Обсуждение: Number of tuples in pg_class - bug or misunderstanding?

I'm just digging a little bit in PostgreSQL internals and I found the
following thing. Shouldn't the reltuples field in pg_class contain the
real number of tuples? It is always a little bit to low ...

irc=# SELECT reltuples FROM pg_class WHERE relname = 'events';
 reltuples
-----------
       188
(1 row)

irc=# SELECT COUNT(*) FROM events;
 count
-------
   195
(1 row)


This annoys me a little bit as I can do
  SELECT reltuples FROM pg_class WHERE relname = $1;
from plpgsql, but not
  SELECT COUNT(*) FROM $1;
--
Martin Neumann
Appartement 201, Emil-Figge-Str. 9, 44227 Dortmund, Germany
mne@mne.de - http://www.mne.de/ - PGP key @ http://www.mne.de/mne/pgp.txt

> I'm just digging a little bit in PostgreSQL internals and I found the
> following thing. Shouldn't the reltuples field in pg_class contain the
> real number of tuples? It is always a little bit to low ...
>
> irc=# SELECT reltuples FROM pg_class WHERE relname = 'events';
>  reltuples
> -----------
>        188
> (1 row)
>
> irc=# SELECT COUNT(*) FROM events;
>  count
> -------
>    195
> (1 row)
>
>
> This annoys me a little bit as I can do
>   SELECT reltuples FROM pg_class WHERE relname = $1;
> from plpgsql, but not
>   SELECT COUNT(*) FROM $1;

reltuples is updated only by VACUUM.  The performance hit is too great
to keep it up-to-date, we think, though if it was accurate, we could
optimize COUNT(*) with no WHERE.

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