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Implement sharable row-level locks, and use them for foreign key references

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to eliminate unnecessary deadlocks.  This commit adds SELECT ... FOR SHARE
paralleling SELECT ... FOR UPDATE.  The implementation uses a new SLRU
data structure (managed much like pg_subtrans) to represent multiple-
transaction-ID sets.  When more than one transaction is holding a shared
lock on a particular row, we create a MultiXactId representing that set
of transactions and store its ID in the row's XMAX.  This scheme allows
an effectively unlimited number of row locks, just as we did before,
while not costing any extra overhead except when a shared lock actually
has to be shared.   Still TODO: use the regular lock manager to control
the grant order when multiple backends are waiting for a row lock.

Alvaro Herrera and Tom Lane.
This commit is contained in:
Tom Lane
2005-04-28 21:47:18 +00:00
parent d902e7d63b
commit bedb78d386
55 changed files with 2802 additions and 439 deletions
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8
src/backend/optimizer/plan/initsplan.c
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@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.104 2004/12/31 22:00:09 pgsql Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.105 2005/04/28 21:47:13 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -323,11 +323,11 @@ mark_baserels_for_outer_join(Query *root, Relids rels, Relids outerrels)
Assert(bms_is_subset(rel->outerjoinset, outerrels));
/*
* Presently the executor cannot support FOR UPDATE marking of
* Presently the executor cannot support FOR UPDATE/SHARE marking of
* rels appearing on the nullable side of an outer join. (It's
* somewhat unclear what that would mean, anyway: what should we
* mark when a result row is generated from no element of the
* nullable relation?) So, complain if target rel is FOR UPDATE.
* nullable relation?) So, complain if target rel is FOR UPDATE/SHARE.
* It's sufficient to make this check once per rel, so do it only
* if rel wasn't already known nullable.
*/
@@ -336,7 +336,7 @@ mark_baserels_for_outer_join(Query *root, Relids rels, Relids outerrels)
if (list_member_int(root->rowMarks, relno))
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("SELECT FOR UPDATE cannot be applied to the nullable side of an outer join")));
errmsg("SELECT FOR UPDATE/SHARE cannot be applied to the nullable side of an outer join")));
}
rel->outerjoinset = outerrels;