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Very slow sql query using while loop and like select

Posted on 2008-06-17
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Last Modified: 2012-05-05
I have the attached SQL query which taking hours to run. It badly needs optimising but I'm no SQL expert!
Any help greatly appreciated!

I think it's the like bit which is slowing the query down as the query strings table is massive.

Thanks,
Tristan
DECLARE @StartTime		DATETIME
DECLARE @EndTime		DATETIME
DECLARE @CatStart		INT
DECLARE @CatEnd			INT
DECLARE @TheDay			DATETIME
DECLARE @TheDayPlusOne	DATETIME
DECLARE @TheCat			INT
DECLARE @Count			INT
DECLARE @Search			VARCHAR(10)
 
SET @StartTime =		'April 17 2008'
SET @EndTime =			'June 17 2008'
SET @CatStart =			1
SET @CatEnd =			11
SET @TheDay =			@startTime
SET @TheDayPlusOne =	        DATEADD(DAY, 1, @TheDay)
SET @TheCat =			@CatStart
 
SET NOCOUNT ON
WHILE @TheDay < @endTime
BEGIN
	WHILE @TheCat < @CatEnd
	BEGIN
		SET @Search =	'%cat=' + CAST(@TheCat AS VARCHAR) + '%'
		SET @Count =	( 
		
		SELECT COUNT (DISTINCT WS.[UNIQUE_IDENTIFIER_ID]) 
		FROM [WAP_SESSIONS] AS WS
		INNER JOIN [PAGE_HITS] AS PH ON WS.[WAP_SESSION_ID] = PH.[WAP_SESSION_ID]
		INNER JOIN [QUERY_STRINGS] AS QS ON PH.[QUERY_STRING_ID] = QS.[QUERY_STRING_ID]
		WHERE QS.[QUERY_STRING] LIKE @Search
		AND WS.[CLIENT_ID] = 50510
		AND WS.[DATE_CREATED] BETWEEN @TheDay AND @TheDayPlusOne
 
		 )
 
		PRINT CAST(@TheCat AS VARCHAR) + ', ' + CAST(@TheDay AS VARCHAR) + ', ' + CAST (@Count AS VARCHAR)
		SET @TheCat =	@TheCat + 1		
	END
	SET @TheCat =		@CatStart
    SET @TheDay =		DATEADD(DAY, 1, @TheDay)
	SET @TheDayPlusOne =DATEADD(DAY, 1, @TheDay)
END
 
SET NOCOUNT OFF

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Question by:Tristan Shortland
2 Comments
 
LVL 92

Accepted Solution

by:
Patrick Matthews earned 500 total points
ID: 21803710
Hello TShortland,

For one thing, make sure you have indices set up on the columns used in the joins and in the WHERE
clause.  For another, any time you use the LIKE operator, having an index on that column does you no
good, as the LIKE operator forces a full table scan.

It may be worthwhile to do a first step where you create a temp table with the query strings you want and
then join to that temp table, bypassing the LIKE operator for the main pass...

Regards,

Patrick
0
 
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Author Comment

by:Tristan Shortland
ID: 21804554
Hi,
I ended up doing a select where query_string like %cat=% into a temp table and then running the above query using that temp table.
This sped it up massively (from returning 10 rows in 18 minutes to 300 in about 5!).
Thanks for your help,
Tristan
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