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>Chapter 38. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Procedural Language</TD
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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-IMPLEMENTATION"
>38.10. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> Under the Hood</A
></H1
><P
> This section discusses some implementation details that are
frequently important for <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> users to know.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-VAR-SUBST"
>38.10.1. Variable Substitution</A
></H2
><P
> When <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> prepares a SQL statement or expression
for execution, any <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> variable name
appearing in the statement or expression is replaced by a parameter symbol,
<TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
>. The current value
of the variable is then provided as the value for the parameter whenever
the statement or expression is executed. As an example, consider the
function
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION logfunc(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp := now();
BEGIN
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;</PRE
><P>
The <TT
CLASS="COMMAND"
>INSERT</TT
> statement will effectively be processed as
</P><PRE
CLASS="PROGRAMLISTING"
>PREPARE <TT
CLASS="REPLACEABLE"
><I
>statement_name</I
></TT
>(text, timestamp) AS
INSERT INTO logtable VALUES ($1, $2);</PRE
><P>
followed on each execution by <TT
CLASS="COMMAND"
>EXECUTE</TT
> with the current
actual values of the two variables. (Note: here we are speaking of
the main SQL engine's
<A
HREF="sql-execute.html"
><I
>EXECUTE</I
></A
> command,
not <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>'s <TT
CLASS="COMMAND"
>EXECUTE</TT
>.)
</P
><P
> <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>The substitution mechanism will replace any token that matches a
known variable's name.</I
></SPAN
> This poses various traps for the unwary.
For example, it is a bad idea
to use a variable name that is the same as any table or column name
that you need to reference in queries within the function, because
what you think is a table or column name will still get replaced.
In the above example, suppose that <TT
CLASS="STRUCTNAME"
>logtable</TT
> has
column names <TT
CLASS="STRUCTFIELD"
>logtxt</TT
> and <TT
CLASS="STRUCTFIELD"
>logtime</TT
>,
and we try to write the <TT
CLASS="COMMAND"
>INSERT</TT
> as
</P><PRE
CLASS="PROGRAMLISTING"
> INSERT INTO logtable (logtxt, logtime) VALUES (logtxt, curtime);</PRE
><P>
This will be fed to the main SQL parser as
</P><PRE
CLASS="PROGRAMLISTING"
> INSERT INTO logtable ($1, logtime) VALUES ($1, $2);</PRE
><P>
resulting in a syntax error like this:
</P><PRE
CLASS="SCREEN"
>ERROR: syntax error at or near "$1"
LINE 1: INSERT INTO logtable ( $1 , logtime) VALUES ( $1 , $2 )
^
QUERY: INSERT INTO logtable ( $1 , logtime) VALUES ( $1 , $2 )
CONTEXT: SQL statement in PL/PgSQL function "logfunc2" near line 5</PRE
><P>
</P
><P
> This example is fairly easy to diagnose, since it leads to an
obvious syntax error. Much nastier are cases where the substitution
is syntactically permissible, since the only symptom may be misbehavior
of the function. In one case, a user wrote something like this:
</P><PRE
CLASS="PROGRAMLISTING"
> DECLARE
val text;
search_key integer;
BEGIN
...
FOR val IN SELECT val FROM table WHERE key = search_key LOOP ...</PRE
><P>
and wondered why all his table entries seemed to be NULL. Of course
what happened here was that the query became
</P><PRE
CLASS="PROGRAMLISTING"
> SELECT $1 FROM table WHERE key = $2</PRE
><P>
and thus it was just an expensive way of assigning <TT
CLASS="LITERAL"
>val</TT
>'s
current value back to itself for each row.
</P
><P
> A commonly used coding rule for avoiding such traps is to use a
different naming convention for <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
variables than you use for table and column names. For example,
if all your variables are named
<TT
CLASS="LITERAL"
>v_<TT
CLASS="REPLACEABLE"
><I
>something</I
></TT
></TT
> while none of your
table or column names start with <TT
CLASS="LITERAL"
>v_</TT
>, you're pretty safe.
</P
><P
> Another workaround is to use qualified (dotted) names for SQL entities.
For instance we could safely have written the above example as
</P><PRE
CLASS="PROGRAMLISTING"
> FOR val IN SELECT table.val FROM table WHERE key = search_key LOOP ...</PRE
><P>
because <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> will not substitute a
variable for a trailing component of a qualified name.
However this solution does not work in every case — you can't
qualify a name in an <TT
CLASS="COMMAND"
>INSERT</TT
>'s column name list, for instance.
Another point is that record and row variable names will be matched to
the first components of qualified names, so a qualified SQL name is
still vulnerable in some cases.
In such cases choosing a non-conflicting variable name is the only way.
</P
><P
> Another technique you can use is to attach a label to the block in
which your variables are declared, and then qualify the variable names
in your SQL commands (see <A
HREF="plpgsql-structure.html"
>Section 38.2</A
>).
For example,
</P><PRE
CLASS="PROGRAMLISTING"
> <<pl>>
DECLARE
val text;
BEGIN
...
UPDATE table SET col = pl.val WHERE ...</PRE
><P>
This is not in itself a solution to the problem of conflicts,
since an unqualified name in a SQL command is still at risk of being
interpreted the <SPAN
CLASS="QUOTE"
>"wrong"</SPAN
> way. But it is useful for clarifying
the intent of potentially-ambiguous code.
</P
><P
> Variable substitution does not happen in the command string given
to <TT
CLASS="COMMAND"
>EXECUTE</TT
> or one of its variants. If you need to
insert a varying value into such a command, do so as part of
constructing the string value, as illustrated in
<A
HREF="plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN"
>Section 38.5.4</A
>.
</P
><P
> Variable substitution currently works only in <TT
CLASS="COMMAND"
>SELECT</TT
>,
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, and <TT
CLASS="COMMAND"
>DELETE</TT
> commands,
because the main SQL engine allows parameter symbols only in these
commands. To use a non-constant name or value in other statement
types (generically called utility statements), you must construct
the utility statement as a string and <TT
CLASS="COMMAND"
>EXECUTE</TT
> it.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-PLAN-CACHING"
>38.10.2. Plan Caching</A
></H2
><P
> The <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> interpreter parses the function's source
text and produces an internal binary instruction tree the first time the
function is called (within each session). The instruction tree
fully translates the
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> statement structure, but individual
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> expressions and <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> commands
used in the function are not translated immediately.
</P
><P
> As each expression and <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> command is first
executed in the function, the <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> interpreter
creates a prepared execution plan (using the
<ACRONYM
CLASS="ACRONYM"
>SPI</ACRONYM
> manager's <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
>
and <CODE
CLASS="FUNCTION"
>SPI_saveplan</CODE
>
functions).<A
NAME="AEN48649"
></A
> Subsequent visits to that expression or command
reuse the prepared plan. Thus, a function with conditional code
that contains many statements for which execution plans might be
required will only prepare and save those plans that are really
used during the lifetime of the database connection. This can
substantially reduce the total amount of time required to parse
and generate execution plans for the statements in a
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function. A disadvantage is that errors
in a specific expression or command cannot be detected until that
part of the function is reached in execution. (Trivial syntax
errors will be detected during the initial parsing pass, but
anything deeper will not be detected until execution.)
</P
><P
> Once <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> has made an execution plan for a particular
command in a function, it will reuse that plan for the life of the
database connection. This is usually a win for performance, but it
can cause some problems if you dynamically
alter your database schema. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION populate() RETURNS integer AS $$
DECLARE
-- declarations
BEGIN
PERFORM my_function();
END;
$$ LANGUAGE plpgsql;</PRE
><P>
If you execute the above function, it will reference the OID for
<CODE
CLASS="FUNCTION"
>my_function()</CODE
> in the execution plan produced for
the <TT
CLASS="COMMAND"
>PERFORM</TT
> statement. Later, if you
drop and recreate <CODE
CLASS="FUNCTION"
>my_function()</CODE
>, then
<CODE
CLASS="FUNCTION"
>populate()</CODE
> will not be able to find
<CODE
CLASS="FUNCTION"
>my_function()</CODE
> anymore. You would then have to
start a new database session so that <CODE
CLASS="FUNCTION"
>populate()</CODE
>
will be compiled afresh, before it will work again. You can avoid
this problem by using <TT
CLASS="COMMAND"
>CREATE OR REPLACE FUNCTION</TT
>
when updating the definition of
<CODE
CLASS="FUNCTION"
>my_function</CODE
>, since when a function is
<SPAN
CLASS="QUOTE"
>"replaced"</SPAN
>, its OID is not changed.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.3 and later, saved plans
will be replaced whenever any schema changes have occurred to any
tables they reference. This eliminates one of the major disadvantages
of saved plans. However, there is no such mechanism for function
references, and thus the above example involving a reference to a
deleted function is still valid.
</P
></BLOCKQUOTE
></DIV
><P
> Because <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> saves execution plans
in this way, SQL commands that appear directly in a
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function must refer to the
same tables and columns on every execution; that is, you cannot use
a parameter as the name of a table or column in an SQL command. To get
around this restriction, you can construct dynamic commands using
the <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> <TT
CLASS="COMMAND"
>EXECUTE</TT
>
statement — at the price of constructing a new execution plan on
every execution.
</P
><P
> Another important point is that the prepared plans are parameterized
to allow the values of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> variables
to change from one use to the next, as discussed in detail above.
Sometimes this means that a plan is less efficient than it would be
if generated for a specific variable value. As an example, consider
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * INTO myrec FROM dictionary WHERE word LIKE search_term;</PRE
><P>
where <TT
CLASS="LITERAL"
>search_term</TT
> is a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
variable. The cached plan for this query will never use an index on
<TT
CLASS="STRUCTFIELD"
>word</TT
>, since the planner cannot assume that the
<TT
CLASS="LITERAL"
>LIKE</TT
> pattern will be left-anchored at run time. To use
an index the query must be planned with a specific constant
<TT
CLASS="LITERAL"
>LIKE</TT
> pattern provided. This is another situation where
<TT
CLASS="COMMAND"
>EXECUTE</TT
> can be used to force a new plan to be
generated for each execution.
</P
><P
> The mutable nature of record variables presents another problem in this
connection. When fields of a record variable are used in
expressions or statements, the data types of the fields must not
change from one call of the function to the next, since each
expression will be planned using the data type that is present
when the expression is first reached. <TT
CLASS="COMMAND"
>EXECUTE</TT
> can be
used to get around this problem when necessary.
</P
><P
> If the same function is used as a trigger for more than one table,
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> prepares and caches plans
independently for each such table — that is, there is a cache
for each trigger function and table combination, not just for each
function. This alleviates some of the problems with varying
data types; for instance, a trigger function will be able to work
successfully with a column named <TT
CLASS="LITERAL"
>key</TT
> even if it happens
to have different types in different tables.
</P
><P
> Likewise, functions having polymorphic argument types have a separate
plan cache for each combination of actual argument types they have been
invoked for, so that data type differences do not cause unexpected
failures.
</P
><P
> Plan caching can sometimes have surprising effects on the interpretation
of time-sensitive values. For example there
is a difference between what these two functions do:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
BEGIN
INSERT INTO logtable VALUES (logtxt, 'now');
END;
$$ LANGUAGE plpgsql;</PRE
><P>
and:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp;
BEGIN
curtime := 'now';
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;</PRE
><P>
</P
><P
> In the case of <CODE
CLASS="FUNCTION"
>logfunc1</CODE
>, the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> main parser knows when
preparing the plan for the <TT
CLASS="COMMAND"
>INSERT</TT
> that the
string <TT
CLASS="LITERAL"
>'now'</TT
> should be interpreted as
<TT
CLASS="TYPE"
>timestamp</TT
>, because the target column of
<CODE
CLASS="CLASSNAME"
>logtable</CODE
> is of that type. Thus,
<TT
CLASS="LITERAL"
>'now'</TT
> will be converted to a constant when the
<TT
CLASS="COMMAND"
>INSERT</TT
> is planned, and then used in all
invocations of <CODE
CLASS="FUNCTION"
>logfunc1</CODE
> during the lifetime
of the session. Needless to say, this isn't what the programmer
wanted.
</P
><P
> In the case of <CODE
CLASS="FUNCTION"
>logfunc2</CODE
>, the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> main parser does not know
what type <TT
CLASS="LITERAL"
>'now'</TT
> should become and therefore
it returns a data value of type <TT
CLASS="TYPE"
>text</TT
> containing the string
<TT
CLASS="LITERAL"
>now</TT
>. During the ensuing assignment
to the local variable <TT
CLASS="VARNAME"
>curtime</TT
>, the
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> interpreter casts this
string to the <TT
CLASS="TYPE"
>timestamp</TT
> type by calling the
<CODE
CLASS="FUNCTION"
>text_out</CODE
> and <CODE
CLASS="FUNCTION"
>timestamp_in</CODE
>
functions for the conversion. So, the computed time stamp is updated
on each execution as the programmer expects.
</P
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