls). This does not have to do anything
beyond returning index statistics, but it might perform bulk cleanup
such as reclaiming empty index pages. <code class="literal">stats</code> is whatever the
last <code class="function">ambulkdelete</code> call returned, or NULL if
<code class="function">ambulkdelete</code> was not called because no tuples needed to be
deleted. If the result is not NULL it must be a palloc'd struct.
The statistics it contains will be used to update <code class="structname">pg_class</code>,
and will be reported by <code class="command">VACUUM</code> if <code class="literal">VERBOSE</code> is given.
It is OK to return NULL if the index was not changed at all during the
<code class="command">VACUUM</code> operation, but otherwise correct stats should
be returned.
</p><p>
As of <span class="productname">PostgreSQL</span> 8.4,
<code class="function">amvacuumcleanup</code> will also be called at completion of an
<code class="command">ANALYZE</code> operation. In this case <code class="literal">stats</code> is always
NULL and any return value will be ignored. This case can be distinguished
by checking <code class="literal">info->analyze_only</code>. It is recommended
that the access method do nothing except post-insert cleanup in such a
call, and that only in an autovacuum worker process.
</p><p>
</p><pre class="programlisting">
bool
amcanreturn (Relation indexRelation, int attno);
</pre><p>
Check whether the index can support <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes"><em class="firstterm">index-only scans</em></a> on
the given column, by returning the indexed column values for an index entry
in the form of an <code class="structname">IndexTuple</code>. The attribute number
is 1-based, i.e. the first column's attno is 1. Returns true if supported,
else false. If the access method does not support index-only scans at all,
the <code class="structfield">amcanreturn</code> field in its <code class="structname">IndexAmRoutine</code>
struct can be set to NULL.
</p><p>
</p><pre class="programlisting">
void
amcostestimate (PlannerInfo *root,
IndexPath *path,
double loop_count,
Cost *indexStartupCost,
Cost *indexTotalCost,
Selectivity *indexSelectivity,
double *indexCorrelation,
double *indexPages);
</pre><p>
Estimate the costs of an index scan. This function is described fully
in <a class="xref" href="index-cost-estimation.html" title="61.6. Index Cost Estimation Functions">Section 61.6</a>, below.
</p><p>
</p><pre class="programlisting">
bytea *
amoptions (ArrayType *reloptions,
bool validate);
</pre><p>
Parse and validate the reloptions array for an index. This is called only
when a non-null reloptions array exists for the index.
<em class="parameter"><code>reloptions</code></em> is a <code class="type">text</code> array containing entries of the
form <em class="replaceable"><code>name</code></em><code class="literal">=</code><em class="replaceable"><code>value</code></em>.
The function should construct a <code class="type">bytea</code> value, which will be copied
into the <code class="structfield">rd_options</code> field of the index's relcache entry.
The data contents of the <code class="type">bytea</code> value are open for the access
method to define; most of the standard access methods use struct
<code class="structname">StdRdOptions</code>.
When <em class="parameter"><code>validate</code></em> is true, the function should report a suitable
error message if any of the options are unrecognized or have invalid
values; when <em class="parameter"><code>validate</code></em> is false, invalid entries should be
silently ignored. (<em class="parameter"><code>validate</code></em> is false when loading options
already stored in <code class="structname">pg_catalog</code>; an invalid entry could only
be found if the access method has changed its rules for options, and in
that case ignoring obsolete entries is appropriate.)
It is OK to return NULL if default behavior is wanted.
</p><p>
</p><pre class="programlisting">
bool
amproperty (Oid index_oid, int attno,
IndexAMProperty prop, const char *propname,
bool *res, bool *isnull);
</pre><p>
The <code class="function">amproperty</code> method allows index access methods to override
the default behavior of <code class="function">pg_index_column_has_property</code>
and related functions.
If the access method does not have any special behavior for index property
inquiries, the <code class="structfield">amproperty</code> field in
its <code class="structname">IndexAmRoutine</code> struct can be set to NULL.
Otherwise, the <code class="function">amproperty</code> method will be called with
<em class="parameter"><code>index_oid</code></em> and <em class="parameter"><code>attno</code></em> both zero for
<code class="function">pg_indexam_has_property</code> calls,
or with <em class="parameter"><code>index_oid</code></em> valid and <em class="parameter"><code>attno</code></em> zero for
<code class="function">pg_index_has_property</code> calls,
or with <em class="parameter"><code>index_oid</code></em> valid and <em class="parameter"><code>attno</code></em> greater than
zero for <code class="function">pg_index_column_has_property</code> calls.
<em class="parameter"><code>prop</code></em> is an enum value identifying the property being tested,
while <em class="parameter"><code>propname</code></em> is the original property name string.
If the core code does not recognize the property name
then <em class="parameter"><code>prop</code></em> is <code class="literal">AMPROP_UNKNOWN</code>.
Access methods can define custom property names by
checking <em class="parameter"><code>propname</code></em> for a match (use <code class="function">pg_strcasecmp</code>
to match, for consistency with the core code); for names known to the core
code, it's better to inspect <em class="parameter"><code>prop</code></em>.
If the <code class="structfield">amproperty</code> method returns <code class="literal">true</code> then
it has determined the property test result: it must set <code class="literal">*res</code>
to the boolean value to return, or set <code class="literal">*isnull</code>
to <code class="literal">true</code> to return a NULL. (Both of the referenced variables
are initialized to <code class="literal">false</code> before the call.)
If the <code class="structfield">amproperty</code> method returns <code class="literal">false</code> then
the core code will proceed with its normal logic for determining the
property test result.
</p><p>
Access methods that support ordering operators should
implement <code class="literal">AMPROP_DISTANCE_ORDERABLE</code> property testing, as the
core code does not know how to do that and will return NULL. It may
also be advantageous to implement <code class="literal">AMPROP_RETURNABLE</code> testing,
if that can be done more cheaply than by opening the index and calling
<code class="structfield">amcanreturn</code>, which is the core code's default behavior.
The default behavior should be satisfactory for all other standard
properties.
</p><p>
</p><pre class="programlisting">
bool
amvalidate (Oid opclassoid);
</pre><p>
Validate the catalog entries for the specified operator class, so far as
the access method can reasonably do that. For example, this might include
testing that all required support functions are provided.
The <code class="function">amvalidate</code> function must return false if the opclass is
invalid. Problems should be reported with <code class="function">ereport</code> messages.
</p><p>
The purpose of an index, of course, is to support scans for tuples matching
an indexable <code class="literal">WHERE</code> condition, often called a
<em class="firstterm">qualifier</em> or <em class="firstterm">scan key</em>. The semantics of
index scanning are described more fully in <a class="xref" href="index-scanning.html" title="61.3. Index Scanning">Section 61.3</a>,
below. An index access method can support <span class="quote">“<span class="quote">plain</span>”</span> index scans,
<span class="quote">“<span class="quote">bitmap</span>”</span> index scans, or both. The scan-related functions that an
index access method must or may provide are:
</p><p>
</p><pre class="programlisting">
IndexScanDesc
ambeginscan (Relation indexRelation,
int nkeys,
int norderbys);
</pre><p>
Prepare for an index scan. The <code class="literal">nkeys</code> and <code class="literal">norderbys</code>
parameters indicate the number of quals and ordering operators that will be
used in the scan; these may be useful for space allocation purposes.
Note that the actual values of the scan keys aren't provided yet.
The result must be a palloc'd struct.
For implementation reasons the index access method
<span class="emphasis"><em>must</em></span> create this struct by calling
<code class="function">RelationGetIndexScan()</code>. In most cases
<code class="function">ambeginscan</code> does little beyond making that call and perhaps
acquiring locks;
the interesting parts of index-scan startup are in <code class="function">amrescan</code>.
</p><p>
</p><pre class="programlisting">
void
amrescan (IndexScanDesc scan,
ScanKey keys,
int nkeys,
ScanKey orderbys,
int norderbys);
</pre><p>
Start or restart an index scan, possibly with new scan keys. (To restart
using previously-passed keys, NULL is passed for <code class="literal">keys</code> and/or
<code class="literal">orderbys</code>.) Note that it is not allowed for
the number of keys or order-by operators to be larger than
what was passed to <code class="function">ambeginscan</code>. In practice the restart
feature is used when a new outer tuple is selected by a nested-loop join
and so a new key comparison value is needed, but the scan key structure
remains the same.
</p><p>
</p><pre class="programlisting">
boolean
amgettuple (IndexScanDesc scan,
ScanDirection direction);
</pre><p>
Fetch the next tuple in the given scan, moving in the given
direction (forward or backward in the index). Returns true if a tuple was
obtained, false if no matching tuples remain. In the true case the tuple
TID is stored into the <code class="literal">scan</code> structure. Note that
<span class="quote">“<span class="quote">success</span>”</span> means only that the index contains an entry that matches
the scan keys, not that the tuple necessarily still exists in the heap or
will pass the caller's snapshot test. On success, <code class="function">amgettuple</code>
must also set <code class="literal">scan->xs_recheck</code> to true or false.
False means it is certain that the index entry matches the scan keys.
true means this is not certain, and the conditions represented by the
scan keys must be rechecked against the heap tuple after fetching it.
This provision supports <span class="quote">“<span class="quote">lossy</span>”</span> index operators.
Note that rechecking will extend only to the scan conditions; a partial
index predicate (if any) is never rechecked by <code class="function">amgettuple</code>
callers.
</p><p>
If the index supports <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">index-only
scans</a> (i.e., <code class="function">amcanreturn</code> returns true for it),
then on success the AM must also check <code class="literal">scan->xs_want_itup</code>,
and if that is true it must return the originally indexed data for the
index entry. The data can be returned in the form of an
<code class="structname">IndexTuple</code> pointer stored at <code class="literal">scan->xs_itup</code>,
with tuple descriptor <code class="literal">scan->xs_itupdesc</code>; or in the form of
a <code class="structname">HeapTuple</code> pointer stored at <code class="literal">scan->xs_hitup</code>,
with tuple descriptor <code class="literal">scan->xs_hitupdesc</code>. (The latter
format should be used when reconstructing data that might possibly not fit
into an <code class="structname">IndexTuple</code>.) In either case,
management of the data referenced by the pointer is the access method's
responsibility. The data must remain good at least until the next
<code class="function">amgettuple</code>, <code class="function">amrescan</code>, or <code class="function">amendscan</code>
call for the scan.
</p><p>
The <code class="function">amgettuple</code> function need only be provided if the access
method supports <span class="quote">“<span class="quote">plain</span>”</span> index scans. If it doesn't, the
<code class="structfield">amgettuple</code> field in its <code class="structname">IndexAmRoutine</code>
struct must be set to NULL.
</p><p>
</p><pre class="programlisting">
int64
amgetbitmap (IndexScanDesc scan,
TIDBitmap *tbm);
</pre><p>
Fetch all tuples in the given scan and add them to the caller-supplied
<code class="type">TIDBitmap</code> (that is, OR the set of tuple IDs into whatever set is already
in the bitmap). The number of tuples fetched is returned (this might be
just an approximate count, for instance some AMs do not detect duplicates).
While inserting tuple IDs into the bitmap, <code class="function">amgetbitmap</code> can
indicate that rechecking of the scan conditions is required for specific
tuple IDs. This is analogous to the <code class="literal">xs_recheck</code> output parameter
of <code class="function">amgettuple</code>. Note: in the current implementation, support
for this feature is conflated with support for lossy storage of the bitmap
itself, and therefore callers recheck both the scan conditions and the
partial index predicate (if any) for recheckable tuples. That might not
always be true, however.
<code class="function">amgetbitmap</code> and
<code class="function">amgettuple</code> cannot be used in the same index scan; there
are other restrictions too when using <code class="function">amgetbitmap</code>, as explained
in <a class="xref" href="index-scanning.html" title="61.3. Index Scanning">Section 61.3</a>.
</p><p>
The <code class="function">amgetbitmap</code> function need only be provided if the access
method supports <span class="quote">“<span class="quote">bitmap</span>”</span> index scans. If it doesn't, the
<code class="structfield">amgetbitmap</code> field in its <code class="structname">IndexAmRoutine</code>
struct must be set to NULL.
</p><p>
</p><pre class="programlisting">
void
amendscan (IndexScanDesc scan);
</pre><p>
End a scan and release resources. The <code class="literal">scan</code> struct itself
should not be freed, but any locks or pins taken internally by the
access method must be released, as well as any other memory allocated
by <code class="function">ambeginscan</code> and other scan-related functions.
</p><p>
</p><pre class="programlisting">
void
ammarkpos (IndexScanDesc scan);
</pre><p>
Mark current scan position. The access method need only support one
remembered scan position per scan.
</p><p>
The <code class="function">ammarkpos</code> function need only be provided if the access
method supports ordered scans. If it doesn't,
the <code class="structfield">ammarkpos</code> field in its <code class="structname">IndexAmRoutine</code>
struct may be set to NULL.
</p><p>
</p><pre class="programlisting">
void
amrestrpos (IndexScanDesc scan);
</pre><p>
Restore the scan to the most recently marked position.
</p><p>
The <code class="function">amrestrpos</code> function need only be provided if the access
method supports ordered scans. If it doesn't,
the <code class="structfield">amrestrpos</code> field in its <code class="structname">IndexAmRoutine</code>
struct may be set to NULL.
</p><p>
In addition to supporting ordinary index scans, some types of index
may wish to support <em class="firstterm">parallel index scans</em>, which allow
multiple backends to cooperate in performing an index scan. The
index access method should arrange things so that each cooperating
process returns a subset of the tuples that would be performed by
an ordinary, non-parallel index scan, but in such a way that the
union of those subsets is equal to the set of tuples that would be
returned by an ordinary, non-parallel index scan. Furthermore, while
there need not be any global ordering of tuples returned by a parallel
scan, the ordering of that subset of tuples returned within each
cooperating backend must match the requested ordering. The following
functions may be implemented to support parallel index scans:
</p><p>
</p><pre class="programlisting">
Size
amestimateparallelscan (void);
</pre><p>
Estimate and return the number of bytes of dynamic shared memory which
the access method will be needed to perform a parallel scan. (This number
is in addition to, not in lieu of, the amount of space needed for
AM-independent data in <code class="structname">ParallelIndexScanDescData</code>.)
</p><p>
It is not necessary to implement this function for access methods which
do not support parallel scans or for which the number of additional bytes
of storage required is zero.
</p><p>
</p><pre class="programlisting">
void
aminitparallelscan (void *target);
</pre><p>
This function will be called to initialize dynamic shared memory at the
beginning of a parallel scan. <em class="parameter"><code>target</code></em> will point to at least
the number of bytes previously returned by
<code class="function">amestimateparallelscan</code>, and this function may use that
amount of space to store whatever data it wishes.
</p><p>
It is not necessary to implement this function for access methods which
do not support parallel scans or in cases where the shared memory space
required needs no initialization.
</p><p>
</p><pre class="programlisting">
void
amparallelrescan (IndexScanDesc scan);
</pre><p>
This function, if implemented, will be called when a parallel index scan
must be restarted. It should reset any shared state set up by
<code class="function">aminitparallelscan</code> such that the scan will be restarted from
the beginning.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index-api.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-scanning.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">61.1. Basic API Structure for Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 61.3. Index Scanning</td></tr></table></div></body></html>
