Re: faster SELECT

On Mar 8, 2004, at 5:40 AM, Chris wrote:

>
>> in a 'SELECT', does postgres read the 'WHERE' condition from left to
>> right.
>
> PostgreSQL (SQL in general?) does NOT define evaluation order (unlike
> programming languages like C).
>

It is a fairly well known optimization in Oracle (at least with the
rule-based optimizer, and IIRC non-indexed clauses) to optimize the
WHERE clause right to left.

>
>> for example
>> 1) select ... where a and b;
>> 2) select ... where b and a;
>>
>> 1 and 2 will use the same cpu time or not ?
>
> Depends on the problem.
> In theory the optimizer should find the best evaluation order
> after analyzing a and b.
>
>
> Bye, Chris.
>
>
>
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Andrew Rawnsley
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