Re: Much Ado About COUNT(*)

Centuries ago, Nostradamus foresaw when mkoi-pg(at)aon(dot)at (Manfred Koizar) would write:
> On Mon, 24 Jan 2005 08:28:09 -0700, "Jonah H. Harris" <jharris(at)tvi(dot)edu>
> wrote:
>> UPDATE pg_user_table_counts
>> SET rowcount = rowcount + 1
>> WHERE schemaname = this_schemaname
>> AND tablename = TG_RELNAME;
>
> This might work for small single user applications. You'll have to keep
> an eye on dead tuples in pg_user_table_counts though.
>
> But as soon as there are several concurrent transactions doing both
> INSERTs and DELETEs, your solution will in the best case serialise
> access to test_tbl or it will break down because of deadlocks.

At that point, what you need to do is to break the process in three:

1. Instead of the above, use...

insert into pg_user_table_counts (rowcount, schemaname,
tablename) values (1, this_schemaname, TG_RELNAME);

The process for DELETEs involves using the value -1, of course...

2. A process needs to run once in a while that does...

create temp table new_counts as
select sum(rowcount), schemaname, tablename from
pg_user_table_counts group by schemaname, tablename;
delete from pg_user_table_counts;
insert into pg_user_table_counts select * from new_counts;

This process "compresses" the table so that it becomes cheaper to
do the aggregate in 3.

3. Querying values is done differently...

select sum(rowcount) from pg_user_table_counts where schemaname =
'this' and tablename = 'that';
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