Re: multivariate statistics v9

Hi,

attached is v9 of the patch series, including mostly these changes:

1) CREATE STATISTICS cleanup

Firstly, I forgot to make the STATISTICS keyword unreserved again.
I've also removed additional stuff from the grammar that turned out
to be unnecessary / could be replaced with existing pieces.

2) making statistics schema-specific

Similarly to the other objects (e.g. types), statistics names are now
unique within a schema. This also means that the statistics may be
created using qualified name, and also may belong to a different
schema than a table.

It seems to me we probably also need to track owner, and only allow
the owner (or superuser / schema owner) to manipulate the statistics.

The initial intention was to inherit all this from the parent table,
but as we're designing this for the multi-table case, it's not
really working anymore.

3) adding IF [NOT] EXISTS to DROP STATISTICS / CREATE STATISTICS

4) basic documentation of the DDL commands

It's really simple at this point and some of the paragraphs are
still empty. I also think that we'll have to add stuff explaining
how to use statistics, not just docs for the DDL commands.

5) various fixes of the regression tests, related to the above

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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