Support snapshot resets in concurrent builds of unique indexes

Previously, concurrent builds if unique index used a fixed snapshot for the entire scan to ensure proper uniqueness checks.

Now reset snapshots periodically during concurrent unique index builds, while still maintaining uniqueness by:
- ignoring SnapshotSelf dead tuples during uniqueness checks in tuplesort as not a guarantee, but a fail-fast mechanics
- adding a uniqueness check in _bt_load that detects multiple alive tuples with the same key values as a guarantee of correctness

Tuples are SnapshotSelf tested only in the case of equal index key values, overwise _bt_load works like before.

Author: michail-nikolaev

src/backend/access/heap/README.HOT

@@ -386,12 +386,12 @@ have the HOT-safety property enforced before we start to build the new
 index.
 
 After waiting for transactions which had the table open, we build the index
-for all rows that are valid in a fresh snapshot.  Any tuples visible in the
-snapshot will have only valid forward-growing HOT chains.  (They might have
-older HOT updates behind them which are broken, but this is OK for the same
-reason it's OK in a regular index build.)  As above, we point the index
-entry at the root of the HOT-update chain but we use the key value from the
-live tuple.
+for all rows that are valid in a fresh snapshot, which is updated every so
+often. Any tuples visible in the snapshot will have only valid forward-growing
+HOT chains.  (They might have older HOT updates behind them which are broken,
+but this is OK for the same reason it's OK in a regular index build.)
+As above, we point the index entry at the root of the HOT-update chain but we
+use the key value from the live tuple.
 
 We mark the index open for inserts (but still not ready for reads) then
 we again wait for transactions which have the table open.  Then we take

src/backend/access/heap/heapam_handler.c

@@ -1236,15 +1236,15 @@ heapam_index_build_range_scan(Relation heapRelation,
 	 * qual checks (because we have to index RECENTLY_DEAD tuples). In a
 	 * concurrent build, or during bootstrap, we take a regular MVCC snapshot
 	 * and index whatever's live according to that while that snapshot is reset
-	 * every so often (in case of non-unique index).
+	 * every so often.
 	 */
 	OldestXmin = InvalidTransactionId;
 
 	/*
-	 * For unique index we need consistent snapshot for the whole scan.
+	 * For concurrent builds of non-system indexes, we may want to periodically
+	 * reset snapshots to allow vacuum to clean up tuples.
 	 */
 	reset_snapshots = indexInfo->ii_Concurrent &&
-					  !indexInfo->ii_Unique &&
 					  !is_system_catalog; /* just for the case */
 
 	/* okay to ignore lazy VACUUMs here */

src/backend/access/nbtree/nbtdedup.c

@@ -148,7 +148,7 @@ _bt_dedup_pass(Relation rel, Buffer buf, IndexTuple newitem, Size newitemsz,
 			_bt_dedup_start_pending(state, itup, offnum);
 		}
 		else if (state->deduplicate &&
-				 _bt_keep_natts_fast(rel, state->base, itup) > nkeyatts &&
+				 _bt_keep_natts_fast(rel, state->base, itup, NULL) > nkeyatts &&
 				 _bt_dedup_save_htid(state, itup))
 		{
 			/*
@@ -374,7 +374,7 @@ _bt_bottomupdel_pass(Relation rel, Buffer buf, Relation heapRel,
 			/* itup starts first pending interval */
 			_bt_dedup_start_pending(state, itup, offnum);
 		}
-		else if (_bt_keep_natts_fast(rel, state->base, itup) > nkeyatts &&
+		else if (_bt_keep_natts_fast(rel, state->base, itup, NULL) > nkeyatts &&
 				 _bt_dedup_save_htid(state, itup))
 		{
 			/* Tuple is equal; just added its TIDs to pending interval */
@@ -789,12 +789,12 @@ _bt_do_singleval(Relation rel, Page page, BTDedupState state,
 	itemid = PageGetItemId(page, minoff);
 	itup = (IndexTuple) PageGetItem(page, itemid);
 
-	if (_bt_keep_natts_fast(rel, newitem, itup) > nkeyatts)
+	if (_bt_keep_natts_fast(rel, newitem, itup, NULL) > nkeyatts)
 	{
 		itemid = PageGetItemId(page, PageGetMaxOffsetNumber(page));
 		itup = (IndexTuple) PageGetItem(page, itemid);
 
-		if (_bt_keep_natts_fast(rel, newitem, itup) > nkeyatts)
+		if (_bt_keep_natts_fast(rel, newitem, itup, NULL) > nkeyatts)
 			return true;
 	}