On 3/22/07, Andreas Tille <tillea@xxxxxx> wrote:
I just try to find out why a simple count(*) might last that long. At first I tried explain, which rather quickly knows how many rows to check, but the final count is two orders of magnitude slower.
You can get the approximate count by selecting reltuples from pg_class. It is valid as of last analyze. As others suggest select count(*) from table is very special case which non-mvcc databases can optimize for. There are many reasons why this is the case and why it explains nothing about the relative performance of the two databases. This is probably #1 most frequenctly asked question to -performance...there is a wealth of information in the archives. merlin
