Fix CLUSTER tuplesorts on abbreviated expressions.

CLUSTER sort won't use the datum1 SortTuple field when clustering
against an index whose leading key is an expression.  This makes it
unsafe to use the abbreviated keys optimization, which was missed by the
logic that sets up SortSupport state.  Affected tuplesorts output tuples
in a completely bogus order as a result (the wrong SortSupport based
comparator was used for the leading attribute).

This issue is similar to the bug fixed on the master branch by recent
commit cc58eec.  But it's a far older issue, that dates back to the
introduction of the abbreviated keys optimization by commit 4ea51cd.

Backpatch to all supported versions.

Author: Peter Geoghegan <pg@bowt.ie>
Author: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA+hUKG+bA+bmwD36_oDxAoLrCwZjVtST2fqe=b4=qZcmU7u89A@mail.gmail.com
Backpatch: 10-

Author: petergeoghegan

src/backend/utils/sort/tuplesort.c

@@ -956,6 +956,7 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 {
 	Tuplesortstate *state = tuplesort_begin_common(workMem, coordinate,
 												   randomAccess);
+	AttrNumber	leading;
 	BTScanInsert indexScanKey;
 	MemoryContext oldcontext;
 	int			i;
@@ -988,6 +989,7 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 	state->abbrevNext = 10;
 
 	state->indexInfo = BuildIndexInfo(indexRel);
+	leading = state->indexInfo->ii_IndexAttrNumbers[0];
 
 	state->tupDesc = tupDesc;	/* assume we need not copy tupDesc */
 
@@ -1026,7 +1028,7 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 			(scanKey->sk_flags & SK_BT_NULLS_FIRST) != 0;
 		sortKey->ssup_attno = scanKey->sk_attno;
 		/* Convey if abbreviation optimization is applicable in principle */
-		sortKey->abbreviate = (i == 0);
+		sortKey->abbreviate = (i == 0 && leading != 0);
 
 		AssertState(sortKey->ssup_attno != 0);
 

src/test/regress/expected/cluster.out

@@ -511,6 +511,13 @@ SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
 COMMIT;
 -- and after clustering on clstr_expression_minus_a
 CLUSTER clstr_expression USING clstr_expression_minus_a;
+WITH rows AS
+  (SELECT ctid, lag(a) OVER (ORDER BY ctid) AS la, a FROM clstr_expression)
+SELECT * FROM rows WHERE la < a;
+ ctid | la | a 
+------+----+---
+(0 rows)
+
 BEGIN;
 SET LOCAL enable_seqscan = false;
 EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
@@ -545,6 +552,13 @@ SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
 COMMIT;
 -- and after clustering on clstr_expression_upper_b
 CLUSTER clstr_expression USING clstr_expression_upper_b;
+WITH rows AS
+  (SELECT ctid, lag(b) OVER (ORDER BY ctid) AS lb, b FROM clstr_expression)
+SELECT * FROM rows WHERE upper(lb) > upper(b);
+ ctid | lb | b 
+------+----+---
+(0 rows)
+
 BEGIN;
 SET LOCAL enable_seqscan = false;
 EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';

src/test/regress/sql/cluster.sql

@@ -245,6 +245,9 @@ COMMIT;
 
 -- and after clustering on clstr_expression_minus_a
 CLUSTER clstr_expression USING clstr_expression_minus_a;
+WITH rows AS
+  (SELECT ctid, lag(a) OVER (ORDER BY ctid) AS la, a FROM clstr_expression)
+SELECT * FROM rows WHERE la < a;
 BEGIN;
 SET LOCAL enable_seqscan = false;
 EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
@@ -255,6 +258,9 @@ COMMIT;
 
 -- and after clustering on clstr_expression_upper_b
 CLUSTER clstr_expression USING clstr_expression_upper_b;
+WITH rows AS
+  (SELECT ctid, lag(b) OVER (ORDER BY ctid) AS lb, b FROM clstr_expression)
+SELECT * FROM rows WHERE upper(lb) > upper(b);
 BEGIN;
 SET LOCAL enable_seqscan = false;
 EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';