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From:Christophe Date:March 20 2014 12:20pm
Subject:Re: Precedence in WHERE clauses.
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Hi Morgan, Harald, and all,

Thanks for the answers.

While reading your answers, I realize one more time this problem does
not seem so obvious ... That's why I asked ;) .

Beyond the specific case I mentioned, my goal is also to get a general
discussion about theorical behaviour of MySQL while handling WHERE clauses.

I don't have access to the application for now, but will try EXPLAIN as
soon as I can .

Regards,
Christophe.

Le 19/03/2014 17:12, Morgan Tocker a écrit :
> Hi Harald,
> 
>> you can call me Harald
> 
> My apologies!
> 
>> the question was "is there any performance impact"
>> and (DWProcessed / Lowest cardinality and indexed)
>>
>> so *yes* it does matter, i have seen MySQL more than once not using any
>> index because a unlucky order of where clauses up to copy a 16 GB
>> table in a table file instead using a existing primary key
> 
> If you have experienced that the order of the where clause has changed which
> execution plans are possible, I suggest taking a look at optimizer trace:
> http://dev.mysql.com/doc/internals/en/optimizer-tracing.html
> 
> This is more detailed than EXPLAIN and will show indexes being considered, and the
> reasons that they are excluded.
> 
> The 1=1 example I gave will also show as eliminated in optimizer trace.
> 
> - Morgan
> 

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