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
PL/pgSQL will evaluate the expression by feeding a query like
to the main SQL engine. While forming the
any occurrences of PL/pgSQL variable names
are replaced by parameters, as discussed in detail in
This allows the query plan for the
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
y, and we write
IF x < y THEN ...
what happens behind the scenes is equivalent to
statement_name(integer, integer) AS SELECT $1 < $2;
and then this prepared statement is
EXECUTEd 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 42.11.2.