Обсуждение: Slow SELECTS after large update cycle
Hello, After fixing the hanging problems I reported here earlier (by uninstalling W2K3 SP1), I'm running into another weird one. After doing a +/- 8hr cycle of updates and inserts (what we call a 'batch'), the first 'reporting' type query on tables involved in that write cycle is very slow. As an example, I have a query which according to EXPLAIN ANALYZE takes about 1.1s taking 46s. After this one hit, everything is back to normal, and subsequent executions of the same query are in fact subsecond. Restarting the appserver and pgsql does not make the slowness re-appear, only running another batch will. During the 'write'/batch cycle, a large number of rows in various tables are inserted and subsequently (repeatedly) updated. The reporting type queries after that are basically searches on those tables. Anybody any ideas? Thanks, jan -- -------------------------------------------------------------- Jan de Visser jdevisser@digitalfairway.com Baruk Khazad! Khazad ai-menu! --------------------------------------------------------------
Jan de Visser wrote: > Hello, > > After fixing the hanging problems I reported here earlier (by uninstalling > W2K3 SP1), I'm running into another weird one. > > After doing a +/- 8hr cycle of updates and inserts (what we call a 'batch'), > the first 'reporting' type query on tables involved in that write cycle is > very slow. As an example, I have a query which according to EXPLAIN ANALYZE > takes about 1.1s taking 46s. After this one hit, everything is back to > normal, and subsequent executions of the same query are in fact subsecond. > Restarting the appserver and pgsql does not make the slowness re-appear, only > running another batch will. > > During the 'write'/batch cycle, a large number of rows in various tables are > inserted and subsequently (repeatedly) updated. The reporting type queries > after that are basically searches on those tables. After a large batch you need to run 'analyze' over the tables involved to get postgresql to update it's statistics so it can work out which indexes etc it should use. -- Postgresql & php tutorials http://www.designmagick.com/
On Wed, 2006-03-15 at 14:39 -0500, Jan de Visser wrote: > After fixing the hanging problems I reported here earlier (by uninstalling > W2K3 SP1), I'm running into another weird one. > > After doing a +/- 8hr cycle of updates and inserts (what we call a 'batch'), > the first 'reporting' type query on tables involved in that write cycle is > very slow. As an example, I have a query which according to EXPLAIN ANALYZE > takes about 1.1s taking 46s. After this one hit, everything is back to > normal, and subsequent executions of the same query are in fact subsecond. > Restarting the appserver and pgsql does not make the slowness re-appear, only > running another batch will. > > During the 'write'/batch cycle, a large number of rows in various tables are > inserted and subsequently (repeatedly) updated. The reporting type queries > after that are basically searches on those tables. > > Anybody any ideas? This is caused by updating the commit status hint bits on each row touched by the SELECTs. This turns the first SELECT into a write operation. Try running a scan of the whole table to take the hit before you give it back to the users. Best Regards, Simon Riggs
On Wednesday 15 March 2006 18:21, Simon Riggs wrote: > On Wed, 2006-03-15 at 14:39 -0500, Jan de Visser wrote: > > After fixing the hanging problems I reported here earlier (by > > uninstalling W2K3 SP1), I'm running into another weird one. > > > > After doing a +/- 8hr cycle of updates and inserts (what we call a > > 'batch'), the first 'reporting' type query on tables involved in that > > write cycle is very slow. As an example, I have a query which according > > to EXPLAIN ANALYZE takes about 1.1s taking 46s. After this one hit, > > everything is back to normal, and subsequent executions of the same query > > are in fact subsecond. Restarting the appserver and pgsql does not make > > the slowness re-appear, only running another batch will. > > > > During the 'write'/batch cycle, a large number of rows in various tables > > are inserted and subsequently (repeatedly) updated. The reporting type > > queries after that are basically searches on those tables. > > > > Anybody any ideas? > > This is caused by updating the commit status hint bits on each row > touched by the SELECTs. This turns the first SELECT into a write > operation. > > Try running a scan of the whole table to take the hit before you give it > back to the users. Thanks Simon. I didn't know about the cause, but I expected the answer to be 'deal with it', as it is. At least I can explain it now... > > Best Regards, Simon Riggs jan -- -------------------------------------------------------------- Jan de Visser jdevisser@digitalfairway.com Baruk Khazad! Khazad ai-menu! --------------------------------------------------------------