From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
        Pg Hackers <pgsql-hackers@postgresql.org>,
        Maksim Milyutin <milyutinma@gmail.com>,
        Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Subject: Re: Proposal: Local indexes for partitioned table
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 <CA+TgmoZ6z8+PqygTR=yC=tFCMJ6uUYWTLFKTSKXaHu=jYCjLLg@mail.gmail.com>
References: <20171023151251.j75uoe27gajdjmlm@alvherre.pgsql>
 <20171027172730.eh2domlkpn4ja62m@alvherre.pgsql>
 <CA+TgmoZ6z8+PqygTR=yC=tFCMJ6uUYWTLFKTSKXaHu=jYCjLLg@mail.gmail.com>
Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Fri, 27 Oct 2017 22:18:06 +0200"
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Date: Fri, 27 Oct 2017 17:25:43 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Oct 27, 2017 at 7:27 PM, Alvaro Herrera <alvherre@alvh.no-ip.org>=
 wrote:
>> I noticed that RelationBuildPartitionKey is generating a partition key
>> in a temp context, then creating a private context and copying the key
>> into that.  That seems leftover from some previous iteration of some
>> other patch; I think it's pretty reasonable to create the new context
>> right from the start and allocate the key there directly instead.  Then
>> there's no need for copy_partition_key at all.

> We could do that, but the motivation for the current system was to
> avoid leaking memory in a long-lived context.  I think the same
> technique is used elsewhere for similar reasons.  I admit I haven't
> checked whether there would actually be a leak here if we did it as
> you propose, but I wouldn't find it at all surprising.

Another key point is to avoid leaving a corrupted relcache entry behind
if you fail partway through.  I think that the current coding is
RelationBuildPartitionKey is safe, although it's far too undercommented
for my taste.  (The ordering of the last few steps is *critical*.)

It might work to build the new key in a context that's initially a
child of CurrentMemoryContext, then reparent it to be a child of
CacheMemoryContext when done.  But you'd have to look at whether that
would end up wasting long-lived memory, if the build process generates
any cruft in the same context.

			regards, tom lane


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