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Wednesday, 6 February 2013

Incorrect Results with Indexed Views

Incorrect Results with Indexed Views

If you use MERGE, indexed views and foreign keys, your queries might return incorrect results. Microsoft have released a fix for incorrect results returned when querying an indexed view. The problem applies to:

  • SQL Server 2012
  • SQL Server 2008 R2
  • SQL Server 2008

The Knowledge Base article does not go into detail, or provide a reproduction script, but this blog post does.

The KB says that reproducing the bug requires these features:

  • An indexed view on two tables that have a foreign key relationship
  • An update performed against the base tables
  • A query executed against the indexed view using a NOEXPAND hint

There are two important details I would like to add:

  • The NOEXPAND hint is not required to reproduce the bug on Enterprise Edition or equivalent
  • The update must be performed by a MERGE statement

The fix is available starting from the following cumulative updates:

  • SQL Server 2012 SP1 CU2 build 11.0.3339
  • SQL Server 2012 RTM CU5 build 11.0.2395
  • SQL Server 2008 R2 SP2 CU4 build 10.50.4270
  • SQL Server 2008 R2 SP1 CU10 build 10.50.2868
  • SQL Server 2008 SP3 CU8 build 10.00.5828

Steps to Reproduce

We will need two tables:

CREATE TABLE dbo.Parent 
(
    parent_id integer IDENTITY NOT NULL,
    [value] varchar(20) NOT NULL,

    CONSTRAINT PK_Parent_id 
        PRIMARY KEY CLUSTERED (parent_id)
);

CREATE TABLE dbo.Child
(
    child_id integer IDENTITY NOT NULL,
    parent_id integer NOT NULL,

    CONSTRAINT PK_Child_id 
        PRIMARY KEY CLUSTERED (child_id)
);

…and a few rows of data:

INSERT dbo.Child
    (parent_id)
SELECT 
    New.parent_id 
FROM 
    (
    INSERT dbo.Parent
    OUTPUT inserted.parent_id 
    VALUES 
        ('Apple'), 
        ('Banana'), 
        ('Cherry')
    ) AS New;

The tables now look like this (parent first):

Tables and data

We can now add the required FOREIGN KEY relationship:

ALTER TABLE dbo.Child
ADD CONSTRAINT FK_Child_Parent
FOREIGN KEY (parent_id)
REFERENCES dbo.Parent (parent_id);

Next, we add an indexed view that joins the two tables. The view could contain more complex features like aggregates, but it is not necessary:

CREATE VIEW dbo.ParentsAndChildren
WITH SCHEMABINDING 
AS
SELECT 
    P.parent_id, 
    P.[value], 
    C.child_id
FROM dbo.Parent AS P
JOIN dbo.Child AS C
    ON C.parent_id = P.parent_id;
GO
CREATE UNIQUE CLUSTERED INDEX cuq 
ON dbo.ParentsAndChildren (child_id);

The final step is to use a MERGE statement to make some changes to the Parent table:

DECLARE @ParentMerge AS TABLE
(
    parent_id integer PRIMARY KEY, 
    [value] varchar(20) NOT NULL
);

INSERT @ParentMerge
    (parent_id, [value])
VALUES
    (1, 'Kiwi Fruit'),
    (4, 'Dragon Fruit');

MERGE dbo.Parent AS P
USING @ParentMerge AS S 
    ON S.parent_id = P.parent_id
WHEN MATCHED THEN 
    UPDATE 
    SET [value] = S.[value]
WHEN NOT MATCHED THEN 
    INSERT ([value])
    VALUES (S.[value])
OUTPUT 
    $action, 
    inserted.parent_id, 
    deleted.[value] AS old_value, 
    inserted.[value] AS new_value;

This MERGE performs two actions:

  1. Updates the value column of parent row 1 from ‘Apple’ to ‘Kiwi Fruit’
  2. Adds a new parent row 4 for ‘Dragon Fruit’

The statement includes an OUTPUT clause to show the changes it makes (this is not required for the repro):

Output

This confirms that the changes have been made as we requested: parent row 1 has changed; and row 4 has been added. The changes are reflected in the base tables:

SELECT P.* FROM dbo.Parent AS P;
SELECT C.* FROM dbo.Child AS C;

Updated values

As highlighted, row 1 has changed from Apple to Kiwi Fruit and row 4 has been added.

We do not expect to see row 4 in the indexed view because there are no child records for that row (the indexed view uses an inner join).

Checking the indexed view using the NOEXPAND table hint (required in non-Enterprise SKUs to use indexes on a view):

SELECT PAC.*
FROM dbo.ParentsAndChildren AS PAC
    WITH (NOEXPAND);

Indexed view result

These results are incorrect. They show the old value of the data for parent row 1 (apple instead of kiwi fruit).

Now we try using the EXPAND VIEWS query hint to force SQL Server to access base tables rather than view indexes:

SELECT PAC.*
FROM dbo.ParentsAndChildren AS PAC
OPTION (EXPAND VIEWS);

Results with view expansion

This query produces correct results.

On SQL Server Enterprise Edition and equivalents, the optimizer chooses whether to access the indexed view or the base tables. For following query, without any hints, the optimizer chooses not to expand the view. It reads the view index and produces incorrect results:

-- Enterprise Edition ONLY
SELECT PAC.* 
FROM dbo.ParentsAndChildren AS PAC;

Enterprise without NOEXPAND hint

Perhaps adding a child row to match the new parent row 4 will somehow fix things up?

INSERT dbo.Child 
    (parent_id) 
VALUES 
    (4);
GO
SELECT PAC.* 
FROM dbo.ParentsAndChildren AS PAC 
    WITH (NOEXPAND);

SELECT PAC.* 
FROM dbo.ParentsAndChildren AS PAC
OPTION (EXPAND VIEWS);

After adding child for parent 4

No. The query plan that accesses the view index still returns an incorrect value for row 1. It seems MERGE has corrupted our indexed view.

Analysis using DBCC CHECKTABLE

Checking the view with DBCC CHECKTABLE returns no errors:

DBCC CHECKTABLE

…unless we use the EXTENDED_LOGICAL_CHECKS option:

DBCC CHECKTABLE (N'dbo.ParentsAndChildren') 
    WITH EXTENDED_LOGICAL_CHECKS;

CHECKTABLE with extended checks

The damage is repairable:

ALTER DATABASE Sandpit 
    SET SINGLE_USER 
    WITH ROLLBACK IMMEDIATE;
GO
DBCC CHECKTABLE (N'dbo.ParentsAndChildren', REPAIR_REBUILD) 
    WITH EXTENDED_LOGICAL_CHECKS;
GO
DBCC CHECKTABLE (N'dbo.ParentsAndChildren')
    WITH EXTENDED_LOGICAL_CHECKS;
GO
ALTER DATABASE Sandpit 
    SET MULTI_USER;

Repairing the errors

Now you probably do not want to set your database to SINGLE_USER mode and run a DBCC repair after every MERGE statement. (We could also rebuild the clustered index on the indexed view to effect a repair).

Cause

For the MERGE statement above, the query optimizer builds a plan that does not update the indexed view:

MERGE plan

In a version of SQL Server with the fix applied, the same MERGE statement produces a plan that does maintain the indexed view:

MERGE plan with fix

The plan operators used to keep the view index in step with the base tables are highlighted. Without these operators, the changes to the base table are not correctly written to any indexes defined on the view.

The root cause of the problem is related to the same simplification that allows the optimizer to remove the reference to the Parent table in this query:

SELECT 
    COUNT_BIG(*)
FROM dbo.Parent AS P
JOIN dbo.Child AS C
    ON C.parent_id = P.parent_id;

Join simplification

The FOREIGN KEY relationship and NOT NULL constraints on the referencing column together mean that the join to Parent cannot affect the result of the query, so the join can be simplified away.

In SQL Server 2012 onward, we can see when this simplification is performed because the following message appears when undocumented trace flag 8619 (with 3604) is enabled during compilation:

Full Join removed for table TBL: dbo.Parent TableID=xxx

The same message is emitted when a MERGE statement contains a WHEN MATCHED THEN UPDATE clause and either a WHEN NOT MATCHED THEN INSERT or WHEN MATCHED THEN DELETE clause.

These conditions combine such that the optimizer incorrectly concludes that a table reference can be removed, when in fact it is needed later on when the update side of the plan is built.

Other details of the query and database can affect whether the simplification can be misapplied. For example, if the FOREIGN KEY constraint contains an ON DELETE CASCADE clause, and the MERGE contains a DELETE clause, the simplification is not performed, the TF 8619 message does not appear, and the bug does not manifest.

The key to determining whether a particular query is vulnerable to this bug (TF 8619 aside) is to check whether the query plan includes operators to maintain the indexed view.

At a minimum, you should see a update operator for the view:

View Clustered Index Update

Sentry One Plan Explorer identifies the operator as applying to a view explicitly. In SSMS you need to click on the graphical operator and inspect the Properties window to see the same information.

Summary

The updated conditions for incorrect results are:

  • An indexed view that joins tables
  • Two tables have a single-column FOREIGN KEY constraint
  • A MERGE statement contains an UPDATE action that affects one of the tables
  • The MERGE statement also contains an INSERT or DELETE action (or both)
  • The optimizer applies a simplification that removes a table reference based on the foreign key relationship
  • As a result, the MERGE execution plan does not contain the operators necessary to correctly maintain the indexed view
  • A subsequent query plan accesses an index on the view, either explicitly or via indexed-view matching (Enterprise Edition)

Note:

  • The simplification is not applied in tempdb
  • The simplification is not applied to multi-column foreign key constraints

Under these conditions, view indexes do not reflect the state of the base tables and incorrect results are returned. Once the fix is applied, the optimizer does not misapply the simplification, and the correct indexed view maintenance features are built into execution plans.

© Paul White
email: SQLkiwi@gmail.com
twitter: @SQL_Kiwi

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