A Silent Bug With Clustered Column Store Indexes And Computed Columns

Oh Boy


This bug has been reported to the proper authorities, and this blog post is not to dunk on the Summer Intern responsible for column store indexes, but to warn you to not do this, because I don’t know what it will break under the covers.

If you read the documentation for column store indexes, it says that column store indexes can’t be created on persisted computed columns.

And that’s true. If we step through this script, creating the column store index will fail.

/*Bye Felicia*/
DROP TABLE IF EXISTS dbo.cci;
GO 

/*Hi Felicia*/
CREATE TABLE dbo.cci(id int, fk int, sd date, ed datetime);
GO 

/*COMPUTER!*/
ALTER TABLE dbo.cci ADD cc AS id * 2;
ALTER TABLE dbo.cci ALTER COLUMN cc ADD PERSISTED;
GO 

/*Rats*/
CREATE CLUSTERED COLUMNSTORE INDEX c ON dbo.cci;
GO

The error message we get is pretty clear:

Msg 35307, Level 16, State 1, Line 76
The statement failed because column 'cc' on table 'cci' is a computed column. 
Columnstore index cannot include a computed column implicitly or explicitly.

80 Blocks from Tiffany’s


However, if we change the order of things a little bit, we’ll find that we can add a persisted computed column to a table with a clustered column store index on it.

/*Bye Felicia*/
DROP TABLE IF EXISTS dbo.cci;
GO 

/*Hi Felicia*/
CREATE TABLE dbo.cci(id int, fk int, sd date, ed datetime, INDEX c CLUSTERED COLUMNSTORE);
GO 

/*???*/
ALTER TABLE dbo.cci ADD cc AS id * 2;
ALTER TABLE dbo.cci ALTER COLUMN cc ADD PERSISTED;
GO 

/*With data?*/
INSERT dbo.cci
(
    id, fk, sd, ed
)
VALUES
(
    10, 10, GETDATE(), GETDATE()  
);

/*yepyepyepyepyepyep*/
SELECT
    c.*
FROM dbo.cci AS c

/*Eh?*/
ALTER INDEX c ON dbo.cci REBUILD;
ALTER INDEX c ON dbo.cci REORGANIZE;
DBCC CHECKTABLE('dbo.cci');

And everything seems to work. However, if we drop the column store index, it can’t be re-created.

The key here is that the computed column is added, and then the persisted attribute is added second. That is where an error should be thrown.

But what do I know? I’m just a bouncer.

Bomber


Again, I’m telling you not to do this. I’m telling you that it might break things in gruesome and horrible ways.

I don’t think that this is a preview of upcoming support for persisted computed columns in clustered column store indexes.

Thanks for reading!

p.s. Really, don’t do this

p.p.s. If you do this you’re asking for trouble

p.p.p.s. This was fixed in CU12



One thought on “A Silent Bug With Clustered Column Store Indexes And Computed Columns

  1. It would still be possible to do the reverse, first drop the persisted attributed, drop and recreate the index and then add the persisted attribute again.
    ALTER TABLE dbo.cci ALTER COLUMN cc DROP PERSISTED;
    DROP INDEX dbo.cci.c;
    CREATE CLUSTERED COLUMNSTORE INDEX c ON dbo.cci;
    ALTER TABLE dbo.cci ALTER COLUMN cc ADD PERSISTED;

    But yeah, probably not a good idea of using it in the first place.

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