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TupleDesc refcounting

From: Neil Conway <neilc(at)samurai(dot)com>
To: pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: TupleDesc refcounting
Date: 2006-01-10 22:41:04
Message-ID: 1136932864.26329.17.camel@localhost.localdomain (view raw, whole thread or download thread mbox)
Lists: pgsql-patches
Attached is a WIP patch that adds reference counting for TupleDescs. Two
issues that I ran into while implementing it:

(1) How should the lifetime of a TupleDesc be managed? The existing
ResourceOwner stuff seems to assume that it is managing "per-query"
resources. A TupleDesc will often live beyond the lifetime of a single
transaction, and might even be created before transactions can be
started (e.g. formrdesc() in relcache.c, when we're creating relcache
entries for nailed-in-cache relations early in InitPostgres()).

In current sources, the lifetime of a TupleDesc is the lifetime of the
memory context in which it is allocated (and/or whenever FreeTupleDesc()
is invoked). We could imitate that behavior by optionally linking a
ResourceOwner with each MemoryContext, and releasing the ResourceOwner
when the MemoryContext is reset or deleted. However, I'm not sure that
that's the right approach...

(2) The existing ResourceOwner users issue a warning if the resource
they are managing is not explicitly released before a transaction
successfully commits (so they elog(WARNING)). I don't see the need to be
that strict for TupleDescs -- as we do with palloc() without a matching
pfree(), I think it should be okay to just silently clean up "leaked"
TupleDescs when releasing a ResourceOwner.



Attachment: tdesc_refcount-2.patch
Description: text/x-patch (22.3 KB)


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Subject: Re: TupleDesc refcounting
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