Craig James wrote:
Tom Lane wrote:
Craig James <craig_james@xxxxxxxxxxxxxx> writes:
Count() on Oracle and MySQL is almost instantaneous, even for very
large tables. So why can't Postgres do what they do?
AFAIK the above claim is false for Oracle. They have the same
transactional issues we do.
My experience doesn't match this claim. When I ported my application
from Oracle to Postgres, this was the single biggest performance
problem. count() in Oracle was always very fast. We're not talking
about a 20% or 50% difference, we're talking about a small fraction of
a second (Oracle) versus a minute (Postgres) -- something like two or
three orders of magnitude.
It may be that Oracle has a way to detect when there's no transaction
and use a faster method. If so, this was a clever optimization -- in
my experience, that represents the vast majority of the times you want
to use count(). It's not very useful to count the rows of a table
that many apps are actively modifying since the result may change the
moment your transaction completes. Most of the time when you use
count(), it's because you're the only one modifying the table, so the
count will be meaningful.
Craig
Oracle will use a btree index on a not null set of columns to do a fast
full index scan, which can be an order of magnitude or faster compared
to a table scan. Also, Oracle can use a bitmap index (in cases where a
bitmap index isn't otherwise silly) for a bitmap fast index scan/bitmap
conversion for similar dramatic results.
For "large" tables, Oracle is not going to be as fast as MyISAM tables
in MySQL, even with these optimizations, since MyISAM doesn't have to
scan even index pages to get a count(*) answer against the full table.
Paul
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