41.4. Expressions #
All expressions used in PL/pgSQL statements are processed using the server's main SQL executor. For example, when you write a PL/pgSQL statement like
IF expression
THEN ...
PL/pgSQL will evaluate the expression by feeding a query like
SELECT expression
to the main SQL engine. While forming the SELECT
command, any occurrences of PL/pgSQL variable names are replaced by query parameters, as discussed in detail in Section 41.11.1. This allows the query plan for the SELECT
to be prepared just once and then reused for subsequent evaluations with different values of the variables. Thus, what really happens on first use of an expression is essentially a PREPARE
command. For example, if we have declared two integer variables x
and y
, and we write
IF x < y THEN ...
what happens behind the scenes is equivalent to
PREPARE statement_name
(integer, integer) AS SELECT $1 < $2;
and then this prepared statement is EXECUTE
d for each execution of the IF
statement, with the current values of the PL/pgSQL variables supplied as parameter values. Normally these details are not important to a PL/pgSQL user, but they are useful to know when trying to diagnose a problem. More information appears in Section 41.11.2.
Since an expression
is converted to a SELECT
command, it can contain the same clauses that an ordinary SELECT
would, except that it cannot include a top-level UNION
, INTERSECT
, or EXCEPT
clause. Thus for example one could test whether a table is non-empty with
IF count(*) > 0 FROM my_table THEN ...
since the expression
between IF
and THEN
is parsed as though it were SELECT count(*) > 0 FROM my_table
. The SELECT
must produce a single column, and not more than one row. (If it produces no rows, the result is taken as NULL.)