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 .
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:
> 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