Fix infer_arbiter_index for partitioned tables

The fix for concurrent index operations in bc32a12e0d started
considering indexes that are not yet marked indisvalid as arbiters for
INSERT ON CONFLICT.  For partitioned tables, this leads to including
indexes that may not exist in partitions, causing a trivially
reproducible "invalid arbiter index list" error to be thrown because of
failure to match the index.  To fix, it suffices to ignore !indisvalid
indexes on partitioned tables.  There should be no risk that the set of
indexes will change for concurrent transactions, because in order for
such an index to be marked valid, an ALTER INDEX ATTACH PARTITION must
run which requires AccessExclusiveLock.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/17622f79-117a-4a44-aa8e-0374e53faaf0%40gmail.com

src/backend/optimizer/util/plancat.c

@@ -999,6 +999,15 @@ infer_arbiter_indexes(PlannerInfo *root)
 		if (!idxForm->indisready)
 			continue;
 
+		/*
+		 * Ignore invalid indexes for partitioned tables.  It's possible that
+		 * some partitions don't have the index (yet), and then we would not
+		 * find a match during ExecInitPartitionInfo.
+		 */
+		if (relation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE &&
+			!idxForm->indisvalid)
+			continue;
+
 		/*
 		 * Note that we do not perform a check against indcheckxmin (like e.g.
 		 * get_relation_info()) here to eliminate candidates, because

src/test/regress/expected/partition_info.out

@@ -349,3 +349,10 @@ SELECT pg_partition_root('ptif_li_child');
 DROP VIEW ptif_test_view;
 DROP MATERIALIZED VIEW ptif_test_matview;
 DROP TABLE ptif_li_parent, ptif_li_child;
+-- Test about selection of arbiter indexes for partitioned tables with
+-- non-valid index on the parent table
+CREATE TABLE pt (a int PRIMARY KEY) PARTITION BY RANGE (a);
+CREATE TABLE p1 PARTITION OF pt FOR VALUES FROM (1) to (2) PARTITION BY RANGE (a);
+CREATE TABLE p1_1 PARTITION OF p1 FOR VALUES FROM (1) TO (2);
+CREATE UNIQUE INDEX ON ONLY p1 (a);
+INSERT INTO p1 VALUES (1) ON CONFLICT (a) DO NOTHING;

src/test/regress/sql/partition_info.sql

@@ -127,3 +127,11 @@ SELECT pg_partition_root('ptif_li_child');
 DROP VIEW ptif_test_view;
 DROP MATERIALIZED VIEW ptif_test_matview;
 DROP TABLE ptif_li_parent, ptif_li_child;
+
+-- Test about selection of arbiter indexes for partitioned tables with
+-- non-valid index on the parent table
+CREATE TABLE pt (a int PRIMARY KEY) PARTITION BY RANGE (a);
+CREATE TABLE p1 PARTITION OF pt FOR VALUES FROM (1) to (2) PARTITION BY RANGE (a);
+CREATE TABLE p1_1 PARTITION OF p1 FOR VALUES FROM (1) TO (2);
+CREATE UNIQUE INDEX ON ONLY p1 (a);
+INSERT INTO p1 VALUES (1) ON CONFLICT (a) DO NOTHING;