Question

Well, I have a table which is 40,000,000+ records but when I try to execute a simple query, it takes ~3 min to finish execution. Since I am using the same query in my c# solution, which it needs to execute over 100+ times, the overall performance of the solution is deeply hit.

This is the query that I am using in a proc

 DECLARE @Id bigint    
 SELECT @Id = MAX(ExecutionID) from ExecutionLog where TestID=50881    
 select @Id  

Any help to improve the performance would be great. Thanks.

Answer 1

What indexes do you have on the table? It sounds like you don’t have anything even close to useful for this particular query, so I’d suggest trying to do:

CREATE INDEX IX_ExecutionLog_TestID ON ExecutionLog (TestID, ExecutionID)

…at the very least. Your query is filtering by TestID, so this needs to be the primary column in the composite index: if you have no indexes on TestID, then SQL Server will resort to scanning the entire table in order to find rows where TestID = 50881.

It may help to think of indexes on SQL tables in the same way as those you’d find in the back of a big book that are hierarchial and multi-level. If you were looking for something, then you’d manually look under ‘T’ for TestID then there’d be a sub-heading under TestID for ExecutionID. Without an index entry for TestID, you’d have to read through the entire book looking for TestID, then see if there’s a mention of ExecutionID with it. This is effectively what SQL Server has to do.

If you don’t have any indexes, then you’ll find it useful to review all the queries that hit the table, and ensure that one of those indexes is a clustered index (rather than non-clustered).