From: David Christensen <david@endpoint.com>
Message-Id: <E75ECBBA-5902-4598-8F09-F36192E184EE@endpoint.com>
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Subject: Re: BUG #16676: SELECT ... FETCH FIRST ROW WITH TIES FOR UPDATE SKIP
 LOCKED locks rows it doesn't return
Date: Wed, 21 Oct 2020 14:12:29 -0500
In-Reply-To: <7EA65D44-7C3A-4FC2-8DE1-1F59BBB10B37@endpoint.com>
Cc: pgsql-bugs@lists.postgresql.org
To: Tom Lane <tgl@sss.pgh.pa.us>,
 Alvaro Herrera <alvherre@alvh.no-ip.org>
References: <7B98FC76-00EE-4441-AF10-A4A805DC021E@endpoint.com>
 <7EA65D44-7C3A-4FC2-8DE1-1F59BBB10B37@endpoint.com>
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> On Oct 19, 2020, at 7:33 PM, David Christensen <david@endpoint.com> =
wrote:
>=20
> Maybe splitting LockRows into two nodes, one for locking and one for =
emitting unlocked nodes then interleaving Limit in between? (Or only =
doing something along these lines for this admittedly narrow use case. )

This does appear to be quite a bit harder than originally hoped for on =
the surface; creating a node to =E2=80=9Cpass-thru=E2=80=9D unlocked =
tuples without locking them would require building out some additional =
infrastructure that does not appear to be present in all AMs; something =
to allow a test for locks without actually calling table_tuple_lock() =
formally. Perhaps adding a param to indicate =E2=80=9Ctest=E2=80=9D =
only=E2=80=94not sure if something like similar to heapam's =
=E2=80=9Ctest_lockmode_for_conflict()=E2=80=9D is something we=E2=80=99d =
need to had.  (Or even a new lock mode which doesn=E2=80=99t create =
RowMarks but skips any conflicting modes.)

That said, I think it might be worth trying to go down the road of =
=E2=80=9Cforbid this behavior=E2=80=9D, particularly if a workaround is =
available.  Perhaps looking into why =C3=81lvaro=E2=80=99s plan was =
generating the multiple LockRows nodes would be more fruitful and less =
likely to cause backpatching pain.

David
--
David Christensen
Senior Software and Database Engineer
End Point Corporation
david@endpoint.com
785-727-1171


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