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 > -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql
