Обсуждение: HashAggregate row estimate = 200

Something I've noticed frequently is that HashAggregate will, especially if the children are Append with one of the
nodesa 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? 
--
-- Christophe Pettus  xof@thebuild.com

On 4 June 2016 at 14:10, Christophe Pettus <xof@thebuild.com> wrote:
> Something I've noticed frequently is that HashAggregate will, especially if the children are Append with one of the
nodesa 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