Why filtered index on IS NULL value is not used?

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:

The KeyLookup is for the WhereColumn IS NULL predicate.

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

2 Answers
2

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:

WhereColumn

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


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.

Thanks for contributing an answer to Database Administrators Stack Exchange!

But avoid …

To learn more, see our tips on writing great answers.

Required, but never shown

Required, but never shown

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.