In COPY, insert tuples to the heap in batches.

This greatly reduces the WAL volume, especially when the table is narrow.
The overhead of locking the heap page is also reduced. Reduced WAL traffic
also makes it scale a lot better, if you run multiple COPY processes at
the same time.

src/include/access/htup.h

@@ -608,6 +608,7 @@ typedef HeapTupleData *HeapTuple;
 /* 0x20 is free, was XLOG_HEAP2_CLEAN_MOVE */
 #define XLOG_HEAP2_CLEANUP_INFO 0x30
 #define XLOG_HEAP2_VISIBLE		0x40
+#define XLOG_HEAP2_MULTI_INSERT	0x50
 
 /*
  * All what we need to find changed tuple
@@ -661,6 +662,36 @@ typedef struct xl_heap_insert
 
 #define SizeOfHeapInsert	(offsetof(xl_heap_insert, all_visible_cleared) + sizeof(bool))
 
+/*
+ * This is what we need to know about a multi-insert. The record consists of
+ * xl_heap_multi_insert header, followed by a xl_multi_insert_tuple and tuple
+ * data for each tuple. 'offsets' array is omitted if the whole page is
+ * reinitialized (XLOG_HEAP_INIT_PAGE)
+ */
+typedef struct xl_heap_multi_insert
+{
+	RelFileNode node;
+	BlockNumber	blkno;
+	bool		all_visible_cleared;
+	uint16		ntuples;
+	OffsetNumber offsets[1];
+
+	/* TUPLE DATA (xl_multi_insert_tuples) FOLLOW AT END OF STRUCT */
+} xl_heap_multi_insert;
+
+#define SizeOfHeapMultiInsert	offsetof(xl_heap_multi_insert, offsets)
+
+typedef struct xl_multi_insert_tuple
+{
+	uint16		datalen;				/* size of tuple data that follows */
+	uint16		t_infomask2;
+	uint16		t_infomask;
+	uint8		t_hoff;
+	/* TUPLE DATA FOLLOWS AT END OF STRUCT */
+} xl_multi_insert_tuple;
+
+#define SizeOfMultiInsertTuple	(offsetof(xl_multi_insert_tuple, t_hoff) + sizeof(uint8))
+
 /* This is what we need to know about update|hot_update */
 typedef struct xl_heap_update
 {