Re: [pgsql-hackers-win32] Poor Performance for large queries
Richard Huxton <[email protected]>
| Newsgroups | gmane.comp.db.postgresql.performance,gmane.comp.db.postgresql.devel.win32 |
|---|---|
| Message-ID | <[email protected]> |
John Meinel wrote: > > So notice that when doing the actual select it is able to do the index > query. But for some reason with a prepared statement, it is not able to > do it. > > Any ideas? In the index-using example, PG knows the value you are comparing to. So, it can make a better estimate of how many rows will be returned. With the prepared/compiled version it has to come up with a plan that makes sense for any value. If you look back at the explain output you'll see PG is guessing 181,923 rows will match with the prepared query but only 1 for the second query. If in fact you returned that many rows, you wouldn't want to use the index - it would mean fetching values twice. The only work-around if you are using plpgsql functions is to use EXECUTE to make sure your queries are planned for each value provided. -- Richard Huxton Archonet Ltd ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings