Обсуждение: 8.x VACUUM overestimates reltuples?

Is VACUUM in 8.x supposed to be overestimating reltuples by 50% of
the number of dead tuples?

CREATE TABLE foo (x integer);
INSERT INTO foo SELECT * FROM generate_series(1, 1000);
UPDATE foo SET x = x;
UPDATE foo SET x = x;
UPDATE foo SET x = x;
VACUUM foo;
SELECT relpages, reltuples FROM pg_class WHERE relname = 'foo';relpages | reltuples 
----------+-----------      22 |      2500

Another VACUUM at this point brings reltuples back to the actual
number of rows in the table:

VACUUM foo;
SELECT relpages, reltuples FROM pg_class WHERE relname = 'foo';relpages | reltuples 
----------+-----------      22 |      1000

Is this intentional?

-- 
Michael Fuhr
http://www.fuhr.org/~mfuhr/

Michael Fuhr <mike@fuhr.org> writes:
> Is VACUUM in 8.x supposed to be overestimating reltuples by 50% of
> the number of dead tuples?

See this thread:
http://archives.postgresql.org/pgsql-hackers/2004-11/msg01043.php
        regards, tom lane

On Sat, Feb 12, 2005 at 01:16:49AM -0500, Tom Lane wrote:
> Michael Fuhr <mike@fuhr.org> writes:
> > Is VACUUM in 8.x supposed to be overestimating reltuples by 50% of
> > the number of dead tuples?
> 
> See this thread:
> http://archives.postgresql.org/pgsql-hackers/2004-11/msg01043.php

Thanks -- not sure how I missed that thread in my searches.  I
wondered if the reason might be what your message talks about but
I wanted to check.

-- 
Michael Fuhr
http://www.fuhr.org/~mfuhr/