To: len@cs.pdx.edu
cc: pgsql-performance@postgresql.org
Subject: Re: Understanding histograms 
In-reply-to: <4817FC00.50609@cs.pdx.edu> 
References: <4817FC00.50609@cs.pdx.edu>
Comments: In-reply-to Len Shapiro <len@cs.pdx.edu>
	message dated "Tue, 29 Apr 2008 21:56:32 -0700"
Date: Wed, 30 Apr 2008 01:19:51 -0400
Message-ID: <14251.1209532791@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Len Shapiro <len@cs.pdx.edu> writes:
> 1. Why does Postgres come up with a negative n_distinct?

It's a fractional representation.  Per the docs:

> stadistinct	float4	 	The number of distinct nonnull data values in the column. A value greater than zero is the actual number of distinct values. A value less than zero is the negative of a fraction of the number of rows in the table (for example, a column in which values appear about twice on the average could be represented by stadistinct = -0.5). A zero value means the number of distinct values is unknown

> The "rows=2" estimate makes sense when const = 1 or 5, but it makes no
> sense to me for other values of const not in the MVC list.
> For example, if I run the query
> EXPLAIN SELECT * from sailors where rank = -1000;
> Postgres still gives an estimate of "row=2".

I'm not sure what estimate you'd expect instead?  The code has a built in
assumption that no value not present in the MCV list can be more
frequent than the last member of the MCV list, so it's definitely not
gonna guess *more* than 2.

			regards, tom lane