Horton

Re: So, is COUNT(*) fast now?

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
Cc: "Robert Haas" <robertmhaas@gmail.com>, pgsql-hackers@postgresql.org
Date: 2011-10-24T17:51:59Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Remove pg_upgrade dependency on the 'postgres' database existing in the

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I had wondered whether it'd be worth optimizing that along the
>> lines of slot_getallattrs().  But most indexes probably have only
>> one column, or anyway not enough to make for a useful savings.
 
>> From a heavily-used production database:
 
> cir=> select indnatts, count(*) from pg_index group by indnatts
> order by indnatts;
>  indnatts | count 
> ----------+-------
>         1 |   200
>         2 |   684
>         3 |   155
>         4 |    76
>         5 |    43
>         6 |    13
>         7 |     2
>         9 |     1
> (8 rows)
 
> This includes system table and TOAST table indexes (which seem to
> have two columns).

Yeah, TOAST indexes are 2-column.  It would be best to exclude those
from your counts, since it seems pretty unlikely that anyone will care
how fast nodeIndexonlyscan.c is for scans on toast tables.

> There are over 400 user tables, each of which
> has a primary key, so most primary keys in our database are more
> than one column.

It doesn't look to me like the mean is above 2 (unless you have many
fewer toast tables than I suspect), so trying to optimize many-column
cases isn't going to help.

			regards, tom lane