Be sure to rewind the tuplestore read pointer in non-leader CTEScan nodes.

tglsfdc · tglsfdc · commit 73df86a37ba5 · 2016-09-22T11:34:44.000-04:00
ExecInitCteScan supposed that it didn't have to do anything to the extra
tuplestore read pointer it gets from tuplestore_alloc_read_pointer.
However, it needs this read pointer to be positioned at the start of the
tuplestore, while tuplestore_alloc_read_pointer is actually defined as
cloning the current position of read pointer 0.  In normal situations
that accidentally works because we initialize the whole plan tree at once,
before anything gets read.  But it fails in an EvalPlanQual recheck, as
illustrated in bug #14328 from Dima Pavlov.  To fix, just forcibly rewind
the pointer after tuplestore_alloc_read_pointer.  The cost of doing so is
negligible unless the tuplestore is already in TSS_READFILE state, which
wouldn't happen in normal cases.  We could consider altering tuplestore's
API to make that case cheaper, but that would make for a more invasive
back-patch and it doesn't seem worth it.

This has been broken probably for as long as we've had CTEs, so back-patch
to all supported branches.

Discussion: &lt;32468.1474548308@sss.pgh.pa.us&gt;
diff --git a/src/backend/executor/nodeCtescan.c b/src/backend/executor/nodeCtescan.c
@@ -224,9 +224,13 @@ ExecInitCteScan(CteScan *node, EState *estate, int eflags)
 	{
 		/* Not the leader */
 		Assert(IsA(scanstate->leader, CteScanState));
+		/* Create my own read pointer, and ensure it is at start */
 		scanstate->readptr =
 			tuplestore_alloc_read_pointer(scanstate->leader->cte_table,
 										  scanstate->eflags);
+		tuplestore_select_read_pointer(scanstate->leader->cte_table,
+									   scanstate->readptr);
+		tuplestore_rescan(scanstate->leader->cte_table);
 	}
 
 	/*
diff --git a/src/test/isolation/expected/eval-plan-qual.out b/src/test/isolation/expected/eval-plan-qual.out
@@ -104,3 +104,24 @@ a              b              c
 2              2              2              
 2              3              0              
 step c2: COMMIT;
+
+starting permutation: wrtwcte readwcte c1 c2
+step wrtwcte: UPDATE table_a SET value = 'tableAValue2' WHERE id = 1;
+step readwcte: 
+	WITH
+	    cte1 AS (
+	      SELECT id FROM table_b WHERE value = 'tableBValue'
+	    ),
+	    cte2 AS (
+	      SELECT * FROM table_a
+	      WHERE id = (SELECT id FROM cte1)
+	      FOR UPDATE
+	    )
+	SELECT * FROM cte2;
+ <waiting ...>
+step c1: COMMIT;
+step c2: COMMIT;
+step readwcte: <... completed>
+id             value          
+
+1              tableAValue2   
diff --git a/src/test/isolation/specs/eval-plan-qual.spec b/src/test/isolation/specs/eval-plan-qual.spec
@@ -16,12 +16,18 @@ setup
  INSERT INTO c1 SELECT 0, a / 3, a % 3 FROM generate_series(0, 9) a;
  INSERT INTO c2 SELECT 1, a / 3, a % 3 FROM generate_series(0, 9) a;
  INSERT INTO c3 SELECT 2, a / 3, a % 3 FROM generate_series(0, 9) a;
+
+ CREATE TABLE table_a (id integer, value text);
+ CREATE TABLE table_b (id integer, value text);
+ INSERT INTO table_a VALUES (1, 'tableAValue');
+ INSERT INTO table_b VALUES (1, 'tableBValue');
 }
 
 teardown
 {
  DROP TABLE accounts;
  DROP TABLE p CASCADE;
+ DROP TABLE table_a, table_b;
 }
 
 session "s1"
@@ -67,15 +73,32 @@ step "returningp1" {
 	WITH u AS ( UPDATE p SET b = b WHERE a > 0 RETURNING * )
 	  SELECT * FROM u;
 }
+step "wrtwcte"	{ UPDATE table_a SET value = 'tableAValue2' WHERE id = 1; }
 step "c2"	{ COMMIT; }
 
 session "s3"
 setup		{ BEGIN ISOLATION LEVEL READ COMMITTED; }
 step "read"	{ SELECT * FROM accounts ORDER BY accountid; }
+
+# this test exercises EvalPlanQual with a CTE, cf bug #14328
+step "readwcte"	{
+	WITH
+	    cte1 AS (
+	      SELECT id FROM table_b WHERE value = 'tableBValue'
+	    ),
+	    cte2 AS (
+	      SELECT * FROM table_a
+	      WHERE id = (SELECT id FROM cte1)
+	      FOR UPDATE
+	    )
+	SELECT * FROM cte2;
+}
+
 teardown	{ COMMIT; }
 
 permutation "wx1" "wx2" "c1" "c2" "read"
 permutation "wy1" "wy2" "c1" "c2" "read"
 permutation "upsert1" "upsert2" "c1" "c2" "read"
 permutation "readp1" "writep1" "readp2" "c1" "c2"
 permutation "writep2" "returningp1" "c1" "c2"
+permutation "wrtwcte" "readwcte" "c1" "c2"

Original file line number	Diff line number	Diff line change
`@@ -224,9 +224,13 @@ ExecInitCteScan(CteScan node, EState estate, int eflags)`
`224`	`224`	`{`
`225`	`225`	`/* Not the leader */`
`226`	`226`	`Assert(IsA(scanstate->leader, CteScanState));`
	`227`	`+ /* Create my own read pointer, and ensure it is at start */`
`227`	`228`	`scanstate->readptr =`
`228`	`229`	`tuplestore_alloc_read_pointer(scanstate->leader->cte_table,`
`229`	`230`	`scanstate->eflags);`
	`231`	`+ tuplestore_select_read_pointer(scanstate->leader->cte_table,`
	`232`	`+ scanstate->readptr);`
	`233`	`+ tuplestore_rescan(scanstate->leader->cte_table);`
`230`	`234`	`}`
`231`	`235`
`232`	`236`	`/*`