Avoid trying to fetch metapage of an SPGist partitioned index.

This is necessary when spgcanreturn() is invoked on a partitioned
index, and the failure might be reachable in other scenarios as
well.  The rest of what spgGetCache() does is perfectly sensible
for a partitioned index, so we should allow it to go through.

I think the main takeaway from this is that we lack sufficient test
coverage for non-btree partitioned indexes.  Therefore, I added
simple test cases for brin and gin as well as spgist (hash and
gist AMs were covered already in indexing.sql).

Per bug #18256 from Alexander Lakhin.  Although the known test case
only fails since v16 (3c569049b), I've got no faith at all that there
aren't other ways to reach this problem; so back-patch to all
supported branches.

Discussion: https://postgr.es/m/18256-0b0e1b6e4a620f1b@postgresql.org

src/backend/access/spgist/spgutils.c

@@ -185,8 +185,6 @@ spgGetCache(Relation index)
 		Oid			atttype;
 		spgConfigIn in;
 		FmgrInfo   *procinfo;
-		Buffer		metabuffer;
-		SpGistMetaPageData *metadata;
 
 		cache = MemoryContextAllocZero(index->rd_indexcxt,
 									   sizeof(SpGistCache));
@@ -254,19 +252,28 @@ spgGetCache(Relation index)
 		fillTypeDesc(&cache->attPrefixType, cache->config.prefixType);
 		fillTypeDesc(&cache->attLabelType, cache->config.labelType);
 
-		/* Last, get the lastUsedPages data from the metapage */
-		metabuffer = ReadBuffer(index, SPGIST_METAPAGE_BLKNO);
-		LockBuffer(metabuffer, BUFFER_LOCK_SHARE);
+		/*
+		 * Finally, if it's a real index (not a partitioned one), get the
+		 * lastUsedPages data from the metapage
+		 */
+		if (index->rd_rel->relkind != RELKIND_PARTITIONED_INDEX)
+		{
+			Buffer		metabuffer;
+			SpGistMetaPageData *metadata;
 
-		metadata = SpGistPageGetMeta(BufferGetPage(metabuffer));
+			metabuffer = ReadBuffer(index, SPGIST_METAPAGE_BLKNO);
+			LockBuffer(metabuffer, BUFFER_LOCK_SHARE);
 
-		if (metadata->magicNumber != SPGIST_MAGIC_NUMBER)
-			elog(ERROR, "index \"%s\" is not an SP-GiST index",
-				 RelationGetRelationName(index));
+			metadata = SpGistPageGetMeta(BufferGetPage(metabuffer));
 
-		cache->lastUsedPages = metadata->lastUsedPages;
+			if (metadata->magicNumber != SPGIST_MAGIC_NUMBER)
+				elog(ERROR, "index \"%s\" is not an SP-GiST index",
+					 RelationGetRelationName(index));
 
-		UnlockReleaseBuffer(metabuffer);
+			cache->lastUsedPages = metadata->lastUsedPages;
+
+			UnlockReleaseBuffer(metabuffer);
+		}
 
 		index->rd_amcache = (void *) cache;
 	}

src/test/regress/expected/indexing.out

@@ -1280,6 +1280,45 @@ select tableoid::regclass, * from idxpart order by a;
  idxpart2 | 857142 | six
 (8 rows)
 
+drop table idxpart;
+-- Test some other non-btree index types
+create table idxpart (a int, b text, c int[]) partition by range (a);
+create table idxpart1 partition of idxpart for values from (0) to (100000);
+set enable_seqscan to off;
+create index idxpart_brin on idxpart using brin(b);
+explain (costs off) select * from idxpart where b = 'abcd';
+                QUERY PLAN                 
+-------------------------------------------
+ Bitmap Heap Scan on idxpart1 idxpart
+   Recheck Cond: (b = 'abcd'::text)
+   ->  Bitmap Index Scan on idxpart1_b_idx
+         Index Cond: (b = 'abcd'::text)
+(4 rows)
+
+drop index idxpart_brin;
+create index idxpart_spgist on idxpart using spgist(b);
+explain (costs off) select * from idxpart where b = 'abcd';
+                QUERY PLAN                 
+-------------------------------------------
+ Bitmap Heap Scan on idxpart1 idxpart
+   Recheck Cond: (b = 'abcd'::text)
+   ->  Bitmap Index Scan on idxpart1_b_idx
+         Index Cond: (b = 'abcd'::text)
+(4 rows)
+
+drop index idxpart_spgist;
+create index idxpart_gin on idxpart using gin(c);
+explain (costs off) select * from idxpart where c @> array[42];
+                  QUERY PLAN                  
+----------------------------------------------
+ Bitmap Heap Scan on idxpart1 idxpart
+   Recheck Cond: (c @> '{42}'::integer[])
+   ->  Bitmap Index Scan on idxpart1_c_idx
+         Index Cond: (c @> '{42}'::integer[])
+(4 rows)
+
+drop index idxpart_gin;
+reset enable_seqscan;
 drop table idxpart;
 -- intentionally leave some objects around
 create table idxpart (a int) partition by range (a);

src/test/regress/sql/indexing.sql

@@ -668,6 +668,26 @@ insert into idxpart values (857142, 'six');
 select tableoid::regclass, * from idxpart order by a;
 drop table idxpart;
 
+-- Test some other non-btree index types
+create table idxpart (a int, b text, c int[]) partition by range (a);
+create table idxpart1 partition of idxpart for values from (0) to (100000);
+set enable_seqscan to off;
+
+create index idxpart_brin on idxpart using brin(b);
+explain (costs off) select * from idxpart where b = 'abcd';
+drop index idxpart_brin;
+
+create index idxpart_spgist on idxpart using spgist(b);
+explain (costs off) select * from idxpart where b = 'abcd';
+drop index idxpart_spgist;
+
+create index idxpart_gin on idxpart using gin(c);
+explain (costs off) select * from idxpart where c @> array[42];
+drop index idxpart_gin;
+
+reset enable_seqscan;
+drop table idxpart;
+
 -- intentionally leave some objects around
 create table idxpart (a int) partition by range (a);
 create table idxpart1 partition of idxpart for values from (0) to (100);