Question

The following simple query takes a very long time (several minutes) to execute.

I have an index:

create index IX on [fctWMAUA] (SourceSystemKey, AsAtDateKey)

SELECT MAX([t0].[AsAtDateKey]) AS [Date], [t0].[SourceSystemKey] AS [SourceSystem]
FROM [fctWMAUA] (NOLOCK) AS [t0]
WHERE SourceSystemKey in (1,2,3,4,5,6,7,8,9)
GROUP BY [t0].[SourceSystemKey]

The statistics are as follows:

• logical reads 1827978
• physical reads 1113
• read aheads 1806459

Taking that exact same query and reformatting it as follows gives me these statistics:

• logical reads 36
• physical reads 0
• read aheads 0

It takes 31ms to execute.

SELECT MAX([t0].[AsAtDateKey]) AS [Date], [t0].[SourceSystemKey] AS [SourceSystem]
 FROM [fctWMAUA] (NOLOCK) AS [t0]
 WHERE SourceSystemKey = 1
 GROUP BY [t0].[SourceSystemKey]
UNION
 SELECT MAX([t0].[AsAtDateKey]) AS [Date], [t0].[SourceSystemKey] AS [SourceSystem]
 FROM [fctWMAUA] (NOLOCK) AS [t0]
 WHERE SourceSystemKey = 2
 GROUP BY [t0].[SourceSystemKey]
UNION
 SELECT MAX([t0].[AsAtDateKey]) AS [Date], [t0].[SourceSystemKey] AS [SourceSystem]
 FROM [fctWMAUA] (NOLOCK) AS [t0]
 WHERE SourceSystemKey = 3
 GROUP BY [t0].[SourceSystemKey]
/* AND SO ON TO 9 */

How do I make an index that does the group by quickly?

Answer 1

Try to tell SQL Server to use the index:

...
FROM [fctWMAUA] (NOLOCK, INDEX(IX)) AS [t0]
...

Make sure the statistics for the table are up to date:

UPDATE STATISTICS [fctWMAUA]

For better answers, turn on the showplan for both queries:

SET SHOWPLAN_TEXT ON

and add the results to your question.

You can also write the query without a GROUP BY. For example, you can use an exclusive LEFT JOIN excluding rows with older dates:

select cur.SourceSystemKey, cur.date
from fctWMAUA cur
left join fctWMAUA next
    on next.SourceSystemKey = next.SourceSystemKey
    and next.date > cur.date
where next.SourceSystemKey is null
and cur.SourceSystemKey in (1,2,3,4,5,6,7,8,9)

This can be surprisingly fast, but I don’t think it could beat the UNION.