tableam: Add table_multi_insert() and revamp/speed-up COPY FROM buffering.

This adds table_multi_insert(), and converts COPY FROM, the only user
of heap_multi_insert, to it.

A simple conversion of COPY FROM use slots would have yielded a
slowdown when inserting into a partitioned table for some
workloads. Different partitions might need different slots (both slot
types and their descriptors), and dropping / creating slots when
there's constant partition changes is measurable.

Thus instead revamp the COPY FROM buffering for partitioned tables to
allow to buffer inserts into multiple tables, flushing only when
limits are reached across all partition buffers. By only dropping
slots when there've been inserts into too many different partitions,
the aforementioned overhead is gone. By allowing larger batches, even
when there are frequent partition changes, we actuall speed such cases
up significantly.

By using slots COPY of very narrow rows into unlogged / temporary
might slow down very slightly (due to the indirect function calls).

Author: David Rowley, Andres Freund, Haribabu Kommi
Discussion:
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
    https://postgr.es/m/20190327054923.t3epfuewxfqdt22e@alap3.anarazel.de

Author: anarazel

src/backend/access/heap/heapam.c

@@ -2106,7 +2106,7 @@ heap_prepare_insert(Relation relation, HeapTuple tup, TransactionId xid,
  * temporary context before calling this, if that's a problem.
  */
 void
-heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
+heap_multi_insert(Relation relation, TupleTableSlot **slots, int ntuples,
 				  CommandId cid, int options, BulkInsertState bistate)
 {
 	TransactionId xid = GetCurrentTransactionId();
@@ -2127,11 +2127,18 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 	saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
 												   HEAP_DEFAULT_FILLFACTOR);
 
-	/* Toast and set header data in all the tuples */
+	/* Toast and set header data in all the slots */
 	heaptuples = palloc(ntuples * sizeof(HeapTuple));
 	for (i = 0; i < ntuples; i++)
-		heaptuples[i] = heap_prepare_insert(relation, tuples[i],
-											xid, cid, options);
+	{
+		HeapTuple	tuple;
+
+		tuple = ExecFetchSlotHeapTuple(slots[i], true, NULL);
+		slots[i]->tts_tableOid = RelationGetRelid(relation);
+		tuple->t_tableOid = slots[i]->tts_tableOid;
+		heaptuples[i] = heap_prepare_insert(relation, tuple, xid, cid,
+											options);
+	}
 
 	/*
 	 * We're about to do the actual inserts -- but check for conflict first,
@@ -2361,13 +2368,9 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
 			CacheInvalidateHeapTuple(relation, heaptuples[i], NULL);
 	}
 
-	/*
-	 * Copy t_self fields back to the caller's original tuples. This does
-	 * nothing for untoasted tuples (tuples[i] == heaptuples[i)], but it's
-	 * probably faster to always copy than check.
-	 */
+	/* copy t_self fields back to the caller's slots */
 	for (i = 0; i < ntuples; i++)
-		tuples[i]->t_self = heaptuples[i]->t_self;
+		slots[i]->tts_tid = heaptuples[i]->t_self;
 
 	pgstat_count_heap_insert(relation, ntuples);
 }

src/backend/access/heap/heapam_handler.c

@@ -2516,6 +2516,7 @@ static const TableAmRoutine heapam_methods = {
 	.tuple_insert = heapam_tuple_insert,
 	.tuple_insert_speculative = heapam_tuple_insert_speculative,
 	.tuple_complete_speculative = heapam_tuple_complete_speculative,
+	.multi_insert = heap_multi_insert,
 	.tuple_delete = heapam_tuple_delete,
 	.tuple_update = heapam_tuple_update,
 	.tuple_lock = heapam_tuple_lock,