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

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.

2 files changed, 7 insertions, 7 deletions

diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index 292b550356..22878c0099 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -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;
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index b542fef61e..2e83a7417d 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.184 2005/04/11 23:06:55 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.185 2005/04/28 21:47:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -635,13 +635,13 @@ grouping_planner(Query *parse, double tuple_fraction)
 		tlist = postprocess_setop_tlist(result_plan->targetlist, tlist);
 
 		/*
-		 * Can't handle FOR UPDATE here (parser should have checked
+		 * Can't handle FOR UPDATE/SHARE here (parser should have checked
 		 * already, but let's make sure).
 		 */
 		if (parse->rowMarks)
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("SELECT FOR UPDATE is not allowed with UNION/INTERSECT/EXCEPT")));
+					 errmsg("SELECT FOR UPDATE/SHARE is not allowed with UNION/INTERSECT/EXCEPT")));
 
 		/*
 		 * Calculate pathkeys that represent result ordering requirements