Re: Re: the opposite of a join?

On Thu, 2007-10-04 at 11:56 +1000, Chris wrote: > Robert Cummings wrote: > > On Thu, 2007-10-04 at 11:23 +1000, Chris wrote: > >> Robert Cummings wrote: > >>> On Wed, 2007-10-03 at 14:49 -0700, Jim Lucas wrote: > >>>> This is only from my own personal testing. Mind you that I have only been > >>>> using PostgreSQL for a > >>>> year or so. But one problem that I have always ran into with MySQL is that > >>>> when JOIN'ing tables > >>>> that have large data sets is a PITA. > >>> Were you doing left joins when you experienced those problems? Left > >>> joins are usually very fast. > >> If indexed properly of course ;) > > > > Yes, but you're not going to get a performance improvement if you use > > anything else if the table isn't properly indexed. > > A subselect could win out in terms of performance especially if the > table in the subselect is reasonably small (eg all fits into memory). But if it fits in memory then it's probably already in memory for a left join also. Cheers, Rob. -- ........................................................... SwarmBuy.com - http://www.swarmbuy.com Leveraging the buying power of the masses! ...........................................................