Fix ALTER TABLE .. ATTACH PARTITION ... DEFAULT

If the table being attached contained values that contradict the default
partition's partition constraint, it would fail to complain, because
CommandCounterIncrement changes in 4dba331c coupled with some bogus
coding in the existing ValidatePartitionConstraints prevented the
partition constraint from being validated after all -- or rather, it
caused to constraint to become an empty one, always succeeding.

Fix by not re-reading the OID of the default partition in
ATExecAttachPartition.  To forestall similar problems, revise the
existing code:
* rename routine from ValidatePartitionConstraints() to
  QueuePartitionConstraintValidation, to better represent what it
  actually does.
* add an Assert() to make sure that when queueing a constraint for a
  partition we're not overwriting a constraint previously queued.
* add an Assert() that we don't try to invoke the special-purpose
  validation of the default partition when attaching the default
  partition itself.

While at it, change some loops to obtain partition OIDs from
partdesc->oids rather than find_all_inheritors; reduce the lock level
of partitions being scanned from AccessExclusiveLock to ShareLock;
rewrite QueuePartitionConstraintValidation in a recursive fashion rather
than repetitive.

Author: Álvaro Herrera.  Tests written by Amit Langote
Reported-by: Rushabh Lathia
Diagnosed-by: Kyotaro HORIGUCHI, who also provided the initial fix.
Reviewed-by: Kyotaro HORIGUCHI, Amit Langote, Jeevan Ladhe
Discussion: https://postgr.es/m/CAGPqQf0W+v-Ci_qNV_5R3A=Z9LsK4+jO7LzgddRncpp_rrnJqQ@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -480,10 +480,9 @@ static void RemoveInheritance(Relation child_rel, Relation parent_rel);
 static ObjectAddress ATExecAttachPartition(List **wqueue, Relation rel,
 					  PartitionCmd *cmd);
 static void AttachPartitionEnsureIndexes(Relation rel, Relation attachrel);
-static void ValidatePartitionConstraints(List **wqueue, Relation scanrel,
-							 List *scanrel_children,
-							 List *partConstraint,
-							 bool validate_default);
+static void QueuePartitionConstraintValidation(List **wqueue, Relation scanrel,
+								   List *partConstraint,
+								   bool validate_default);
 static void CloneRowTriggersToPartition(Relation parent, Relation partition);
 static ObjectAddress ATExecDetachPartition(Relation rel, RangeVar *name);
 static ObjectAddress ATExecAttachPartitionIdx(List **wqueue, Relation rel,
@@ -13939,29 +13938,23 @@ PartConstraintImpliedByRelConstraint(Relation scanrel,
 }
 
 /*
- * ValidatePartitionConstraints
+ * QueuePartitionConstraintValidation
  *
- * Check whether all rows in the given table obey the given partition
- * constraint; if so, it can be attached as a partition.  We do this by
- * scanning the table (or all of its leaf partitions) row by row, except when
- * the existing constraints are sufficient to prove that the new partitioning
- * constraint must already hold.
+ * Add an entry to wqueue to have the given partition constraint validated by
+ * Phase 3, for the given relation, and all its children.
+ *
+ * We first verify whether the given constraint is implied by pre-existing
+ * relation constraints; if it is, there's no need to scan the table to
+ * validate, so don't queue in that case.
  */
 static void
-ValidatePartitionConstraints(List **wqueue, Relation scanrel,
-							 List *scanrel_children,
-							 List *partConstraint,
-							 bool validate_default)
+QueuePartitionConstraintValidation(List **wqueue, Relation scanrel,
+								   List *partConstraint,
+								   bool validate_default)
 {
-	bool		found_whole_row;
-	ListCell   *lc;
-
-	if (partConstraint == NIL)
-		return;
-
 	/*
-	 * Based on the table's existing constraints, determine if we can skip
-	 * scanning the table to validate the partition constraint.
+	 * Based on the table's existing constraints, determine whether or not we
+	 * may skip scanning the table.
 	 */
 	if (PartConstraintImpliedByRelConstraint(scanrel, partConstraint))
 	{
@@ -13976,68 +13969,53 @@ ValidatePartitionConstraints(List **wqueue, Relation scanrel,
 		return;
 	}
 
-	/* Constraints proved insufficient, so we need to scan the table. */
-	foreach(lc, scanrel_children)
+	/*
+	 * Constraints proved insufficient. For plain relations, queue a validation
+	 * item now; for partitioned tables, recurse to process each partition.
+	 */
+	if (scanrel->rd_rel->relkind == RELKIND_RELATION)
 	{
 		AlteredTableInfo *tab;
-		Oid			part_relid = lfirst_oid(lc);
-		Relation	part_rel;
-		List	   *my_partconstr = partConstraint;
 
-		/* Lock already taken */
-		if (part_relid != RelationGetRelid(scanrel))
-			part_rel = heap_open(part_relid, NoLock);
-		else
-			part_rel = scanrel;
+		/* Grab a work queue entry. */
+		tab = ATGetQueueEntry(wqueue, scanrel);
+		Assert(tab->partition_constraint == NULL);
+		tab->partition_constraint = (Expr *) linitial(partConstraint);
+		tab->validate_default = validate_default;
+	}
+	else if (scanrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+	{
+		PartitionDesc partdesc = RelationGetPartitionDesc(scanrel);
+		int			i;
 
-		/*
-		 * Skip if the partition is itself a partitioned table.  We can only
-		 * ever scan RELKIND_RELATION relations.
-		 */
-		if (part_rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		for (i = 0; i < partdesc->nparts; i++)
 		{
-			if (part_rel != scanrel)
-				heap_close(part_rel, NoLock);
-			continue;
-		}
+			Relation	part_rel;
+			bool		found_whole_row;
+			List	   *thisPartConstraint;
+
+			/*
+			 * This is the minimum lock we need to prevent concurrent data
+			 * additions.
+			 */
+			part_rel = heap_open(partdesc->oids[i], ShareLock);
 
-		if (part_rel != scanrel)
-		{
 			/*
 			 * Adjust the constraint for scanrel so that it matches this
 			 * partition's attribute numbers.
 			 */
-			my_partconstr = map_partition_varattnos(my_partconstr, 1,
-													part_rel, scanrel,
-													&found_whole_row);
+			thisPartConstraint =
+				map_partition_varattnos(partConstraint, 1,
+										part_rel, scanrel, &found_whole_row);
 			/* There can never be a whole-row reference here */
 			if (found_whole_row)
-				elog(ERROR, "unexpected whole-row reference found in partition key");
+				elog(ERROR, "unexpected whole-row reference found in partition constraint");
 
-			/* Can we skip scanning this part_rel? */
-			if (PartConstraintImpliedByRelConstraint(part_rel, my_partconstr))
-			{
-				if (!validate_default)
-					ereport(INFO,
-							(errmsg("partition constraint for table \"%s\" is implied by existing constraints",
-									RelationGetRelationName(part_rel))));
-				else
-					ereport(INFO,
-							(errmsg("updated partition constraint for default partition \"%s\" is implied by existing constraints",
-									RelationGetRelationName(part_rel))));
-				heap_close(part_rel, NoLock);
-				continue;
-			}
+			QueuePartitionConstraintValidation(wqueue, part_rel,
+											   thisPartConstraint,
+											   validate_default);
+			heap_close(part_rel, NoLock);	/* keep lock till commit */
 		}
-
-		/* Grab a work queue entry. */
-		tab = ATGetQueueEntry(wqueue, part_rel);
-		tab->partition_constraint = (Expr *) linitial(my_partconstr);
-		tab->validate_default = validate_default;
-
-		/* keep our lock until commit */
-		if (part_rel != scanrel)
-			heap_close(part_rel, NoLock);
 	}
 }
 
@@ -14067,8 +14045,8 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
 	ListCell   *l;
 
 	/*
-	 * We must lock the default partition, because attaching a new partition
-	 * will change its partition constraint.
+	 * We must lock the default partition if one exists, because attaching a
+	 * new partition will change its partition constraint.
 	 */
 	defaultPartOid =
 		get_default_oid_from_partdesc(RelationGetPartitionDesc(rel));
@@ -14133,17 +14111,17 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
 	 *
 	 * We do that by checking if rel is a member of the list of attachrel's
 	 * partitions provided the latter is partitioned at all.  We want to avoid
-	 * having to construct this list again, so we request the strongest lock
-	 * on all partitions.  We need the strongest lock, because we may decide
-	 * to scan them if we find out that the table being attached (or its leaf
-	 * partitions) may contain rows that violate the partition constraint. If
-	 * the table has a constraint that would prevent such rows, which by
-	 * definition is present in all the partitions, we need not scan the
-	 * table, nor its partitions.  But we cannot risk a deadlock by taking a
-	 * weaker lock now and the stronger one only when needed.
+	 * having to construct this list again, so we request a lock on all
+	 * partitions.  We need ShareLock, preventing data changes, because we
+	 * may decide to scan them if we find out that the table being attached (or
+	 * its leaf partitions) may contain rows that violate the partition
+	 * constraint.  If the table has a constraint that would prevent such rows,
+	 * which by definition is present in all the partitions, we need not scan
+	 * the table, nor its partitions.  But we cannot risk a deadlock by taking
+	 * a weaker lock now and the stronger one only when needed.
 	 */
 	attachrel_children = find_all_inheritors(RelationGetRelid(attachrel),
-											 AccessExclusiveLock, NULL);
+											 ShareLock, NULL);
 	if (list_member_oid(attachrel_children, RelationGetRelid(rel)))
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_TABLE),
@@ -14291,9 +14269,7 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
 		/*
 		 * Run the partition quals through const-simplification similar to
 		 * check constraints.  We skip canonicalize_qual, though, because
-		 * partition quals should be in canonical form already; also, since
-		 * the qual is in implicit-AND format, we'd have to explicitly convert
-		 * it to explicit-AND format and back again.
+		 * partition quals should be in canonical form already.
 		 */
 		partConstraint =
 			(List *) eval_const_expressions(NULL,
@@ -14314,32 +14290,30 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
 				 "unexpected whole-row reference found in partition key");
 
 		/* Validate partition constraints against the table being attached. */
-		ValidatePartitionConstraints(wqueue, attachrel, attachrel_children,
-									 partConstraint, false);
+		QueuePartitionConstraintValidation(wqueue, attachrel, partConstraint,
+										   false);
 	}
 
 	/*
-	 * Check whether default partition has a row that would fit the partition
-	 * being attached.
+	 * If we're attaching a partition other than the default partition and a
+	 * default one exists, then that partition's partition constraint changes,
+	 * so add an entry to the work queue to validate it, too.  (We must not
+	 * do this when the partition being attached is the default one; we
+	 * already did it above!)
 	 */
-	defaultPartOid =
-		get_default_oid_from_partdesc(RelationGetPartitionDesc(rel));
 	if (OidIsValid(defaultPartOid))
 	{
 		Relation	defaultrel;
-		List	   *defaultrel_children;
 		List	   *defPartConstraint;
 
-		/* We already have taken a lock on default partition. */
+		Assert(!cmd->bound->is_default);
+
+		/* we already hold a lock on the default partition */
 		defaultrel = heap_open(defaultPartOid, NoLock);
 		defPartConstraint =
 			get_proposed_default_constraint(partBoundConstraint);
-		defaultrel_children =
-			find_all_inheritors(defaultPartOid,
-								AccessExclusiveLock, NULL);
-		ValidatePartitionConstraints(wqueue, defaultrel,
-									 defaultrel_children,
-									 defPartConstraint, true);
+		QueuePartitionConstraintValidation(wqueue, defaultrel,
+										   defPartConstraint, true);
 
 		/* keep our lock until commit. */
 		heap_close(defaultrel, NoLock);

src/test/regress/expected/alter_table.out

@@ -3887,3 +3887,19 @@ ALTER TABLE attmp ALTER COLUMN i RESET (n_distinct_inherited);
 ANALYZE attmp;
 DROP TABLE attmp;
 DROP USER regress_alter_table_user1;
+-- check that violating rows are correctly reported when attaching as the
+-- default partition
+create table defpart_attach_test (a int) partition by list (a);
+create table defpart_attach_test1 partition of defpart_attach_test for values in (1);
+create table defpart_attach_test_d (like defpart_attach_test);
+insert into defpart_attach_test_d values (1), (2);
+-- error because its constraint as the default partition would be violated
+-- by the row containing 1
+alter table defpart_attach_test attach partition defpart_attach_test_d default;
+ERROR:  partition constraint is violated by some row
+delete from defpart_attach_test_d where a = 1;
+alter table defpart_attach_test_d add check (a > 1);
+-- should be attached successfully and without needing to be scanned
+alter table defpart_attach_test attach partition defpart_attach_test_d default;
+INFO:  partition constraint for table "defpart_attach_test_d" is implied by existing constraints
+drop table defpart_attach_test;

src/test/regress/sql/alter_table.sql

@@ -2564,3 +2564,21 @@ ANALYZE attmp;
 DROP TABLE attmp;
 
 DROP USER regress_alter_table_user1;
+
+-- check that violating rows are correctly reported when attaching as the
+-- default partition
+create table defpart_attach_test (a int) partition by list (a);
+create table defpart_attach_test1 partition of defpart_attach_test for values in (1);
+create table defpart_attach_test_d (like defpart_attach_test);
+insert into defpart_attach_test_d values (1), (2);
+
+-- error because its constraint as the default partition would be violated
+-- by the row containing 1
+alter table defpart_attach_test attach partition defpart_attach_test_d default;
+delete from defpart_attach_test_d where a = 1;
+alter table defpart_attach_test_d add check (a > 1);
+
+-- should be attached successfully and without needing to be scanned
+alter table defpart_attach_test attach partition defpart_attach_test_d default;
+
+drop table defpart_attach_test;