On 3/22/07, Andreas Tille <tillea(at)rki(dot)de> 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.
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