Fix assorted bugs in pg_get_partition_constraintdef().

It failed if passed a nonexistent relation OID, or one that was a non-heap
relation, because of blindly applying heap_open to a user-supplied OID.
This is not OK behavior for a SQL-exposed function; we have a project
policy that we should return NULL in such cases.  Moreover, since
pg_get_partition_constraintdef ought now to work on indexes, restricting
it to heaps is flat wrong anyway.

The underlying function generate_partition_qual() wasn't on board with
indexes having partition quals either, nor for that matter with rels
having relispartition set but yet null relpartbound.  (One wonders
whether the person who wrote the function comment blocks claiming that
these functions allow a missing relpartbound had ever tested it.)

Fix by testing relispartition before opening the rel, and by using
relation_open not heap_open.  (If any other relkinds ever grow the
ability to have relispartition set, the code will work with them
automatically.)  Also, don't reject null relpartbound in
generate_partition_qual.

Back-patch to v11, and all but the null-relpartbound change to v10.
(It's not really necessary to change generate_partition_qual at all
in v10, but I thought s/heap_open/relation_open/ would be a good
idea anyway just to keep the code in sync with later branches.)

Per report from Justin Pryzby.

Discussion: https://postgr.es/m/20180927200020.GJ776@telsasoft.com

src/backend/utils/cache/lsyscache.c

@@ -1846,6 +1846,30 @@ get_rel_relkind(Oid relid)
 		return '\0';
 }
 
+/*
+ * get_rel_relispartition
+ *
+ *		Returns the relispartition flag associated with a given relation.
+ */
+bool
+get_rel_relispartition(Oid relid)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_class reltup = (Form_pg_class) GETSTRUCT(tp);
+		bool		result;
+
+		result = reltup->relispartition;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		return false;
+}
+
 /*
  * get_rel_tablespace
  *

src/backend/utils/cache/partcache.c

@@ -797,31 +797,38 @@ RelationGetPartitionQual(Relation rel)
  * get_partition_qual_relid
  *
  * Returns an expression tree describing the passed-in relation's partition
- * constraint. If there is no partition constraint returns NULL; this can
- * happen if the default partition is the only partition.
+ * constraint.
+ *
+ * If the relation is not found, or is not a partition, or there is no
+ * partition constraint, return NULL.  We must guard against the first two
+ * cases because this supports a SQL function that could be passed any OID.
+ * The last case can happen even if relispartition is true, when a default
+ * partition is the only partition.
  */
 Expr *
 get_partition_qual_relid(Oid relid)
 {
-	Relation	rel = heap_open(relid, AccessShareLock);
 	Expr	   *result = NULL;
-	List	   *and_args;
 
-	/* Do the work only if this relation is a partition. */
-	if (rel->rd_rel->relispartition)
+	/* Do the work only if this relation exists and is a partition. */
+	if (get_rel_relispartition(relid))
 	{
+		Relation	rel = relation_open(relid, AccessShareLock);
+		List	   *and_args;
+
 		and_args = generate_partition_qual(rel);
 
+		/* Convert implicit-AND list format to boolean expression */
 		if (and_args == NIL)
 			result = NULL;
 		else if (list_length(and_args) > 1)
 			result = makeBoolExpr(AND_EXPR, and_args, -1);
 		else
 			result = linitial(and_args);
-	}
 
-	/* Keep the lock. */
-	heap_close(rel, NoLock);
+		/* Keep the lock, to allow safe deparsing against the rel by caller. */
+		relation_close(rel, NoLock);
+	}
 
 	return result;
 }
@@ -845,7 +852,6 @@ generate_partition_qual(Relation rel)
 	MemoryContext oldcxt;
 	Datum		boundDatum;
 	bool		isnull;
-	PartitionBoundSpec *bound;
 	List	   *my_qual = NIL,
 			   *result = NIL;
 	Relation	parent;
@@ -859,8 +865,8 @@ generate_partition_qual(Relation rel)
 		return copyObject(rel->rd_partcheck);
 
 	/* Grab at least an AccessShareLock on the parent table */
-	parent = heap_open(get_partition_parent(RelationGetRelid(rel)),
-					   AccessShareLock);
+	parent = relation_open(get_partition_parent(RelationGetRelid(rel)),
+						   AccessShareLock);
 
 	/* Get pg_class.relpartbound */
 	tuple = SearchSysCache1(RELOID, RelationGetRelid(rel));
@@ -871,13 +877,17 @@ generate_partition_qual(Relation rel)
 	boundDatum = SysCacheGetAttr(RELOID, tuple,
 								 Anum_pg_class_relpartbound,
 								 &isnull);
-	if (isnull)
-		elog(ERROR, "null relpartbound for relation %u", RelationGetRelid(rel));
-	bound = castNode(PartitionBoundSpec,
-					 stringToNode(TextDatumGetCString(boundDatum)));
-	ReleaseSysCache(tuple);
+	if (!isnull)
+	{
+		PartitionBoundSpec *bound;
+
+		bound = castNode(PartitionBoundSpec,
+						 stringToNode(TextDatumGetCString(boundDatum)));
 
-	my_qual = get_qual_from_partbound(rel, parent, bound);
+		my_qual = get_qual_from_partbound(rel, parent, bound);
+	}
+
+	ReleaseSysCache(tuple);
 
 	/* Add the parent's quals to the list (if any) */
 	if (parent->rd_rel->relispartition)
@@ -903,7 +913,7 @@ generate_partition_qual(Relation rel)
 	MemoryContextSwitchTo(oldcxt);
 
 	/* Keep the parent locked until commit */
-	heap_close(parent, NoLock);
+	relation_close(parent, NoLock);
 
 	return result;
 }

src/include/utils/lsyscache.h

@@ -128,6 +128,7 @@ extern char *get_rel_name(Oid relid);
 extern Oid	get_rel_namespace(Oid relid);
 extern Oid	get_rel_type_id(Oid relid);
 extern char get_rel_relkind(Oid relid);
+extern bool get_rel_relispartition(Oid relid);
 extern Oid	get_rel_tablespace(Oid relid);
 extern char get_rel_persistence(Oid relid);
 extern Oid	get_transform_fromsql(Oid typid, Oid langid, List *trftypes);

src/test/regress/expected/indexing.out

@@ -75,6 +75,25 @@ Indexes:
     "idxpart1_a_idx" btree (a)
     "idxpart1_b_c_idx" btree (b, c)
 
+\d+ idxpart1_a_idx
+                 Index "public.idxpart1_a_idx"
+ Column |  Type   | Key? | Definition | Storage | Stats target 
+--------+---------+------+------------+---------+--------------
+ a      | integer | yes  | a          | plain   | 
+Partition of: idxparti 
+No partition constraint
+btree, for table "public.idxpart1"
+
+\d+ idxpart1_b_c_idx
+                Index "public.idxpart1_b_c_idx"
+ Column |  Type   | Key? | Definition | Storage  | Stats target 
+--------+---------+------+------------+----------+--------------
+ b      | integer | yes  | b          | plain    | 
+ c      | text    | yes  | c          | extended | 
+Partition of: idxparti2 
+No partition constraint
+btree, for table "public.idxpart1"
+
 drop table idxpart;
 -- If a partition already has an index, don't create a duplicative one
 create table idxpart (a int, b int) partition by range (a, b);

src/test/regress/sql/indexing.sql

@@ -44,6 +44,8 @@ create table idxpart1 (like idxpart);
 \d idxpart1
 alter table idxpart attach partition idxpart1 for values from (0) to (10);
 \d idxpart1
+\d+ idxpart1_a_idx
+\d+ idxpart1_b_c_idx
 drop table idxpart;
 
 -- If a partition already has an index, don't create a duplicative one