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Add a little more material to the new section about evaluation order.

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Tom Lane
2002-06-15 21:28:55 +00:00
parent eb1ad5b4b5
commit 2da3742cf5
1 changed files with 25 additions and 6 deletions
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31
doc/src/sgml/syntax.sgml
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@@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.61 2002/06/01 20:56:55 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.62 2002/06/15 21:28:55 tgl Exp $
-->
<chapter id="sql-syntax">
@@ -1435,14 +1435,13 @@ FROM states;
<para>
The order of evaluation of subexpressions is not defined. In
particular, subexpressions are not necessarily evaluated
left-to-right, right-to-left, or according to the lexical
precedence rules.
particular, the inputs of an operator or function are not necessarily
evaluated left-to-right or in any other fixed order.
</para>
<para>
Furthermore, if the result of an expression can be determined by
evaluating only some parts of it, then some subexpressions
evaluating only some parts of it, then other subexpressions
might not be evaluated at all. For instance, if one wrote
<programlisting>
SELECT true OR somefunc();
@@ -1459,7 +1458,27 @@ SELECT somefunc() OR true;
<para>
As a consequence, it is unwise to use functions with side effects
as part of complex expressions.
as part of complex expressions. It is particularly dangerous to
rely on side effects or evaluation order in WHERE and HAVING clauses,
since those clauses are extensively reprocessed as part of
developing an execution plan. Boolean
expressions (AND/OR/NOT combinations) in those clauses may be reorganized
in any manner allowed by the laws of Boolean algebra.
</para>
<para>
When it is essential to force evaluation order, a CASE construct may
be used. For example, this is an untrustworthy way of trying to
avoid division by zero in a WHERE clause:
<programlisting>
SELECT ... WHERE x <> 0 AND y/x > 1.5;
</programlisting>
but this is safe:
<programlisting>
SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
</programlisting>
A CASE construct used in this fashion will defeat optimization attempts,
so it should only be done when necessary.
</para>
</sect2>
</sect1>