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Avoid mathematical inconsistency in example about avoiding division by

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zero with a CASE expression.  Per gripe from Russell Smith.
This commit is contained in:
Tom Lane 2008-01-23 19:51:29 +00:00
parent 8984c8bbe2
commit 216e63bbce
1 changed files with 4 additions and 4 deletions
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doc/src/sgml/syntax.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.120 2007/12/11 18:30:20 mha Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.121 2008/01/23 19:51:29 tgl Exp $ -->
<chapter id="sql-syntax"> <chapter id="sql-syntax">
<title>SQL Syntax</title> <title>SQL Syntax</title>
@ -1740,15 +1740,15 @@ SELECT somefunc() OR true;
used. For example, this is an untrustworthy way of trying to used. For example, this is an untrustworthy way of trying to
avoid division by zero in a <literal>WHERE</> clause: avoid division by zero in a <literal>WHERE</> clause:
<programlisting> <programlisting>
SELECT ... WHERE x &lt;&gt; 0 AND y/x &gt; 1.5; SELECT ... WHERE x &gt; 0 AND y/x &gt; 1.5;
</programlisting> </programlisting>
But this is safe: But this is safe:
<programlisting> <programlisting>
SELECT ... WHERE CASE WHEN x &lt;&gt; 0 THEN y/x &gt; 1.5 ELSE false END; SELECT ... WHERE CASE WHEN x &gt; 0 THEN y/x &gt; 1.5 ELSE false END;
</programlisting> </programlisting>
A <literal>CASE</> construct used in this fashion will defeat optimization A <literal>CASE</> construct used in this fashion will defeat optimization
attempts, so it should only be done when necessary. (In this particular attempts, so it should only be done when necessary. (In this particular
example, it would be best to sidestep the problem by writing example, it would be better to sidestep the problem by writing
<literal>y &gt; 1.5*x</> instead.) <literal>y &gt; 1.5*x</> instead.)
</para> </para>
</sect2> </sect2>