11

Assume we have a table definition like this:

CREATE TABLE MyTab (
    ID INT IDENTITY(1,1) CONSTRAINT PK_MyTab_ID PRIMARY KEY
    ,GroupByColumn NVARCHAR(10) NOT NULL
    ,WhereColumn DATETIME NULL
    )

And a filtered non-clustered index like this:

CREATE NONCLUSTERED INDEX IX_MyTab_GroupByColumn ON MyTab 
    (GroupByColumn)
WHERE (WhereColumn IS NULL) 

Why this index is not "covering" for this query:

SELECT 
    GroupByColumn
    ,COUNT(*)
FROM MyTab
WHERE WhereColumn IS NULL
GROUP BY GroupByColumn

I'm getting this execution plan:

enter image description here

The KeyLookup is for the WhereColumn IS NULL predicate.

Here is the plan: https://www.brentozar.com/pastetheplan/?id=SJcbLHxO7

16

Why this index is not "covering" for this query:

No good reason. That is a covering index for that query.

Please vote for the feeback item here: https://feedback.azure.com/forums/908035-sql-server/suggestions/32896348-filtered-index-not-used-when-is-null-and-key-looku

And as a workaround include the WhereColumn in the filtered index:

CREATE NONCLUSTERED INDEX IX_MyTab_GroupByColumn 
ON MyTab (GroupByColumn) include (WhereColumn)
WHERE (WhereColumn IS NULL) 
1

I had the same issue I think when doing some testing weeks ago. I have a query with a primary predicate that requires that results returned have a NULL closedatetime and I thought about using a filtered index as 25K of 2M+ records are NULL and this figure will decrease very soon.

The filtered index didn't get used - I assumed due to 'non-uniqueness' or commonality - until I found a Microsoft support article that says:

To resolve this issue, include the column that is tested as NULL in the returned columns. Or, add this column as include columns in the index.

So adding the column to the Index (or Include) seems to be the official MS response.

  • 1
    It's a viable workaround, until when (and if) they fix it. – ypercubeᵀᴹ Jan 13 at 12:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.