On 1 Jul 2003 at 15:50, Howard Oblowitz wrote: > The documentation says that Effective Cache Size "sets the optimizer's > assumption > about the effective size of the disk cache ( that is, the portion of the > kernel's disk > cache that will be used for PostgreSQL data files ). > > What then will be the effect of setting this too high? > > And too low?
Let's say postgresql is preparing a plan for a scan and it estimates data set size as 100MB whereas your shared buffers+effective cache is 80M. So postgresql would deduce that it would be better off with sequential scan rather than index scan. Where in fact you have much more memory to make a file system cache and the machine can afford index scan. There is nothing too low or too high of a setting. This isn't exactly performance tuning paramter as other. This is more of information to postgresql. The closer it gets to truer, the plans produced would get optimal. About how to set this parameter, it is roughly effective cache size= (Physical RAM size-shared buffers-requirement for other apps) * 0.8 This is very very rough. You need to make sure that some setting does not trigger a swap avelanche HTH Bye Shridhar -- Slous' Contention: If you do a job too well, you'll get stuck with it. ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings