Re: PoC/WIP: Extended statistics on expressions

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