Fix ALTER COLUMN TYPE failure with a partial exclusion constraint.

ATExecAlterColumnType failed to consider the possibility that an index
that needs to be rebuilt might be a child of a constraint that needs to be
rebuilt.  We missed this so far because usually a constraint index doesn't
have a direct dependency on its table, just on the constraint object.
But if there's a WHERE clause, then dependency analysis of the WHERE
clause results in direct dependencies on the column(s) mentioned in WHERE.
This led to trying to drop and rebuild both the constraint and its
underlying index.

In v11/HEAD, we successfully drop both the index and the constraint,
and then try to rebuild both, and of course the second rebuild hits a
duplicate-index-name problem.  Before v11, it fails with obscure messages
about a missing relation OID, due to trying to drop the index twice.

This is essentially the same kind of problem noted in commit
20bef2c3: the possible dependency linkages are broader than what
ATExecAlterColumnType was designed for.  It was probably OK when
written, but it's certainly been broken since the introduction of
partial exclusion constraints.  Fix by adding an explicit check
for whether any of the indexes-to-be-rebuilt belong to any of the
constraints-to-be-rebuilt, and ignoring any that do.

In passing, fix a latent bug introduced by commit 8b08f7d4: in
get_constraint_index() we must "continue" not "break" when rejecting
a relation of a wrong relkind.  This is harmless today because we don't
expect that code path to be taken anyway; but if there ever were any
relations to be ignored, the existing coding would have an extremely
undesirable dependency on the order of pg_depend entries.

Also adjust a couple of obsolete comments.

Per bug #15835 from Yaroslav Schekin.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/15835-32d9b7a76c06a7a9@postgresql.org

src/backend/catalog/pg_depend.c

@@ -779,8 +779,8 @@ getOwnedSequence(Oid relid, AttrNumber attnum)
 
 /*
  * get_constraint_index
- *		Given the OID of a unique or primary-key constraint, return the
- *		OID of the underlying unique index.
+ *		Given the OID of a unique, primary-key, or exclusion constraint,
+ *		return the OID of the underlying index.
  *
  * Return InvalidOid if the index couldn't be found; this suggests the
  * given OID is bogus, but we leave it to caller to decide what to do.
@@ -827,10 +827,13 @@ get_constraint_index(Oid constraintId)
 		{
 			char		relkind = get_rel_relkind(deprec->objid);
 
-			/* This is pure paranoia; there shouldn't be any such */
+			/*
+			 * This is pure paranoia; there shouldn't be any other relkinds
+			 * dependent on a constraint.
+			 */
 			if (relkind != RELKIND_INDEX &&
 				relkind != RELKIND_PARTITIONED_INDEX)
-				break;
+				continue;
 
 			indexId = deprec->objid;
 			break;
@@ -845,8 +848,9 @@ get_constraint_index(Oid constraintId)
 
 /*
  * get_index_constraint
- *		Given the OID of an index, return the OID of the owning unique or
- *		primary-key constraint, or InvalidOid if no such constraint.
+ *		Given the OID of an index, return the OID of the owning unique,
+ *		primary-key, or exclusion constraint, or InvalidOid if there
+ *		is no owning constraint.
  */
 Oid
 get_index_constraint(Oid indexId)

src/backend/commands/tablecmds.c

@@ -10508,6 +10508,9 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 	SysScanDesc scan;
 	HeapTuple	depTup;
 	ObjectAddress address;
+	ListCell   *lc;
+	ListCell   *prev;
+	ListCell   *next;
 
 	/*
 	 * Clear all the missing values if we're rewriting the table, since this
@@ -10646,14 +10649,20 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 					if (relKind == RELKIND_INDEX ||
 						relKind == RELKIND_PARTITIONED_INDEX)
 					{
+						/*
+						 * Indexes that are directly dependent on the table
+						 * might be regular indexes or constraint indexes.
+						 * Constraint indexes typically have only indirect
+						 * dependencies; but there are exceptions, notably
+						 * partial exclusion constraints.  Hence we must check
+						 * whether the index depends on any constraint that's
+						 * due to be rebuilt, which we'll do below after we've
+						 * found all such constraints.
+						 */
 						Assert(foundObject.objectSubId == 0);
-						if (!list_member_oid(tab->changedIndexOids, foundObject.objectId))
-						{
-							tab->changedIndexOids = lappend_oid(tab->changedIndexOids,
-																foundObject.objectId);
-							tab->changedIndexDefs = lappend(tab->changedIndexDefs,
-															pg_get_indexdef_string(foundObject.objectId));
-						}
+						tab->changedIndexOids =
+							list_append_unique_oid(tab->changedIndexOids,
+												   foundObject.objectId);
 					}
 					else if (relKind == RELKIND_SEQUENCE)
 					{
@@ -10819,6 +10828,41 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 
 	systable_endscan(scan);
 
+	/*
+	 * Check the collected index OIDs to see which ones belong to the
+	 * constraint(s) of the table, and drop those from the list of indexes
+	 * that we need to process; rebuilding the constraints will handle them.
+	 */
+	prev = NULL;
+	for (lc = list_head(tab->changedIndexOids); lc; lc = next)
+	{
+		Oid			indexoid = lfirst_oid(lc);
+		Oid			conoid;
+
+		next = lnext(lc);
+
+		conoid = get_index_constraint(indexoid);
+		if (OidIsValid(conoid) &&
+			list_member_oid(tab->changedConstraintOids, conoid))
+			tab->changedIndexOids = list_delete_cell(tab->changedIndexOids,
+													 lc, prev);
+		else
+			prev = lc;
+	}
+
+	/*
+	 * Now collect the definitions of the indexes that must be rebuilt.  (We
+	 * could merge this into the previous loop, but it'd be more complicated
+	 * for little gain.)
+	 */
+	foreach(lc, tab->changedIndexOids)
+	{
+		Oid			indexoid = lfirst_oid(lc);
+
+		tab->changedIndexDefs = lappend(tab->changedIndexDefs,
+										pg_get_indexdef_string(indexoid));
+	}
+
 	/*
 	 * Now scan for dependencies of this column on other things.  The only
 	 * thing we should find is the dependency on the column datatype, which we

src/test/regress/expected/alter_table.out

@@ -1899,6 +1899,46 @@ select * from anothertab;
  f      | IT WAS NULL!
 (3 rows)
 
+drop table anothertab;
+-- Test alter table column type with constraint indexes (cf. bug #15835)
+create table anothertab(f1 int primary key, f2 int unique, f3 int, f4 int);
+alter table anothertab
+  add exclude using btree (f3 with =);
+alter table anothertab
+  add exclude using btree (f4 with =) where (f4 is not null);
+\d anothertab
+             Table "public.anothertab"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ f1     | integer |           | not null | 
+ f2     | integer |           |          | 
+ f3     | integer |           |          | 
+ f4     | integer |           |          | 
+Indexes:
+    "anothertab_pkey" PRIMARY KEY, btree (f1)
+    "anothertab_f2_key" UNIQUE CONSTRAINT, btree (f2)
+    "anothertab_f3_excl" EXCLUDE USING btree (f3 WITH =)
+    "anothertab_f4_excl" EXCLUDE USING btree (f4 WITH =) WHERE (f4 IS NOT NULL)
+
+alter table anothertab alter column f1 type bigint;
+alter table anothertab
+  alter column f2 type bigint,
+  alter column f3 type bigint,
+  alter column f4 type bigint;
+\d anothertab
+            Table "public.anothertab"
+ Column |  Type  | Collation | Nullable | Default 
+--------+--------+-----------+----------+---------
+ f1     | bigint |           | not null | 
+ f2     | bigint |           |          | 
+ f3     | bigint |           |          | 
+ f4     | bigint |           |          | 
+Indexes:
+    "anothertab_pkey" PRIMARY KEY, btree (f1)
+    "anothertab_f2_key" UNIQUE CONSTRAINT, btree (f2)
+    "anothertab_f3_excl" EXCLUDE USING btree (f3 WITH =)
+    "anothertab_f4_excl" EXCLUDE USING btree (f4 WITH =) WHERE (f4 IS NOT NULL)
+
 drop table anothertab;
 create table another (f1 int, f2 text);
 insert into another values(1, 'one');

src/test/regress/sql/alter_table.sql

@@ -1317,6 +1317,23 @@ select * from anothertab;
 
 drop table anothertab;
 
+-- Test alter table column type with constraint indexes (cf. bug #15835)
+create table anothertab(f1 int primary key, f2 int unique, f3 int, f4 int);
+alter table anothertab
+  add exclude using btree (f3 with =);
+alter table anothertab
+  add exclude using btree (f4 with =) where (f4 is not null);
+
+\d anothertab
+alter table anothertab alter column f1 type bigint;
+alter table anothertab
+  alter column f2 type bigint,
+  alter column f3 type bigint,
+  alter column f4 type bigint;
+\d anothertab
+
+drop table anothertab;
+
 create table another (f1 int, f2 text);
 
 insert into another values(1, 'one');