Date: Sat, 16 Jan 2021 17:22:08 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>, pgsql-hackers@postgresql.org
Subject: Re: PoC/WIP: Extended statistics on expressions
Message-ID: <20210116232208.GB8560@telsasoft.com>
References: 
 <CAEZATCXAPrrOKwEsyZKQ4uzzJQWBCt6QAvOcgqRGdWwT1zb+rQ@mail.gmail.com>
 <329203a4-7cad-a0e3-d5a8-ce70872c8156@enterprisedb.com>
 <CAEZATCWGYooC_Tzt8sQo7760ZEsguj0JGoMLhkANrDwJ9CY20Q@mail.gmail.com>
 <2567ae7e-e1dd-48b5-054d-ed6ff81c0335@enterprisedb.com>
 <CAEZATCW_7nfeHmLtV6TzSfmFG+i=io_Qo0NL8BiBnmEqq3CQnQ@mail.gmail.com>
 <CAEZATCUfMM43sv+RFDqyczQpYMP7SZLd4EexFEEUX4buWRNnrw@mail.gmail.com>
 <afcd65d7-7701-a469-b064-9c62d63ef437@enterprisedb.com>
 <20210108023537.GA19743@telsasoft.com>
 <d7c93270-20a0-93cb-f15e-46d7b51b35c1@enterprisedb.com>
 <5305f9cf-3927-06c4-cd5b-e46cbc58bc7f@enterprisedb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5305f9cf-3927-06c4-cd5b-e46cbc58bc7f@enterprisedb.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

On Sat, Jan 16, 2021 at 05:48:43PM +0100, Tomas Vondra wrote:
> +      <entry role="catalog_table_entry"><para role="column_definition">
> +       <structfield>expr</structfield> <type>text</type>
> +      </para>
> +      <para>
> +       Expression the extended statistics is defined on
> +      </para></entry>

Expression the extended statistics ARE defined on
Or maybe say "on which the extended statistics are defined"

> +  <para>
> +   The <command>CREATE STATISTICS</command> command has two basic forms. The
> +   simple variant allows to build statistics for a single expression, does

.. ALLOWS BUILDING statistics for a single expression, AND does (or BUT does)

> +   Expression statistics are per-expression and are similar to creating an
> +   index on the expression, except that they avoid the overhead of the index.

Maybe say "overhead of index maintenance"

> +   All functions and operators used in a statistics definition must be
> +   <quote>immutable</quote>, that is, their results must depend only on
> +   their arguments and never on any outside influence (such as
> +   the contents of another table or the current time).  This restriction

say "outside factor" or "external factor"

> +   results of those expression, and uses default estimates as illustrated
> +   by the first query.  The planner also does not realize the value of the

realize THAT

> +   second column fully defines the value of the other column, because date
> +   truncated to day still identifies the month. Then expression and
> +   ndistinct statistics are built on those two columns:

I got an error doing this:

CREATE TABLE t AS SELECT generate_series(1,9) AS i;
CREATE STATISTICS s ON (i+1) ,(i+1+0) FROM t;
ANALYZE t;
SELECT i+1 FROM t GROUP BY 1;
ERROR:  corrupt MVNDistinct entry

-- 
Justin