Andrew Ballard wrote:
> On Thu, Apr 9, 2009 at 1:39 PM, PJ <af.gourmet@xxxxxxxxxxxx> wrote:
> [snip]
>
>> Now the silly questions = often I am curious what effect the differences
>> in code will have on performance.
>> For instance, why are you using author.last_name LIKE '{$Auth}%' rather
>> than LEFT('$Auth') ? As I have mentioned, it is not important what
>> follows the A
>>
>
> The query optimizer can use an index on author.last_name to evaluate
> "WHERE author.last_name LIKE '{$Auth}%' ". It simply seeks/scans the
> index since the last_name values are ordered, and starts returning
> values when it reaches the first row that begins with $Auth and stops
> returning rows when it finds a value is greater than the pattern.
>
> To evaluate "WHERE LEFT('$Auth', LENGTH('$Auth')) = '$Auth' ", the
> server has to calculate a substring of last_name for every row in the
> table and then compare those substrings to the value $Auth.
>
> Andrew
>
>
Thanks Andrew. Just what I wanted to know. ;-)
--
unheralded genius: "A clean desk is the sign of a dull mind. "
-------------------------------------------------------------
Phil Jourdan --- pj@xxxxxxxxxxxxx
http://www.ptahhotep.com
http://www.chiccantine.com/andypantry.php
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