Sql-server – parallelism repartitions, ordering, and hash matches

You should find explanations for parallelism in SQL Server in these two questions:

• A query submitted from different applications has differing DOP and

• What is the meaning of DOP in the context of sql server?

To enable the use of parallelism in first server you have two options:

• enable it at query level ( use option OPTION (MAXDOP 8) to enable the query to use all CPUs)

• enable it at server level (use the system stored procedure sp_configure or Management Studio - advanced server properties.

Questions regarding the issue:

• do you have statistics updated in both places?
• did you check the explain plans for the query in both places to see what's the difference?

Your ix_hugetable looks quite useless because:

• it is the clustered index (PK)
• the INCLUDE makes no difference because a clustered index INCLUDEs all non-key columns (non-key values at lowest leaf = INCLUDEd = what a clustered index is)

In addition: - added or fk should be first - ID is first = not much use

Try changing the clustered key to (added, fk, id) and drop ix_hugetable. You've already tried (fk, added, id). If nothing else, you'll save a lot of disk space and index maintenance

Another option might be to try the FORCE ORDER hint with table order boh ways and no JOIN/INDEX hints. I try not to use JOIN/INDEX hints personally because you remove options for the optimiser. Many years ago I was told (seminar with a SQL Guru) that FORCE ORDER hint can help when you have huge table JOIN small table: YMMV 7 years later...

Oh, and let us know where the DBA lives so we can arrange for some percussion adjustment

Edit, after 02 Jun update

The 4th column is not part of the non-clustered index so it uses the clustered index.

Try changing the NC index to INCLUDE the value column so it doesn't have to access the value column for the clustered index

create nonclustered index ix_hugetable on dbo.hugetable (
    fk asc, added asc
) include(value)

Note: If value is not nullable then it is the same as COUNT(*) semantically. But for SUM it need the actual value, not existence.

As an example, if you change COUNT(value) to COUNT(DISTINCT value) without changing the index it should break the query again because it has to process value as a value, not as existence.

The query needs 3 columns: added, fk, value. The first 2 are filtered/joined so are key columns. value is just used so can be included. Classic use of a covering index.