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POSimple Database Query - Is there a faster way without a cursor?
 

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    <p>I have 2 tables that I'm trying to query. The first has a list of meters. The second, has the data for those meters. I want to get the newest reading for each meter.</p> <p>Originally, this was in a group by statement, but it ended up processing all 7 million rows in our database, and took a little over a second. A subquery and a number of other ways of writing it had the same result.</p> <p>I have a clustered index that covers the EndTime and the MeterDataConfigurationId columns in the MeterRecordings table.</p> <p>Ultimately, this is what I wrote, which performs in about 20 milliseconds. It seems like SQL should be smart enough to perform the "group by" query in the same time.</p> <pre><code>Declare @Meters Table ( MeterId Integer, LastValue float, LastTimestamp DateTime ) Declare MeterCursor Cursor For Select Id From MeterDataConfiguration Declare @MeterId Int Open MeterCursor Fetch Next From MeterCursor Into @MeterId While @@FETCH_STATUS = 0 Begin Declare @LastValue int Declare @LastTimestamp DateTime Select @LastValue = mr.DataValue, @LastTimestamp = mr.EndTime From MeterRecording mr Where mr.MeterDataConfigurationId = @MeterId And mr.EndTime = (Select MAX(EndTime) from MeterRecording mr2 Where mr2.MeterDataConfigurationId = @MeterId) Insert Into @Meters Select @MeterId, @LastValue, @LastTimestamp Fetch Next From MeterCursor Into @MeterId End Deallocate MeterCursor Select * From @Meters </code></pre> <p>Here is an example of the same query that performs horribly:</p> <pre><code>select mdc.id, mr.EndTime from MeterDataConfiguration mdc inner join MeterRecording mr on mr.MeterDataConfigurationId = mdc.Id and mr.EndTime = (select MAX(EndTime) from MeterRecording mr2 where MeterDataConfigurationId = mdc.Id) </code></pre>
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    1. USJason Young
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    1. COI think you would get better performance if you just clustered on mr.MeterDataConfigurationId and put a nonclustered covering index on that field and mr.endtime.
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      1. USJNK
 

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