From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
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To: | Chapman Flack <chap(at)anastigmatix(dot)net> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Is a syscache tuple more like an on-disk tuple or a freshly made one? |
Date: | 2016-04-15 22:03:01 |
Message-ID: | 20160415220301.GA469536@alvherre.pgsql |
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Lists: | pgsql-hackers |
Chapman Flack wrote:
> I am tempted to apply HeapTupleGetDatum to a tuple retrieved from
> the syscache (as I already have code for processing a tuple presented
> as a Datum).
>
> But I see a comment on HeapTupleHeaderGetDatum: "This must *not* get
> applied to an on-disk tuple; the tuple should be freshly made by
> heap_form_tuple or some wrapper ..."
I suppose you could create a copy of the tuple (SysCacheSearchCopy) and
use that for HeapTupleGetDatum. The problem with the syscache tuple is
that it can go away as soon as you do the ReleaseSysCache -- it lives in
shared_buffers memory, so when it's released the buffer might get
evicted.
heap_form_tuple returns a newly palloc'd tuple, which is what you want.
> ... and here I confess I'm unsure whether a tuple retrieved from
> the syscache is more like an on-disk one, or a freshly-made one,
> for purposes of the warning in that comment.
A "syscache tuple" is definitely an on-disk tuple.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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