Re: extended stats on partitioned tables

+ * XXX Can't we simply look at rte->inh?
+ */
+ inh = root->append_rel_array == NULL ? false :
+ root->append_rel_array[onerel->relid]->parent_relid != 0;

I think so. That's what I came up with while trying to figured this out, and
it's no great surprise that it needed to be cleaned up - thanks.

In your 0003 patch, the "if inh: break" isn't removed from examine_variable(),
but the corresponding thing is removed everywhere else.

In 0003, mcv_clauselist_selectivity still uses simple_rte_array rather than
rt_fetch.

The regression tests changed as a result of not populating stx_data; I think
it's may be better to update like this:

SELECT stxname, stxdndistinct, stxddependencies, stxdmcv, stxoid IS NULL
FROM pg_statistic_ext s LEFT JOIN pg_statistic_ext_data d
ON d.stxoid = s.oid
WHERE s.stxname = 'ab1_a_b_stats';

There's this part about documentation for the changes in backbranches:

On Sun, Sep 26, 2021 at 03:25:50PM -0500, Justin Pryzby wrote:
> Also, I think in backbranches we should document what's being stored in
> pg_statistic_ext, since it's pretty unintuitive:
> - noninherted stats (FROM ONLY) for inheritence parents;
> - inherted stats (FROM *) for partitioned tables;

spellcheck: inheritence should be inheritance.

All for now. I'm going to update the regression tests for dependencies and the
other code paths.

--
Justin