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[Bug] Duplicate results for inheritance and FOR UPDATE.

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: pgsql-hackers(at)postgresql(dot)org
Subject: [Bug] Duplicate results for inheritance and FOR UPDATE.
Date: 2014-12-11 10:27:21
Message-ID: 20141211.192721.173418729.horiguchi.kyotaro@lab.ntt.co.jp (view raw or whole thread)
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Hello, we found (and maybe fixed) two wrong behavior related to
inheritance and FOR UPDATE. This report is about one of them.

This behavior could be observed on master and back to 8.4 but 8.4
behaves a bit more funny. I haven't checked on 8.3.

This issue is that some query returns duplicate rows after FOR
UPDATE was blocked, in other words, after getting
HeapTupleUpdated in ExecLockRows.

- Reproducing the symptom

This behavior is easy to reproduce as following.

A=# DROP TABLE IF EXISTS p CASCADE;
A=# CREATE TABLE p (a int, b int, c int);
A=# CREATE TABLE c1 (LIKE p INCLUDING INDEXES) INHERITS (p);
A=# CREATE TABLE c2 (LIKE p INCLUDING INDEXES) INHERITS (p);
A=# CREATE TABLE c3 (LIKE p INCLUDING INDEXES) INHERITS (p);
A=# INSERT INTO c1 (SELECT 0, a / 3, a % 3 FROM generate_series(0, 9) a);
A=# INSERT INTO c2 (SELECT 1, a / 3, a % 3 FROM generate_series(0, 9) a);
A=# INSERT INTO c3 (SELECT 2, a / 3, a % 3 FROM generate_series(0, 9) a);
A=# ANALYZE;
A=# BEGIN;
A=# SELECT tableoid, ctid, * FROM p WHERE b IN (0, 1) AND c = 0 FOR UPDATE;
A=# UPDATE p SET b = -1 WHERE a = 1 AND b = 1 AND c = 0;
B=# SELECT tableoid, ctid, * FROM p WHERE b IN (0, 1) AND c = 0 FOR UPDATE;
-- the above is blocked by session A until it commits.
A=# COMMIT;

Where A and B are individual sessions.

You will get the follwing result on sesison B just after the
COMMIT on session A.

|  tableoid | ctid  | a | b | c 
| ----------+-------+---+---+---
*     33834 | (0,1) | 0 | 0 | 0
|     33834 | (0,4) | 0 | 1 | 0
|     33838 | (0,1) | 1 | 0 | 0
*     33834 | (0,1) | 0 | 0 | 0
|     33842 | (0,1) | 2 | 0 | 0
|     33842 | (0,4) | 2 | 1 | 0
| (6 rows)

The lines prefixed with '*' appear twice in the result.

The plan for the query is as following,

 LockRows
  ->  Result
   ->  Append
    ->  Seq Scan on p
         Filter: ((b = ANY ('{0,1}'::integer[])) AND (c = 0))
    ->  Seq Scan on c1
         Filter: ((b = ANY ('{0,1}'::integer[])) AND (c = 0))
    ->  Seq Scan on c2
         Filter: ((b = ANY ('{0,1}'::integer[])) AND (c = 0))
    ->  Seq Scan on c3
         Filter: ((b = ANY ('{0,1}'::integer[])) AND (c = 0))



- Analysys and solution

This seems to be caused in ExecScanFetch under EvalPlanQualNext,
when es_epqTuleSet[scanrelid - 1] is false (p and c1 in the above
example).  In that case, execution confluents onto non-EPQ
route. As the result, the accessMtd (= SeqScan, IndexScan will
behaves as the same) retunes null slot at the first iteration on
p. Then the second iteration on c1, it reuturns the c1's first
tuple (33834:(0,1)) and EvalPlanQualNext is satisfied with the
wrong tuple.

In the EPQ block in ExecScanFetch, it seems should return NULL if
the relation does not has test tuple. The attached patch does so
on the current master and the problem has disappeared.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Attachment: 0001-Fix-duplicate-tuples-for-inheritance-and-FOR-UPDATE.patch
Description: text/x-patch (686 bytes)

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Next:From: Kyotaro HORIGUCHIDate: 2014-12-11 10:29:38
Subject: [Bug] Inconsistent result for inheritance and FOR UPDATE.
Previous:From: Heikki LinnakangasDate: 2014-12-11 08:56:09
Subject: Re: Too strict check when starting from a basebackup taken off a standby

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