PostgreSQL 8.3.1 Documentation
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# 38.4. 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 parameters, as discussed in detail in Section 38.10.1. 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

    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. The query plan prepared in this way is saved for the life of the database connection, as described in Section 38.10.2. Normally these details are not important to a PL/pgSQL user, but they are useful to know when trying to diagnose a problem.

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