Avoid O(N^2) cost in ExecFindRowMark().

If there are many ExecRowMark structs, we spent O(N^2) time in
ExecFindRowMark during executor startup.  Once upon a time this was
not of great concern, but the addition of native partitioning has
squeezed out enough other costs that this can become the dominant
overhead in some use-cases for tables with many partitions.

To fix, simply replace that List data structure with an array.

This adds a little bit of cost to execCurrentOf(), but not much,
and anyway that code path is neither of large importance nor very
efficient now.  If we ever decide it is a bottleneck, constructing a
hash table for lookup-by-tableoid would likely be the thing to do.

Per complaint from Amit Langote, though this is different from
his fix proposal.

Discussion: https://postgr.es/m/468c85d9-540e-66a2-1dde-fec2b741e688@lab.ntt.co.jp

src/backend/executor/execCurrent.c

@@ -91,21 +91,22 @@ execCurrentOf(CurrentOfExpr *cexpr,
 	 * the other code can't, while the non-FOR-UPDATE case allows use of WHERE
 	 * CURRENT OF with an insensitive cursor.
 	 */
-	if (queryDesc->estate->es_rowMarks)
+	if (queryDesc->estate->es_rowmarks)
 	{
 		ExecRowMark *erm;
-		ListCell   *lc;
+		Index		i;
 
 		/*
 		 * Here, the query must have exactly one FOR UPDATE/SHARE reference to
 		 * the target table, and we dig the ctid info out of that.
 		 */
 		erm = NULL;
-		foreach(lc, queryDesc->estate->es_rowMarks)
+		for (i = 0; i < queryDesc->estate->es_range_table_size; i++)
 		{
-			ExecRowMark *thiserm = (ExecRowMark *) lfirst(lc);
+			ExecRowMark *thiserm = queryDesc->estate->es_rowmarks[i];
 
-			if (!RowMarkRequiresRowShareLock(thiserm->markType))
+			if (thiserm == NULL ||
+				!RowMarkRequiresRowShareLock(thiserm->markType))
 				continue;		/* ignore non-FOR UPDATE/SHARE items */
 
 			if (thiserm->relid == table_oid)

src/backend/executor/execMain.c

@@ -909,61 +909,68 @@ InitPlan(QueryDesc *queryDesc, int eflags)
 	}
 
 	/*
-	 * Next, build the ExecRowMark list from the PlanRowMark(s), if any.
+	 * Next, build the ExecRowMark array from the PlanRowMark(s), if any.
 	 */
-	estate->es_rowMarks = NIL;
-	foreach(l, plannedstmt->rowMarks)
+	if (plannedstmt->rowMarks)
 	{
-		PlanRowMark *rc = (PlanRowMark *) lfirst(l);
-		Oid			relid;
-		Relation	relation;
-		ExecRowMark *erm;
-
-		/* ignore "parent" rowmarks; they are irrelevant at runtime */
-		if (rc->isParent)
-			continue;
-
-		/* get relation's OID (will produce InvalidOid if subquery) */
-		relid = exec_rt_fetch(rc->rti, estate)->relid;
-
-		/* open relation, if we need to access it for this mark type */
-		switch (rc->markType)
+		estate->es_rowmarks = (ExecRowMark **)
+			palloc0(estate->es_range_table_size * sizeof(ExecRowMark *));
+		foreach(l, plannedstmt->rowMarks)
 		{
-			case ROW_MARK_EXCLUSIVE:
-			case ROW_MARK_NOKEYEXCLUSIVE:
-			case ROW_MARK_SHARE:
-			case ROW_MARK_KEYSHARE:
-			case ROW_MARK_REFERENCE:
-				relation = ExecGetRangeTableRelation(estate, rc->rti);
-				break;
-			case ROW_MARK_COPY:
-				/* no physical table access is required */
-				relation = NULL;
-				break;
-			default:
-				elog(ERROR, "unrecognized markType: %d", rc->markType);
-				relation = NULL;	/* keep compiler quiet */
-				break;
+			PlanRowMark *rc = (PlanRowMark *) lfirst(l);
+			Oid			relid;
+			Relation	relation;
+			ExecRowMark *erm;
+
+			/* ignore "parent" rowmarks; they are irrelevant at runtime */
+			if (rc->isParent)
+				continue;
+
+			/* get relation's OID (will produce InvalidOid if subquery) */
+			relid = exec_rt_fetch(rc->rti, estate)->relid;
+
+			/* open relation, if we need to access it for this mark type */
+			switch (rc->markType)
+			{
+				case ROW_MARK_EXCLUSIVE:
+				case ROW_MARK_NOKEYEXCLUSIVE:
+				case ROW_MARK_SHARE:
+				case ROW_MARK_KEYSHARE:
+				case ROW_MARK_REFERENCE:
+					relation = ExecGetRangeTableRelation(estate, rc->rti);
+					break;
+				case ROW_MARK_COPY:
+					/* no physical table access is required */
+					relation = NULL;
+					break;
+				default:
+					elog(ERROR, "unrecognized markType: %d", rc->markType);
+					relation = NULL;	/* keep compiler quiet */
+					break;
+			}
+
+			/* Check that relation is a legal target for marking */
+			if (relation)
+				CheckValidRowMarkRel(relation, rc->markType);
+
+			erm = (ExecRowMark *) palloc(sizeof(ExecRowMark));
+			erm->relation = relation;
+			erm->relid = relid;
+			erm->rti = rc->rti;
+			erm->prti = rc->prti;
+			erm->rowmarkId = rc->rowmarkId;
+			erm->markType = rc->markType;
+			erm->strength = rc->strength;
+			erm->waitPolicy = rc->waitPolicy;
+			erm->ermActive = false;
+			ItemPointerSetInvalid(&(erm->curCtid));
+			erm->ermExtra = NULL;
+
+			Assert(erm->rti > 0 && erm->rti <= estate->es_range_table_size &&
+				   estate->es_rowmarks[erm->rti - 1] == NULL);
+
+			estate->es_rowmarks[erm->rti - 1] = erm;
 		}
-
-		/* Check that relation is a legal target for marking */
-		if (relation)
-			CheckValidRowMarkRel(relation, rc->markType);
-
-		erm = (ExecRowMark *) palloc(sizeof(ExecRowMark));
-		erm->relation = relation;
-		erm->relid = relid;
-		erm->rti = rc->rti;
-		erm->prti = rc->prti;
-		erm->rowmarkId = rc->rowmarkId;
-		erm->markType = rc->markType;
-		erm->strength = rc->strength;
-		erm->waitPolicy = rc->waitPolicy;
-		erm->ermActive = false;
-		ItemPointerSetInvalid(&(erm->curCtid));
-		erm->ermExtra = NULL;
-
-		estate->es_rowMarks = lappend(estate->es_rowMarks, erm);
 	}
 
 	/*
@@ -2394,13 +2401,12 @@ ExecUpdateLockMode(EState *estate, ResultRelInfo *relinfo)
 ExecRowMark *
 ExecFindRowMark(EState *estate, Index rti, bool missing_ok)
 {
-	ListCell   *lc;
-
-	foreach(lc, estate->es_rowMarks)
+	if (rti > 0 && rti <= estate->es_range_table_size &&
+		estate->es_rowmarks != NULL)
 	{
-		ExecRowMark *erm = (ExecRowMark *) lfirst(lc);
+		ExecRowMark *erm = estate->es_rowmarks[rti - 1];
 
-		if (erm->rti == rti)
+		if (erm)
 			return erm;
 	}
 	if (!missing_ok)
@@ -3131,6 +3137,7 @@ EvalPlanQualStart(EPQState *epqstate, EState *parentestate, Plan *planTree)
 	estate->es_range_table_array = parentestate->es_range_table_array;
 	estate->es_range_table_size = parentestate->es_range_table_size;
 	estate->es_relations = parentestate->es_relations;
+	estate->es_rowmarks = parentestate->es_rowmarks;
 	estate->es_plannedstmt = parentestate->es_plannedstmt;
 	estate->es_junkFilter = parentestate->es_junkFilter;
 	estate->es_output_cid = parentestate->es_output_cid;
@@ -3148,7 +3155,6 @@ EvalPlanQualStart(EPQState *epqstate, EState *parentestate, Plan *planTree)
 	}
 	/* es_result_relation_info must NOT be copied */
 	/* es_trig_target_relations must NOT be copied */
-	estate->es_rowMarks = parentestate->es_rowMarks;
 	estate->es_top_eflags = parentestate->es_top_eflags;
 	estate->es_instrument = parentestate->es_instrument;
 	/* es_auxmodifytables must NOT be copied */

src/backend/executor/execUtils.c

@@ -113,6 +113,7 @@ CreateExecutorState(void)
 	estate->es_range_table_array = NULL;
 	estate->es_range_table_size = 0;
 	estate->es_relations = NULL;
+	estate->es_rowmarks = NULL;
 	estate->es_plannedstmt = NULL;
 
 	estate->es_junkFilter = NULL;
@@ -142,8 +143,6 @@ CreateExecutorState(void)
 
 	estate->es_tupleTable = NIL;
 
-	estate->es_rowMarks = NIL;
-
 	estate->es_processed = 0;
 	estate->es_lastoid = InvalidOid;
 
@@ -709,6 +708,12 @@ ExecInitRangeTable(EState *estate, List *rangeTable)
 	 */
 	estate->es_relations = (Relation *)
 		palloc0(estate->es_range_table_size * sizeof(Relation));
+
+	/*
+	 * es_rowmarks is also parallel to the es_range_table_array, but it's
+	 * allocated only if needed.
+	 */
+	estate->es_rowmarks = NULL;
 }
 
 /*

src/include/nodes/execnodes.h

@@ -34,6 +34,7 @@
 
 struct PlanState;				/* forward references in this file */
 struct ParallelHashJoinState;
+struct ExecRowMark;
 struct ExprState;
 struct ExprContext;
 struct RangeTblEntry;			/* avoid including parsenodes.h here */
@@ -491,6 +492,8 @@ typedef struct EState
 	Index		es_range_table_size;	/* size of the range table arrays */
 	Relation   *es_relations;	/* Array of per-range-table-entry Relation
 								 * pointers, or NULL if not yet opened */
+	struct ExecRowMark **es_rowmarks;	/* Array of per-range-table-entry
+										 * ExecRowMarks, or NULL if none */
 	PlannedStmt *es_plannedstmt;	/* link to top of plan tree */
 	const char *es_sourceText;	/* Source text from QueryDesc */
 
@@ -537,8 +540,6 @@ typedef struct EState
 
 	List	   *es_tupleTable;	/* List of TupleTableSlots */
 
-	List	   *es_rowMarks;	/* List of ExecRowMarks */
-
 	uint64		es_processed;	/* # of tuples processed */
 	Oid			es_lastoid;		/* last oid processed (by INSERT) */
 
@@ -607,7 +608,9 @@ typedef struct EState
  * node that sources the relation (e.g., for a foreign table the FDW can use
  * ermExtra to hold information).
  *
- * EState->es_rowMarks is a list of these structs.
+ * EState->es_rowmarks is an array of these structs, indexed by RT index,
+ * with NULLs for irrelevant RT indexes.  es_rowmarks itself is NULL if
+ * there are no rowmarks.
  */
 typedef struct ExecRowMark
 {
@@ -629,7 +632,7 @@ typedef struct ExecRowMark
  *	   additional runtime representation of FOR [KEY] UPDATE/SHARE clauses
  *
  * Each LockRows and ModifyTable node keeps a list of the rowmarks it needs to
- * deal with.  In addition to a pointer to the related entry in es_rowMarks,
+ * deal with.  In addition to a pointer to the related entry in es_rowmarks,
  * this struct carries the column number(s) of the resjunk columns associated
  * with the rowmark (see comments for PlanRowMark for more detail).  In the
  * case of ModifyTable, there has to be a separate ExecAuxRowMark list for
@@ -638,7 +641,7 @@ typedef struct ExecRowMark
  */
 typedef struct ExecAuxRowMark
 {
-	ExecRowMark *rowmark;		/* related entry in es_rowMarks */
+	ExecRowMark *rowmark;		/* related entry in es_rowmarks */
 	AttrNumber	ctidAttNo;		/* resno of ctid junk attribute, if any */
 	AttrNumber	toidAttNo;		/* resno of tableoid junk attribute, if any */
 	AttrNumber	wholeAttNo;		/* resno of whole-row junk attribute, if any */