I think we do know, have you reviewed the results in the briefing?
- Luke Sent from my GoodLink synchronized handheld (www.good.com) -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Tuesday, July 25, 2006 01:09 AM Eastern Standard Time To: Tom Lane Cc: Bruce Momjian; Jie Zhang; Hannu Krosing; Gavin Sherry; email@example.com; Luke Lonergan Subject: Re: [HACKERS] On-disk bitmap index patch On Tue, Jul 25, 2006 at 12:36:42AM -0400, Tom Lane wrote: > [EMAIL PROTECTED] writes: > > Reading 1/4, for a larger table, has a good chance of being faster than > > reading 4/4 of the table. :-) > Really? > > If you have to hit one tuple out of four, it's pretty much guaranteed > that you will need to fetch every heap page. So using an index provides > zero I/O savings on the heap side, and any fetches needed to read the > index are pure cost. Now you have to demonstrate that the CPU costs > involved in processing the index are significantly cheaper than the cost > of just testing the WHERE qual at every heap tuple --- not a bet that's > likely to win at a one-in-four ratio. Haha. Of course - but that's assuming uniform spread of the values. Next I would try clustering the table on the bitmap index... :-) My databases aren't as large as many of yours. Most or all of them will fit in 1 Gbytes of RAM. The I/O cost isn't substantial for these, but the WHERE clause might be. But yeah - we don't know. Waste of code or performance boost. Cheers, mark -- [EMAIL PROTECTED] / [EMAIL PROTECTED] / [EMAIL PROTECTED] __________________________ . . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder |\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ | | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them... http://mark.mielke.cc/ ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings