Re: speeding up planning with partitions

On Sun, 31 Mar 2019 at 05:50, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Sat, Mar 30, 2019 at 12:16 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> > Fwiw, I had complained when reviewing the run-time pruning patch that
> > creating those maps in the planner and putting them in
> > PartitionPruneInfo might not be a good idea, but David insisted that
> > it'd be good for performance (in the context of using cached plans) to
> > compute this information during planning.
>
> Well, he's not wrong about that, I expect.

I'm aware that there have been combinations of objects to either
having these arrays and/or editing them during execution.

I don't recall Amit's complaint, but I do recall Tom's. He suggested
we not resequence the arrays in the executor and just maintain NULL
elements in the Append/MergeAppend subplans. I did consider this when
writing run-time pruning but found that performance suffers. I
demonstrated this on a thread somewhere.

IIRC, I wrote this code because there was no way to translate the
result of the pruning code into Append/MergeAppend subplan indexes.
Robert has since added a map of Oids to allow the executor to have
those details, so it perhaps would be possible to take the result of
the pruning code then lookup the Oids of the partitions that survived
pruning, then map those to the subplans using the array Robert added.
Using the array for that wouldn't be very efficient due to a lookup
being O(n) per surviving partition. Maybe it could be thrown into a
hashtable to make that faster. This solution would need to take into
account mixed hierarchy Appends. e.g SELECT * FROM partitioned_table
WHERE partkey = $1 UNION ALL SELECT * FROM something_else; so it would
likely need to be a hashtable per partitioned table. If the pruning
code returned a partition whose Oid we didn't know about, then it must
be from a partition that was added concurrently since the plan was
built... However, that shouldn't happen today since Robert went to
great lengths for it not to.

Further discussions are likely best put in their own thread. As far as
I know, nothing is broken with the code today.

--
David Rowley http://www.2ndQuadrant.com/
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