When I was investigating this report:
http://archives.postgresql.org/pgsql-bugs/2011-03/msg00349.php
besides providing a straightforward fix here:
http://archives.postgresql.org/pgsql-bugs/2011-03/msg00352.php
I noted that there was nearby code which needed review, as it didn't
seem safe when in a subtransaction. Further review confirmed this
and didn't turn up any other problems in that section of code. So,
a fix for this overreaching optimization is attached. Note that it
is a one-line fix except for some additional comments to explain the
limitation.
This patch is in addition to and orthogonal to the first patch cited
above.
I will add this one to the 9.1 open items list.
-Kevin
*** a/src/backend/storage/lmgr/predicate.c
--- b/src/backend/storage/lmgr/predicate.c
***************
*** 3635,3649 **** CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
offsetof(PREDICATELOCK,
targetLink));
sxact = predlock->tag.myXact;
! if (sxact == MySerializableXact)
{
/*
! * If we're getting a write lock on the tuple, we don't
need a
! * predicate (SIREAD) lock. At this point our
transaction already
! * has an ExclusiveRowLock on the relation, so we are
OK to drop
! * the predicate lock on the tuple, if found, without
fearing that
! * another write against the tuple will occur before
the MVCC
! * information makes it to the buffer.
*/
if (GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
{
--- 3635,3654 ----
offsetof(PREDICATELOCK,
targetLink));
sxact = predlock->tag.myXact;
! if (sxact == MySerializableXact && !IsSubTransaction())
{
/*
! * If we're getting a write lock on the tuple and we're
not in a
! * subtransaction, we don't need a predicate (SIREAD)
lock. We
! * can't use this optimization within a subtransaction
because
! * the subtransaction could be rolled back, and we
would be left
! * without any lock at the top level.
! *
! * At this point our transaction already has an
ExclusiveRowLock
! * on the relation, so we are OK to drop the predicate
lock on
! * the tuple, if found, without fearing that another
write
! * against the tuple will occur before the MVCC
information
! * makes it to the buffer.
*/
if (GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
{
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