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
SELECT command, any occurrences of PL/pgSQL variable names are replaced by query parameters, as discussed in detail in Section 43.11.1. 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
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 43.11.2.
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
EXCEPT clause. Thus for example one could test whether a table is non-empty with
IF count(*) > 0 FROM my_table THEN ...
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.)