Re: [HACKERS] Bug#48582: psql spends hours computing results it already knows (fwd)

Oliver (& Brian) -
Hmm, that happens to not be the case. The rows=XXXX number is drawn
from the statistics for the table, which are only updated on VACUUM
ANALYZE of that table. Easily tested: just INSERT a couple rows and do
the EXPLAIN again. The rows=XXX won't change. Ah, here's an example:
a table I've never vacuumed, until now (it's in a test copy of a db)

test=> select count(*) from "Personnel";
count
-----
177
(1 row)

test=> explain select count(*) from "Personnel";
NOTICE: QUERY PLAN:

Aggregate (cost=43.00 rows=1000 width=4)
-> Seq Scan on Personnel (cost=43.00 rows=1000 width=4)

EXPLAIN
test=> vacuum analyze "Personnel";
VACUUM
test=> explain select count(*) from "Personnel";
NOTICE: QUERY PLAN:

Aggregate (cost=7.84 rows=177 width=4)
-> Seq Scan on Personnel (cost=7.84 rows=177 width=4)

EXPLAIN
test=>

Ross

On Thu, Oct 28, 1999 at 08:53:31PM +0100, Oliver Elphick wrote:
> This is a Debian bug report, which needs upstream attention.
>
> ------- Forwarded Message
>
> Date: Thu, 28 Oct 1999 13:45:18 -0400
> From: Brian Ristuccia <brianr(at)osiris(dot)978(dot)org>
> To: submit(at)bugs(dot)debian(dot)org
> Subject: Bug#48582: psql spends hours computing results it already knows
>
> Package: postgresql
> Version: 6.5.2-3
>
> massive_db=> explain select count(*) from huge_table;
> NOTICE: QUERY PLAN:
>
> Aggregate (cost=511.46 rows=9923 width=12)
> -> Seq Scan on huge_table (cost=511.46 rows=9923 width=12)
>
> EXPLAIN
>
> If huge_table really is huge -- like 9,000,000 rows instead of 9923, after
> postgresql already knows the number of rows (that's how it determines the
> cost), it proceeds to do a very long and CPU/IO intensive seq scan to
> determine the count().
>
> - --
> Brian Ristuccia
> brianr(at)osiris(dot)978(dot)org
> bristucc(at)nortelnetworks(dot)com
> bristucc(at)cs(dot)uml(dot)edu
>