List:General Discussion« Previous MessageNext Message »
From:Michael Dykman Date:March 18 2014 7:28pm
Subject:Re: Precedence in WHERE clauses.
View as plain text  
My advice to you is to make use of the EXPLAIN facility which
porovides the most accurate information about how MySQL is going to
treat your query.

Also, as you currently have it, the expression DATE_SUB(NOW(),
INTERVAL 24 is going to be executed once for every single candidate
row.  I would suggest you temporarily memoize that like so:

select into DATE_SUB(NOW(), INTERVAL 24 HOUR) INTO @yesterday  ;

SELECT * FROM Status WHERE DWProcessed = 0 AND PreviousStatus NOT IN
('PENDING', 'ACCEPTED') AND SubscribeDate < @yesterday);

On Tue, Mar 18, 2014 at 3:10 PM, Christophe <tech@stripped> wrote:
> Hi list,
>
> I'd like to get your advice about precedence in where clauses in MySQL
> (5.0.51, and 5.1.66 in this case / from lenny and squeeze Debian
> packages ).
>
> Considering the following simple query :
>
> SELECT * FROM Status WHERE DWProcessed = 0 AND PreviousStatus NOT IN
> ('PENDING', 'ACCEPTED') AND SubscribeDate < DATE_SUB(NOW(), INTERVAL 24
> HOUR);
>
> Which of these filters are processed first ?
>
> I'd like the first filter (DWProcessed / Lowest cardinality and indexed)
> being processed first, but I can't really find any useful information
> about this .
>
> Is there any performance impact on query processing, about the order of
> WHERE clauses ?
>
> Regards,
> Christophe.
>
>
> --
> MySQL General Mailing List
> For list archives: http://lists.mysql.com/mysql
> To unsubscribe:    http://lists.mysql.com/mysql
>



-- 
 - michael dykman
 - mdykman@stripped

 May the Source be with you.
Thread
Precedence in WHERE clauses.Christophe18 Mar 2014
  • Re: Precedence in WHERE clauses.Michael Dykman18 Mar 2014
    • Re: Precedence in WHERE clauses.Christophe20 Mar 2014
      • Re: Precedence in WHERE clauses.Roy Lyseng20 Mar 2014
  • Re: Precedence in WHERE clauses.Glyn Astill19 Mar 2014
  • Re: Precedence in WHERE clauses.Morgan Tocker19 Mar 2014
    • Re: Precedence in WHERE clauses.Reindl Harald19 Mar 2014
      • Re: Precedence in WHERE clauses.Morgan Tocker19 Mar 2014
        • Re: Precedence in WHERE clauses.Reindl Harald19 Mar 2014
          • Re: Precedence in WHERE clauses.Morgan Tocker19 Mar 2014
            • Re: Precedence in WHERE clauses.Christophe20 Mar 2014
              • Re: Precedence in WHERE clauses.Christophe23 Mar 2014
                • Re: Precedence in WHERE clauses.shawn l.green25 Mar 2014
    • Re: Precedence in WHERE clauses.Mogens Melander19 Mar 2014
      • Re: Precedence in WHERE clauses.shawn l.green20 Mar 2014