Re: speeding up planning with partitions

Hi,

I don't think I can't help with code review, but I loaded our largest
customer's schema into pg12dev and tested with this patch.  It's working well -
thanks for your work.

Details follow.

We have tooo many tables+indices so this vastly improves planning speed.  Our
large customers have ~300 parent tables and total ~20k child tables with total
~80k indices.  Our largest tables heirarchies have ~1200 child tables, which
may have as many as 6-8 indices each.  And 5-10 of the largest heirarchies are
unioned together in a view.

Running pg11.1, explaining query for the largest view with condition eliminates
all but today's tables can take several minutes with a cold cache, due to not
only stat()ing every file in every table in a partition heirarchy, before
pruning, but also actually open()ing all their indices.

Running 11dev with your v10 patch applied, this takes 2244ms with empty buffer
cache after postmaster restarted on a totally untuned instance (and a new
backend, with no cached opened files).

I was curious why it took even 2sec, and why it did so many opens() (but not
20k of them that PG11 does):

[pryzbyj@database postgresql]$ cut -d'(' -f1 /tmp/strace-12dev-explain |sort |uniq -c |sort -nr
   2544 lseek
   1263 open
   ...

It turns out 1050 open()s are due to historic data which is no longer being
loaded and therefor never converted to relkind=p (but hasn't exceeded the
retention interval so not yet DROPped, either).

Cheers,
Justin