Speed up CREATE INDEX CONCURRENTLY's TID sort.

Encode TIDs as 64-bit integers to speed up comparisons.  This seems to
speed things up on all platforms, but is even more beneficial when
8-byte integers are passed by value.

Peter Geoghegan.  Design suggestions and review by Tom Lane.  Review
also by Simon Riggs and by me.

src/backend/catalog/index.c

@@ -109,6 +109,8 @@ static void index_update_stats(Relation rel,
 static void IndexCheckExclusion(Relation heapRelation,
 					Relation indexRelation,
 					IndexInfo *indexInfo);
+static inline int64 itemptr_encode(ItemPointer itemptr);
+static inline void itemptr_decode(ItemPointer itemptr, int64 encoded);
 static bool validate_index_callback(ItemPointer itemptr, void *opaque);
 static void validate_index_heapscan(Relation heapRelation,
 						Relation indexRelation,
@@ -2832,7 +2834,13 @@ validate_index(Oid heapId, Oid indexId, Snapshot snapshot)
 	ivinfo.num_heap_tuples = heapRelation->rd_rel->reltuples;
 	ivinfo.strategy = NULL;
 
-	state.tuplesort = tuplesort_begin_datum(TIDOID, TIDLessOperator,
+	/*
+	 * Encode TIDs as int8 values for the sort, rather than directly sorting
+	 * item pointers.  This can be significantly faster, primarily because TID
+	 * is a pass-by-reference type on all platforms, whereas int8 is
+	 * pass-by-value on most platforms.
+	 */
+	state.tuplesort = tuplesort_begin_datum(INT8OID, Int8LessOperator,
 											InvalidOid, false,
 											maintenance_work_mem,
 											false);
@@ -2871,6 +2879,46 @@ validate_index(Oid heapId, Oid indexId, Snapshot snapshot)
 	heap_close(heapRelation, NoLock);
 }
 
+/*
+ * itemptr_encode - Encode ItemPointer as int64/int8
+ *
+ * This representation must produce values encoded as int64 that sort in the
+ * same order as their corresponding original TID values would (using the
+ * default int8 opclass to produce a result equivalent to the default TID
+ * opclass).
+ *
+ * As noted in validate_index(), this can be significantly faster.
+ */
+static inline int64
+itemptr_encode(ItemPointer itemptr)
+{
+	BlockNumber		block = ItemPointerGetBlockNumber(itemptr);
+	OffsetNumber	offset = ItemPointerGetOffsetNumber(itemptr);
+	int64			encoded;
+
+	/*
+	 * Use the 16 least significant bits for the offset.  32 adjacent bits are
+	 * used for the block number.  Since remaining bits are unused, there
+	 * cannot be negative encoded values (We assume a two's complement
+	 * representation).
+	 */
+	encoded = ((uint64) block << 16) | (uint16) offset;
+
+	return encoded;
+}
+
+/*
+ * itemptr_decode - Decode int64/int8 representation back to ItemPointer
+ */
+static inline void
+itemptr_decode(ItemPointer itemptr, int64 encoded)
+{
+	BlockNumber		block = (BlockNumber) (encoded >> 16);
+	OffsetNumber	offset = (OffsetNumber) (encoded & 0xFFFF);
+
+	ItemPointerSet(itemptr, block, offset);
+}
+
 /*
  * validate_index_callback - bulkdelete callback to collect the index TIDs
  */
@@ -2878,8 +2926,9 @@ static bool
 validate_index_callback(ItemPointer itemptr, void *opaque)
 {
 	v_i_state  *state = (v_i_state *) opaque;
+	int64		encoded = itemptr_encode(itemptr);
 
-	tuplesort_putdatum(state->tuplesort, PointerGetDatum(itemptr), false);
+	tuplesort_putdatum(state->tuplesort, Int64GetDatum(encoded), false);
 	state->itups += 1;
 	return false;				/* never actually delete anything */
 }
@@ -2911,6 +2960,7 @@ validate_index_heapscan(Relation heapRelation,
 
 	/* state variables for the merge */
 	ItemPointer indexcursor = NULL;
+	ItemPointerData		decoded;
 	bool		tuplesort_empty = false;
 
 	/*
@@ -3020,13 +3070,26 @@ validate_index_heapscan(Relation heapRelation,
 				 */
 				if (ItemPointerGetBlockNumber(indexcursor) == root_blkno)
 					in_index[ItemPointerGetOffsetNumber(indexcursor) - 1] = true;
-				pfree(indexcursor);
 			}
 
 			tuplesort_empty = !tuplesort_getdatum(state->tuplesort, true,
 												  &ts_val, &ts_isnull);
 			Assert(tuplesort_empty || !ts_isnull);
-			indexcursor = (ItemPointer) DatumGetPointer(ts_val);
+			if (!tuplesort_empty)
+			{
+				itemptr_decode(&decoded, DatumGetInt64(ts_val));
+				indexcursor = &decoded;
+
+				/* If int8 is pass-by-ref, free (encoded) TID Datum memory */
+#ifndef USE_FLOAT8_BYVAL
+				pfree(DatumGetPointer(ts_val));
+#endif
+			}
+			else
+			{
+				/* Be tidy */
+				indexcursor = NULL;
+			}
 		}
 
 		/*