query performance tuning


In the database we have a table where all the file names are stored.This table is queried every 1 minute by the application to see a file name already exists or not, So the query is designed as,

    select filename from the filenametable where filename like ('%abctext%')
The filename is around 190 charecters in length and there are about 100k records in the table and it is effecting performance, I have put an index on the filename, but still it does not show a significant change on the perfromance. All 100k records needs to stay in the table, for a month or so. but even for first 100k itself, i see it cracking up. any way we can fine tune this query?
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asked Aug 31, 2011 at 01:33 PM in Default

Katie 1 gravatar image

Katie 1
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Can you share the query plan? (if it doesnt have any sensitive info, ofcourse)
Aug 31, 2011 at 01:41 PM DaniSQL

SQL Server when dealing with character-based columns, starts the index search based on the letters starting from the left (if we're talking about English). Therefore, LIKE 'abctext%' would potentially use an index but that assumes the file name starts with abctext. As soon as you start with % (LIKE '%abctext' or LIKE '%abctext%'), you're telling SQL Server 0 or more characters before abctext, at which point it's going to have to look at the entry for that column for each row.

Think about trying to go through the phonebook knowing that "hit" was in the name, but you didn't know what characters were before or after. You could match this with White, Whit, Whitehead, Chittering, or Doughit. How exactly do you find all matches in the phone book without going through every entry? You don't. You have to consider every single one. And that's why SQL Server won't choose to use an index or be able to make good use for one even if it does select it.
Sep 01, 2011 at 12:42 PM K. Brian Kelley
How long is it taking? I have a 264,000 filename table and the same query takes less than 100ms
Sep 02, 2011 at 04:14 PM Scot Hauder
The problem is, the application pings every 1 min to that table and trys to check if there are any files existing. This is causing a overhead. As we have a requirement to process the files as soon as the file is available, so i cannot mess with the timing.
Sep 05, 2011 at 03:48 PM Katie 1
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7 answers: sort oldest

Are the file searches all wild card searches. I can just type in 'a' and you're going to find aardvark, apple, can, zap...? For 100,000 records? Talk to the business. Convince them that the only way to get for sure performance is to modify the requirement so that if you type in 'a' the search will only return aardvark & apple.

That, or, if this is really the way this must behave, you may want to look at one of the NoSQL databases out there that are specifically targeted to doing wild card text searches. Especially if you're only doing a name/value pair search as the example query implies.
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answered Sep 01, 2011 at 07:49 AM

Grant Fritchey gravatar image

Grant Fritchey ♦♦
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The index you created on filename can not be used since your pridicate is not [sargable][1]. Does your filename start with 'abc'? If yes your index will be used since the where clause will be sargable, just modify your query to

select filename from the filenametable where filename like 'abctext%'
[1]: http://en.wikipedia.org/wiki/Sargable
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answered Aug 31, 2011 at 01:39 PM

DaniSQL gravatar image

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So actually there are two queries select filename from the filenametable where filename like '%abctext1%' and select filename from the filenametable where filename like '%abctext2%' , all other charecters in the filename change every minute.
Aug 31, 2011 at 01:54 PM Katie 1
You mean SARGable. :-)
Sep 01, 2011 at 12:37 PM K. Brian Kelley
@K. Brian Edited! good catch :-)
Sep 01, 2011 at 01:08 PM DaniSQL
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There's little to nothing you can do without changing the query. If you have a way of determining within SQL server what portion of the filename is variable and what portion is not, for example if the abctext portion always is 7 characters beginning after a period, then you could make a computed column that has abctext in it and apply an index to that: http://msdn.microsoft.com/en-us/library/ms189292.aspx

Otherwise, you could split a filename that looks like [variablePrefix][staticName][variableNumber][variableSuffix] into several columns in your table, apply an index to the static portion and the variable number, and adjust the application to split the filename accordingly when it is adding or querying.
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answered Aug 31, 2011 at 03:13 PM

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I'd look into using a [full-text index][1] for this query. If you have the server power to keep that index up-to-date, performance would be much faster than the %something% search for the reason @DaniSQL brought up. Rebuilding the index can be pretty slow, but it might be worth giving this a try.

[1]: http://msdn.microsoft.com/en-us/library/ms142571.aspx
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answered Sep 01, 2011 at 10:49 AM

Kevin Feasel gravatar image

Kevin Feasel
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Is there a pattern to the filenames that could be used to create a computed column from? You say the files all have the same pattern inside them, the rest is different every minute. If that is so, you could create the computed column to fish out the relevant filename part, then index that column. You then change your query to access the computed column and it should fly.

@Kevin feasel's suggestion of full text indexing would be my next step if my suggestion didn't work out.
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answered Sep 01, 2011 at 12:41 PM

WilliamD gravatar image

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just realised that ist exactly what @weitzera wrote almost a whole day ago! Stupid me, must read all answers before posting
Sep 01, 2011 at 12:43 PM WilliamD
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asked: Aug 31, 2011 at 01:33 PM

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Last Updated: Aug 31, 2011 at 01:33 PM