Performance when predicate is < Greater than ...
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My table consists of 4 location IDs, each with a sensor reading value for each minute (approx) of the day, over a period of 1 month. (178,000 rows +/-)
CREATE TABLE [CJ_LBSVTP].[IP21_Import_PowerTier](
[locationID] [char](3) NOT NULL,
[stamp] [datetime] NOT NULL,
[value] [float] NULL
)
... and my query has to create a start & end time from the above.
So, if the table rows are thus ...
Location Time Value
1234 2013-03-01 00:01 98
1234 2013-03-01 00:02 97
The results should be ...
Location StartTime EndTime
1234 2013-03-01 00:01 2013-03-01 00:02
My query code to do this is below, but it is taking a horrendously long time to execute.
Have been playing with indexes for some time now and simply cannot get a decent time out of this one.
Are there any 'tips' when doing such a query utilizing a '<' in the predicate ?
Many Thanks
--------- Query Code ---
SELECT
AA.LocationID
, AA.stamp AS startStamp
, MIN(BB.stamp) AS endStamp
FROM
myTable AS BB
INNER JOIN
myTable AS AA
ON
BB.LocationID = AA.LocationID AND
BB.stamp > AA.stamp
GROUP BY
AA.LocationID
, AA.stamp
-
What you have here is known by some as a triangular join. It is in fact horribly slow and inefficient. Take a look at this article that explains what you have going on here.
http://www.sqlservercentral.com/articles/T-SQL/61539/%5B/url%5D
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You might try using a derived table and getting TOP 1 with an ORDER BY to get the latest version instead of the > operator. Something like the code I have listed [/url]in this article. It might help.
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SQL Server Execution Plans
SQL Server Query Performance Tuning -
Any chance you're on SQL Server 2012?
SELECT LocationID,stamp AS startStamp,
FIRST_VALUE(stamp) OVER(PARTITION BY LocationID
ORDER BY stamp
ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING) AS endStamp
FROM myTable
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Deja View - The strange feeling that somewhere, sometime you've optimised this query before
How to get the best help on a forum
http://www.sqlservercentral.com/articles/Best+Practices/61537
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