From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Robert Haas <robertmhaas@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: hyrax vs. RelationBuildPartitionDesc
In-reply-to: <20190313182145.GA488@alvherre.pgsql>
References: <20190313182145.GA488@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Wed, 13 Mar 2019 15:21:45 -0300"
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Date: Wed, 13 Mar 2019 14:36:13 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> You do?  Unless I misunderstood, your options are:

> 1. (the patch you attached) create a temporary memory context that is
> used for everything, then at the end copy the good stuff to CacheMemCxt
> (or a sub-context thereof).  This still needs to copy.

> 2. create a temp memory context, do everything there, do retail freeing
> of everything we don't want, reparenting the context to CacheMemCxt.
> Hope we didn't forget to pfree anything.

> How is any of those superior to what I propose?

I doubt that what you're suggesting is terribly workable.  It's not
just RelationBuildPartitionDesc that's at issue.  Part of what will
be the long-lived data structure is made by partition_bounds_create,
and that invokes quite a boatload of code that both makes the final
data structure and leaks a lot of intermediate junk.  Having to be
very careful about permanent vs temporary data throughout all of that
seems like a recipe for bugs.

The existing code in RelationBuildPartitionDesc is already pretty good
about avoiding copying of data other than the output of
partition_bounds_create.  In fact, I think it's already close to what
you're suggesting other than that point.  So I think --- particularly
given that we need something we can back-patch into v11 --- that we
shouldn't try to do anything much more complicated than what Robert is
suggesting.

			regards, tom lane