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POOptimizing Levenshtein distance algorithm
 

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  1. POOptimizing Levenshtein distance algorithm
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    <p>I have a stored procedure that uses Levenshtein distance to determine the result closest to what the user typed. The only thing really affecting the speed is the function that calculates the Levenshtein distance for all the records before selecting the record with the lowest distance (I've verified this by putting a 0 in place of the call to the Levenshtein function). The table has 1.5 million records, so even the slightest adjustment may shave off a few seconds. Right now the entire thing runs over 10 minutes. Here's the method I'm using:</p> <pre><code>ALTER function dbo.Levenshtein ( @Source nvarchar(200), @Target nvarchar(200) ) RETURNS int AS BEGIN DECLARE @Source_len int, @Target_len int, @i int, @j int, @Source_char nchar, @Dist int, @Dist_temp int, @Distv0 varbinary(8000), @Distv1 varbinary(8000) SELECT @Source_len = LEN(@Source), @Target_len = LEN(@Target), @Distv1 = 0x0000, @j = 1, @i = 1, @Dist = 0 WHILE @j &lt;= @Target_len BEGIN SELECT @Distv1 = @Distv1 + CAST(@j AS binary(2)), @j = @j + 1 END WHILE @i &lt;= @Source_len BEGIN SELECT @Source_char = SUBSTRING(@Source, @i, 1), @Dist = @i, @Distv0 = CAST(@i AS binary(2)), @j = 1 WHILE @j &lt;= @Target_len BEGIN SET @Dist = @Dist + 1 SET @Dist_temp = CAST(SUBSTRING(@Distv1, @j+@j-1, 2) AS int) + CASE WHEN @Source_char = SUBSTRING(@Target, @j, 1) THEN 0 ELSE 1 END IF @Dist &gt; @Dist_temp BEGIN SET @Dist = @Dist_temp END SET @Dist_temp = CAST(SUBSTRING(@Distv1, @j+@j+1, 2) AS int)+1 IF @Dist &gt; @Dist_temp SET @Dist = @Dist_temp BEGIN SELECT @Distv0 = @Distv0 + CAST(@Dist AS binary(2)), @j = @j + 1 END END SELECT @Distv1 = @Distv0, @i = @i + 1 END RETURN @Dist END </code></pre> <p>Where should I go from here?</p>
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    1. USMatt
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    1. COHave you profiled this yet and looked at your indices?
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    2. COstore the calculated value in each row, and update if target column chnages....
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      1. USMitch Wheat
    3. CONo I haven't profiled it... I'll have to look up how to do that, this is the first time I've tried optimizing a stored proc before. I cannot store the calculated value, this is being used for a search and inputs on the search will rarely be repeated.
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