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I am using SQL Server 2012. We have a table that specifies the incoming requests to be processed. Some of the columns are (total columns are 10),

requestId
requestStatus (Enum - Processed, NotProcessed, Failed, etc)
requestedTime
requestCompletionTime

The query to pick the new requests is

select top 10000 
from RequestProcessingTable 
where requestStatus = 'NotProcessed' 
order by requestId

The total number of rows in this table is 300 million.

We have 2 indexes, one clustered (requestId - primary key of this table) and one non-clustered (requestStatus).

Since we want to process the requests based on the requestId we need to do the order by. When we introduce the order by it is taking 3 times more time to select. Is there a best way to create indexes or do it in a different approach so that the performance of the SQL query can be improved.

Thanks a lot for your help in advance.

  • Can you share the execution plan of the query? – ypercubeᵀᴹ Dec 23 '15 at 7:35
  • Also include what you're selecting from the table. Is it just requestId or something else too – James Z Dec 23 '15 at 10:16
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Create a filtered index on requestId with the predicate where requestStatus = 'NotProcessed'. The optimiser is likely to favour this as long as the query conditions exactly match the index definition.

It may help to use a small tinyint for the status instead of a longer character string. There will be more rows per data page and less IO.

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Share the execution plan, like @YperSillyCubeᵀᴹ said.

I think you can improve the performance of this query by creating a nonclustered index on requestStatus, requestId.

However: the fact that you are not specifying the column list (I'm guessing SELECT *) may change the plan by introducing Key Lookup or Clustered Index Scan (probably CIS on such a big table). If Clustered Index Scan is inserted to compensate for the lookup, the nonclustered index will not be helpful.

I can answer the question more precisely if you update the question with the plan.

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