Add UNIQUE null treatment option

The SQL standard has been ambiguous about whether null values in
unique constraints should be considered equal or not.  Different
implementations have different behaviors.  In the SQL:202x draft, this
has been formalized by making this implementation-defined and adding
an option on unique constraint definitions UNIQUE [ NULLS [NOT]
DISTINCT ] to choose a behavior explicitly.

This patch adds this option to PostgreSQL.  The default behavior
remains UNIQUE NULLS DISTINCT.  Making this happen in the btree code
is pretty easy; most of the patch is just to carry the flag around to
all the places that need it.

The CREATE UNIQUE INDEX syntax extension is not from the standard,
it's my own invention.

I named all the internal flags, catalog columns, etc. in the negative
("nulls not distinct") so that the default PostgreSQL behavior is the
default if the flag is false.

Reviewed-by: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/84e5ee1b-387e-9a54-c326-9082674bde78@enterprisedb.com

src/backend/access/nbtree/nbtinsert.c

@@ -398,9 +398,9 @@ _bt_search_insert(Relation rel, BTInsertState insertstate)
  * _bt_findinsertloc() to reuse most of the binary search work we do
  * here.
  *
- * Do not call here when there are NULL values in scan key.  NULL should be
- * considered unequal to NULL when checking for duplicates, but we are not
- * prepared to handle that correctly.
+ * This code treats NULLs as equal, unlike the default semantics for unique
+ * indexes.  So do not call here when there are NULL values in scan key and
+ * the index uses the default NULLS DISTINCT mode.
  */
 static TransactionId
 _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,

src/backend/access/nbtree/nbtsort.c

@@ -89,6 +89,7 @@ typedef struct BTSpool
 	Relation	heap;
 	Relation	index;
 	bool		isunique;
+	bool		nulls_not_distinct;
 } BTSpool;
 
 /*
@@ -106,6 +107,7 @@ typedef struct BTShared
 	Oid			heaprelid;
 	Oid			indexrelid;
 	bool		isunique;
+	bool		nulls_not_distinct;
 	bool		isconcurrent;
 	int			scantuplesortstates;
 
@@ -206,6 +208,7 @@ typedef struct BTLeader
 typedef struct BTBuildState
 {
 	bool		isunique;
+	bool		nulls_not_distinct;
 	bool		havedead;
 	Relation	heap;
 	BTSpool    *spool;
@@ -307,6 +310,7 @@ btbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 #endif							/* BTREE_BUILD_STATS */
 
 	buildstate.isunique = indexInfo->ii_Unique;
+	buildstate.nulls_not_distinct = indexInfo->ii_NullsNotDistinct;
 	buildstate.havedead = false;
 	buildstate.heap = heap;
 	buildstate.spool = NULL;
@@ -380,6 +384,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 	btspool->heap = heap;
 	btspool->index = index;
 	btspool->isunique = indexInfo->ii_Unique;
+	btspool->nulls_not_distinct = indexInfo->ii_NullsNotDistinct;
 
 	/* Save as primary spool */
 	buildstate->spool = btspool;
@@ -429,6 +434,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 	 */
 	buildstate->spool->sortstate =
 		tuplesort_begin_index_btree(heap, index, buildstate->isunique,
+									buildstate->nulls_not_distinct,
 									maintenance_work_mem, coordinate,
 									false);
 
@@ -468,7 +474,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 		 * full, so we give it only work_mem
 		 */
 		buildstate->spool2->sortstate =
-			tuplesort_begin_index_btree(heap, index, false, work_mem,
+			tuplesort_begin_index_btree(heap, index, false, false, work_mem,
 										coordinate2, false);
 	}
 
@@ -1554,6 +1560,7 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	btshared->heaprelid = RelationGetRelid(btspool->heap);
 	btshared->indexrelid = RelationGetRelid(btspool->index);
 	btshared->isunique = btspool->isunique;
+	btshared->nulls_not_distinct = btspool->nulls_not_distinct;
 	btshared->isconcurrent = isconcurrent;
 	btshared->scantuplesortstates = scantuplesortstates;
 	ConditionVariableInit(&btshared->workersdonecv);
@@ -1747,6 +1754,7 @@ _bt_leader_participate_as_worker(BTBuildState *buildstate)
 	leaderworker->heap = buildstate->spool->heap;
 	leaderworker->index = buildstate->spool->index;
 	leaderworker->isunique = buildstate->spool->isunique;
+	leaderworker->nulls_not_distinct = buildstate->spool->nulls_not_distinct;
 
 	/* Initialize second spool, if required */
 	if (!btleader->btshared->isunique)
@@ -1846,6 +1854,7 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 	btspool->heap = heapRel;
 	btspool->index = indexRel;
 	btspool->isunique = btshared->isunique;
+	btspool->nulls_not_distinct = btshared->nulls_not_distinct;
 
 	/* Look up shared state private to tuplesort.c */
 	sharedsort = shm_toc_lookup(toc, PARALLEL_KEY_TUPLESORT, false);
@@ -1928,6 +1937,7 @@ _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
 	btspool->sortstate = tuplesort_begin_index_btree(btspool->heap,
 													 btspool->index,
 													 btspool->isunique,
+													 btspool->nulls_not_distinct,
 													 sortmem, coordinate,
 													 false);
 
@@ -1950,13 +1960,14 @@ _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
 		coordinate2->nParticipants = -1;
 		coordinate2->sharedsort = sharedsort2;
 		btspool2->sortstate =
-			tuplesort_begin_index_btree(btspool->heap, btspool->index, false,
+			tuplesort_begin_index_btree(btspool->heap, btspool->index, false, false,
 										Min(sortmem, work_mem), coordinate2,
 										false);
 	}
 
 	/* Fill in buildstate for _bt_build_callback() */
 	buildstate.isunique = btshared->isunique;
+	buildstate.nulls_not_distinct = btshared->nulls_not_distinct;
 	buildstate.havedead = false;
 	buildstate.heap = btspool->heap;
 	buildstate.spool = btspool;

src/backend/access/nbtree/nbtutils.c

@@ -165,6 +165,13 @@ _bt_mkscankey(Relation rel, IndexTuple itup)
 			key->anynullkeys = true;
 	}
 
+	/*
+	 * In NULLS NOT DISTINCT mode, we pretend that there are no null keys, so
+	 * that full uniqueness check is done.
+	 */
+	if (rel->rd_index->indnullsnotdistinct)
+		key->anynullkeys = false;
+
 	return key;
 }