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.

1 files changed, 20 insertions, 8 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 8facabbadad..3e2b986034c 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -26,7 +26,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.246 2005/04/14 01:38:17 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.247 2005/04/28 21:47:12 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -560,9 +560,10 @@ InitPlan(QueryDesc *queryDesc, bool explainOnly)
 	}
 
 	/*
-	 * Have to lock relations selected for update
+	 * Have to lock relations selected FOR UPDATE/FOR SHARE
 	 */
 	estate->es_rowMark = NIL;
+	estate->es_forUpdate = parseTree->forUpdate;
 	if (parseTree->rowMarks != NIL)
 	{
 		ListCell   *l;
@@ -986,7 +987,7 @@ ExecEndPlan(PlanState *planstate, EState *estate)
 		heap_close(estate->es_into_relation_descriptor, NoLock);
 
 	/*
-	 * close any relations selected FOR UPDATE, again keeping locks
+	 * close any relations selected FOR UPDATE/FOR SHARE, again keeping locks
 	 */
 	foreach(l, estate->es_rowMark)
 	{
@@ -1126,6 +1127,9 @@ lnext:	;
 										 * ctid!! */
 				tupleid = &tuple_ctid;
 			}
+			/*
+			 * Process any FOR UPDATE or FOR SHARE locking requested.
+			 */
 			else if (estate->es_rowMark != NIL)
 			{
 				ListCell   *l;
@@ -1137,6 +1141,7 @@ lnext:	;
 					Buffer		buffer;
 					HeapTupleData tuple;
 					TupleTableSlot *newSlot;
+					LockTupleMode	lockmode;
 					HTSU_Result		test;
 
 					if (!ExecGetJunkAttribute(junkfilter,
@@ -1151,9 +1156,15 @@ lnext:	;
 					if (isNull)
 						elog(ERROR, "\"%s\" is NULL", erm->resname);
 
+					if (estate->es_forUpdate)
+						lockmode = LockTupleExclusive;
+					else
+						lockmode = LockTupleShared;
+
 					tuple.t_self = *((ItemPointer) DatumGetPointer(datum));
-					test = heap_mark4update(erm->relation, &tuple, &buffer,
-											estate->es_snapshot->curcid);
+					test = heap_lock_tuple(erm->relation, &tuple, &buffer,
+										  estate->es_snapshot->curcid,
+										  lockmode);
 					ReleaseBuffer(buffer);
 					switch (test)
 					{
@@ -1189,7 +1200,7 @@ lnext:	;
 							goto lnext;
 
 						default:
-							elog(ERROR, "unrecognized heap_mark4update status: %u",
+							elog(ERROR, "unrecognized heap_lock_tuple status: %u",
 								 test);
 							return (NULL);
 					}
@@ -1574,8 +1585,8 @@ ExecUpdate(TupleTableSlot *slot,
 	 * If we generate a new candidate tuple after EvalPlanQual testing, we
 	 * must loop back here and recheck constraints.  (We don't need to
 	 * redo triggers, however.	If there are any BEFORE triggers then
-	 * trigger.c will have done mark4update to lock the correct tuple, so
-	 * there's no need to do them again.)
+	 * trigger.c will have done heap_lock_tuple to lock the correct tuple,
+	 * so there's no need to do them again.)
 	 */
 lreplace:;
 	if (resultRelationDesc->rd_att->constr)
@@ -2088,6 +2099,7 @@ EvalPlanQualStart(evalPlanQual *epq, EState *estate, evalPlanQual *priorepq)
 		epqstate->es_param_exec_vals = (ParamExecData *)
 			palloc0(estate->es_topPlan->nParamExec * sizeof(ParamExecData));
 	epqstate->es_rowMark = estate->es_rowMark;
+	epqstate->es_forUpdate = estate->es_forUpdate;
 	epqstate->es_instrument = estate->es_instrument;
 	epqstate->es_select_into = estate->es_select_into;
 	epqstate->es_into_oids = estate->es_into_oids;