Re: HashAggregate row estimate = 200

From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
To: Christophe Pettus <xof(at)thebuild(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: HashAggregate row estimate = 200
Date: 2016-06-04 02:26:57
Message-ID: CAKJS1f9Re58SGE11r=GmnZO8PUHU-kj5vVsii6Nvi22h6WZtww@mail.gmail.com
Views: Raw Message | Whole Thread | Download mbox | Resend email
Thread:
Lists: pgsql-hackers

On 4 June 2016 at 14:10, Christophe Pettus <xof(at)thebuild(dot)com> wrote:
> Something I've noticed frequently is that HashAggregate will, especially if the children are Append with one of the nodes a non-seqscan, estimate 200 rows rather than a calculated vlaue. Where is that value coming from? The statistics target, a hardwired constant, or something else?

Most likely from DEFAULT_NUM_DISTINCT in selfuncs.h. This is the
fallback, when nothing else can be used.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

In response to

Browse pgsql-hackers by date

  From Date Subject
Next Message Amit Kapila 2016-06-04 03:12:04 Re: XTM & parallel search
Previous Message Masahiko Sawada 2016-06-04 02:25:52 Re: Reviewing freeze map code