On Wed, Jul 26, 2017 at 2:13 AM, Amit Langote
<langote_amit...@lab.ntt.co.jp> wrote:
> Sorry to be responding this late to the Amit's make_resultrel_ordered
> patch itself, but I agree that we should teach the planner to *always*
> expand partitioned tables in the partition bound order.

Sounds like we have unanimous agreement on that point.  Yesterday, I
was discussing with Beena Emerson, who is working on run-time
partition pruning, that it would also be useful for that purpose, if
you're trying to prune based on a range query.

> I checked that we get the same result relation order with both the
> patches, but I would like to highlight a notable difference here between
> the approaches taken by our patches.  In my patch, I have now taught
> RelationGetPartitionDispatchInfo() to lock *only* the partitioned tables
> in the tree, because we need to look at its partition descriptor to
> collect partition OIDs and bounds.  We can defer locking (and opening the
> relation descriptor of) leaf partitions to a point where planner has
> determined that the partition will be accessed after all (not pruned),
> which will be done in a separate patch of course.

That's very desirable, but I believe it introduces a deadlock risk
which Amit's patch avoids.  A transaction using the code you've
written here is eventually going to lock all partitions, BUT it's
going to move the partitioned ones to the front of the locking order
vs. what find_all_inheritors would do.  So, when multi-level
partitioning is in use, I think it could happen that some other
transaction is accessing the table using a different code path that
uses the find_all_inheritors order without modification.  If those
locks conflict (e.g. query vs. DROP) then there's a deadlock risk.

Unfortunately I don't see any easy way around that problem, but maybe
somebody else has an idea.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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