Re: count(*) slow on large tables

In article <3F7D172E.3060107@persistent.co.in>,
Shridhar Daithankar <shridhar_daithankar@persistent.co.in> writes:

> Dror Matalon wrote:
>> I smell a religious war in the aii:-). Can you go several days in a
>> row without doing select count(*) on any
>> of your tables? I suspect that this is somewhat a domain specific
>> issue. In some areas
>> you don't need to know the total number of rows in your tables, in
>> others you do.

> If I were you, I would have an autovacuum daemon running and rather
> than doing select count(*), I would look at stats generated by
> vacuums. They give approximate number of tuples and it should be good
> enough it is accurate within a percent.

The stats might indeed be a good estimate presumed there were not many
changes since the last VACUUM.  But how about a variant of COUNT(*)
using an index?  It would not be quite exact since it might contain
tuples not visible in the current transaction, but it might be a much
better estimate than the stats.