Minimize slot creation for multi-inserts of pg_shdepend

When doing multiple insertions in pg_shdepend for the copy of
dependencies from a template database in CREATE DATABASE, the same
number of slots would have been created and used all the time.  As the
number of items to insert is not known in advance, this makes most of
the slots created for nothing.  This improves the slot handling so as
slot creation only happens when needed, minimizing the overhead of the
operation.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200731024148.GB3317@paquier.xyz

src/backend/catalog/pg_shdepend.c

@@ -809,15 +809,19 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
 	int			slotCount;
 	CatalogIndexState indstate;
 	TupleTableSlot **slot;
-	int			nslots;
+	int			nslots,
+				max_slots;
+	bool		slot_init = true;
 
 	sdepRel = table_open(SharedDependRelationId, RowExclusiveLock);
 	sdepDesc = RelationGetDescr(sdepRel);
 
-	nslots = MAX_PGSHDEPEND_INSERT_BYTES / sizeof(FormData_pg_shdepend);
-	slot = palloc(sizeof(TupleTableSlot *) * nslots);
-	for (int i = 0; i < nslots; i++)
-		slot[i] = MakeSingleTupleTableSlot(sdepDesc, &TTSOpsHeapTuple);
+	/*
+	 * Allocate the slots to use, but delay initialization until we know that
+	 * they will be used.
+	 */
+	max_slots = MAX_PGSHDEPEND_INSERT_BYTES / sizeof(FormData_pg_shdepend);
+	slot = palloc(sizeof(TupleTableSlot *) * max_slots);
 
 	indstate = CatalogOpenIndexes(sdepRel);
 
@@ -842,6 +846,9 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
 	{
 		Form_pg_shdepend shdep;
 
+		if (slot_init)
+			slot[slotCount] = MakeSingleTupleTableSlot(sdepDesc, &TTSOpsHeapTuple);
+
 		ExecClearTuple(slot[slotCount]);
 
 		shdep = (Form_pg_shdepend) GETSTRUCT(tup);
@@ -858,10 +865,11 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
 		slotCount++;
 
 		/* If slots are full, insert a batch of tuples */
-		if (slotCount == nslots)
+		if (slotCount == max_slots)
 		{
 			CatalogTuplesMultiInsertWithInfo(sdepRel, slot, slotCount, indstate);
 			slotCount = 0;
+			slot_init = false;
 		}
 	}
 
@@ -874,6 +882,8 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
 	CatalogCloseIndexes(indstate);
 	table_close(sdepRel, RowExclusiveLock);
 
+	/* Drop only the number of slots used */
+	nslots = slot_init ? slotCount : max_slots;
 	for (int i = 0; i < nslots; i++)
 		ExecDropSingleTupleTableSlot(slot[i]);
 	pfree(slot);