On Thu, Oct 30, 2003 at 12:34:15 +0100, Cestmir Hybl <[EMAIL PROTECTED]> wrote: > Are you seeing this question as totally off-topic in this list, or there is > really no one who knows something about indexing "is null" bits in postgres?
There was some talk about IS NULL not being able to use indexes (unless you specifically created a partial index using that condition) a number of months ago. You could search through the archives if you are interested in what was said. My memory is that people thought it would be a good idea but that it wasn't that important to get done. > > > Hi, > > > > suppose, for simplicity, there is a table with index like this: > > > > create table TABLE1 ( > > A integer > > ); > > create index TABLE1_A on TABLE1 (A); > > > > My question is: why psql (7.3.3) does not use index when filtering by A IS > > NULL, A IS NOT > > NULL expressions? That is a Postgres limitation. If there are only a few null values, but you query for them a lot it may be worth creating a partial index. > > > > In fact, I need to filter by expression ((A is null) or (A > const)). This is a whole different matter. Using an indexed search on > is not necessarily a good idea. Unless you know only a small fraction of the table (often 10% is quoted) is greater than the constant, a sequential scan is probably a better plan than an index scan. If you know that there is only a small fraction of the values above constant and you know some large value greater than all values, you can try using a between comparison to coax the planner into doing an index scan. ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly