pgsql: Improve test coverage for multi-column MCV lists

Improve test coverage for multi-column MCV lists

The regression tests for extended statistics were not testing a couple
of important cases for the MCV lists:

* IS NOT NULL clauses - We did have queries with IS NULL clauses, but
not the negative case.

* clauses with variable on the right - All the clauses had the Var on
the left, i.e. (Var op Const), so this adds (Const op Var) too.

* columns with fixed-length types passed by reference - All columns
were using either by-value or varlena types, so add a test with
UUID columns too. This matters for (de)serialization.

* NULL-only dimension - When one of the columns contains only NULL
values, we treat it a a special case during (de)serialization.

* arrays containing NULL - When the constant parameter contains NULL
value, we need to handle it correctly during estimation, for all
IN, ANY and ALL clauses.

Discussion: https://www.postgresql.org/message-id/flat/20200113230008(dot)g67iyk4cs3xbnjju(at)development
Author: Tomas Vondra

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d8cfa82d51f812f886d06bdc7bb73470e55d74b7

Modified Files
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src/test/regress/expected/stats_ext.out | 237 +++++++++++++++++++++++++++++++-
src/test/regress/sql/stats_ext.sql | 102 +++++++++++++-
2 files changed, 335 insertions(+), 4 deletions(-)