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 cc58eecc5d.  But it's a far older issue, that dates back to the
introduction of the abbreviated keys optimization by commit 4ea51cdfe8.

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-

src/backend/utils/sort/tuplesort.c

@@ -884,6 +884,7 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 {
 	Tuplesortstate *state = tuplesort_begin_common(workMem, coordinate,
 												   randomAccess);
+	AttrNumber	leading;
 	ScanKey		indexScanKey;
 	MemoryContext oldcontext;
 	int			i;
@@ -916,6 +917,7 @@ 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 */
 
@@ -954,7 +956,7 @@ 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

@@ -466,10 +466,131 @@ where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
 
 reset enable_indexscan;
 reset maintenance_work_mem;
+-- test CLUSTER on expression index
+CREATE TABLE clstr_expression(id serial primary key, a int, b text COLLATE "C");
+INSERT INTO clstr_expression(a, b) SELECT g.i % 42, 'prefix'||g.i FROM generate_series(1, 133) g(i);
+CREATE INDEX clstr_expression_minus_a ON clstr_expression ((-a), b);
+CREATE INDEX clstr_expression_upper_b ON clstr_expression ((upper(b)));
+-- verify indexes work before cluster
+BEGIN;
+SET LOCAL enable_seqscan = false;
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_upper_b on clstr_expression
+   Index Cond: (upper(b) = 'PREFIX3'::text)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+ id | a |    b    
+----+---+---------
+  3 | 3 | prefix3
+(1 row)
+
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_minus_a on clstr_expression
+   Index Cond: ((- a) = '-3'::integer)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+ id  | a |     b     
+-----+---+-----------
+ 129 | 3 | prefix129
+   3 | 3 | prefix3
+  45 | 3 | prefix45
+  87 | 3 | prefix87
+(4 rows)
+
+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';
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_upper_b on clstr_expression
+   Index Cond: (upper(b) = 'PREFIX3'::text)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+ id | a |    b    
+----+---+---------
+  3 | 3 | prefix3
+(1 row)
+
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_minus_a on clstr_expression
+   Index Cond: ((- a) = '-3'::integer)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+ id  | a |     b     
+-----+---+-----------
+ 129 | 3 | prefix129
+   3 | 3 | prefix3
+  45 | 3 | prefix45
+  87 | 3 | prefix87
+(4 rows)
+
+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';
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_upper_b on clstr_expression
+   Index Cond: (upper(b) = 'PREFIX3'::text)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+ id | a |    b    
+----+---+---------
+  3 | 3 | prefix3
+(1 row)
+
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+                          QUERY PLAN                           
+---------------------------------------------------------------
+ Index Scan using clstr_expression_minus_a on clstr_expression
+   Index Cond: ((- a) = '-3'::integer)
+(2 rows)
+
+SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+ id  | a |     b     
+-----+---+-----------
+ 129 | 3 | prefix129
+   3 | 3 | prefix3
+  45 | 3 | prefix45
+  87 | 3 | prefix87
+(4 rows)
+
+COMMIT;
 -- clean up
 DROP TABLE clustertest;
 DROP TABLE clstr_1;
 DROP TABLE clstr_2;
 DROP TABLE clstr_3;
 DROP TABLE clstr_4;
+DROP TABLE clstr_expression;
 DROP USER regress_clstr_user;

src/test/regress/sql/cluster.sql

@@ -222,10 +222,53 @@ where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
 reset enable_indexscan;
 reset maintenance_work_mem;
 
+-- test CLUSTER on expression index
+CREATE TABLE clstr_expression(id serial primary key, a int, b text COLLATE "C");
+INSERT INTO clstr_expression(a, b) SELECT g.i % 42, 'prefix'||g.i FROM generate_series(1, 133) g(i);
+CREATE INDEX clstr_expression_minus_a ON clstr_expression ((-a), b);
+CREATE INDEX clstr_expression_upper_b ON clstr_expression ((upper(b)));
+
+-- verify indexes work before cluster
+BEGIN;
+SET LOCAL enable_seqscan = false;
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+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;
+BEGIN;
+SET LOCAL enable_seqscan = false;
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+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);
+BEGIN;
+SET LOCAL enable_seqscan = false;
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+SELECT * FROM clstr_expression WHERE upper(b) = 'PREFIX3';
+EXPLAIN (COSTS OFF) SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+SELECT * FROM clstr_expression WHERE -a = -3 ORDER BY -a, b;
+COMMIT;
+
 -- clean up
 DROP TABLE clustertest;
 DROP TABLE clstr_1;
 DROP TABLE clstr_2;
 DROP TABLE clstr_3;
 DROP TABLE clstr_4;
+DROP TABLE clstr_expression;
+
 DROP USER regress_clstr_user;