If you have no active transactions and want a cron delete, an example is : * Create table tmp as select * from your_table where ... <- here indexes are used * drop indexes * delete from you_table where ... * insert into your_table select * from tmp * create index on you_table.
You must test it to unsure that index creation is not slow when you have a lot of indexes. You can also disable constraints when deleting and optimize your table at the end of the deletion. In myisam storage, since an update,insert or delete means lock table there is a big transactional problem. Innodb offers row loocking, but you seem having a problem using it. Unfortunaltly ! To simulate transaction, you must split your queries. I remember had worked on a load problem which take days to finish (or not) because the load operation was combined with a lot of select (verify) data. My solution was to do a lot of selects (using indexes), spool results to files, delete rows, and load data from files. It took 1.5 hour to finish a 650Mo data with all the checking operations. Mathias -----Original Message----- From: Almar van Pel [mailto:[EMAIL PROTECTED] Sent: dimanche 24 avril 2005 18:44 To: mysql@lists.mysql.com Cc: 'Jigal van Hemert'; 'mathias fatene' Subject: RE: Performance issues when deleting and reading on large table Hi Jigal, Mathias, Thanks the time you took to reply to my issue's! I would like to clear out some things. > It's a probably a case of not having the cardinality of indexes right and thus making wrong decisions for queries. - Currently there is not a single query in the application that does not use the correct index. We only have key-reads. Wich would mean that MySQL is creating these incorrect indexes? > Depending on the size of the resulting record sets, your system must have enough memory to handle it. Otherwise a lot of temporary tables will end up on disk (slow) and also indexes cannot be loaded in memory (slow). - The system runs with a key-buffer of 382 M, wich is most of the time not filled 100 %. Created temp. tables is very low. > Deleting a lot of records will have impact on the indexes, so it's quite a job. The inserts/updates/deletes will also block the table for reading in case of MyISAM. - During deletion of records from the table there is no user interaction. The only person manipulating the table/database is me. That's the reason why i'm finding this 'strange'. Changing to Innodb would be a great risk I think. Maybe we should think this over again, but the way the system is configured right now should in my opion be sufficient enough. Mathias, what do you mean by: > If you want to do a massive delete with a cron, it's better to : > * select the rows to delete (using indexes) > * delete indexes > * delete rows (already marked) > * recreate indexes I don't really understand how you 'mark' the records for deletion before deleting indexes. However I'm very interested. Regards, Almar van Pel -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
