Re: PoC/WIP: Extended statistics on expressions

Hi,

Attached is an updated version of the patch series, merging some of the
changes proposed by Justin. I've kept the bootstrap patches separate, at
least for now.

As for the individual 0004-0007 patches:

1) 0004 - merged as is

2) 0005 - I've merged some of the docs changes, but some of the wording
was copied from CREATE INDEX docs in which case I've kept that. I've
also not merged changed to pre-existing docs, like the t1 example which
is unrelated to this patch.

OTOH I've corrected the t3 example description, which was somewhat bogus
and unrelated to what the example actually did. I've also removed the
irrelevant para which originally described index predicates and was
copied from CREATE INDEX docs by mistake.

3) 0006 - I've committed something similar / less invasive, achieving
the same goals (I think), and I've added a couple regression tests.

4) 0007 - I agreed we need a way to expose the stats, but including this
in pg_stats_ext seems rather inconvenient (table in a table is difficult
to work with). Instead I've added a new catalog pg_stats_ext_exprs with
structure similar to pg_stats. I've also added the expressions to the
pg_stats_ext catalog, which was only showing the attributes, and some
basic docs for the catalog changes.

regards

--
Tomas Vondra
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