The order of tables in the from does NOT make any difference. The optimizer (normally) will "rewrite" the kwiri to suit its needs. Also the order in the WHERE does not make any difference.
You should put all restrictions into the WHERE clause, which is MUCH faster than putting them into the HAVING. Why? To apply having conditions you first need to have a result set. So if you already can limit the size of the result set with WHEREs makes it all that faster. Order in the FROM does matter when you use STRAIGHT_JOIN. Please also note that the optimizer only uses keys when it calculates that using key scan will not require more than scanning at least about 30% of the table records (MyISAM). So with very small tables indexes will almost never be uses and only slow down inserts/updates/deletes. Cheers /rudy -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: woensdag 9 juli 2003 18:18 To: [EMAIL PROTECTED] Subject: order of table joins or where clauses relevant? As we're on this topic in another thread right now: Say I have a SELECT query from more than one table and with some conditions, does it matter in what order I enter the tables in the FROM clause and in what order the WHERE conditions appear in my query? Or does it make any difference if I use WHERE or HAVING? (I see that MS Access likes those HAVINGs...) Of course my tables contain (maybe very much) more than some 100 records and are well-indexed, I believe.... but that's not my question for now. I guess, the MySQL optimizer reads the table and column names in the specified order and tries to process them the same way, right? Or it joins the tables in my given order... And when are the resulting records reduced by matching against my conditions? Maybe someone can tell me a little bit about performance gains just by doing some 'manual query optimization' :) Yves Goergen www.unclassified.de -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED] -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
