On Thu, Dec 17, 2009 at 4:22 PM, Karl Larsson <karl.larsson47@gmail.com> wrote: > Hello. > > I have a problem I don't understand. I hope it's a simple problem and I'm > just stupid. > > When I make a subquery Postgres don't care about my indexes and makes > a seq scan instead of a index scan. Why?
PostgreSQL uses an intelligent query planner that predicets how many rows it will get back for each plan and chooses accordingly. Since a few dozen rows will all likely fit in the same block, it's way faster to sequentially scan the table than to use an index scan.
Note that pgsql always has to go back to the original table to get the rows anyway, since visibility info is not stored in the indexes.
I forgot to mention that I have a reel problem with 937(and growing) rows of data. My test tables and test query is just to exemplify my problem. But I'll extend table_two and see if it change anything.