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POIterate over characters in string in mysql
 

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    <p>First at all I have a very concrete question, but maybe an alternative approach to my problem (second part) could also help me.</p> <p><strong>Is there a way to address a character in a string via its index in mysql. (i.e. in PHP $var[2] will give you the 3rd charater)?</strong></p> <p>The obvious way is <code>SUBSTRING(var, 3,1 )</code> but since my strings are 1024 character long I assume this is not the fastest solution. As displayed in the code sample using substring to retrieve the tail of the string also gain no performance difference. Is there maybe a way to iterate over a string? (Shift the first element?)</p> <pre><code>CREATE FUNCTION hashDiff( hash1 TEXT(1024), hash2 TEXT(1024), threshold INT) RETURNS INT DETERMINISTIC BEGIN DECLARE diff, x, b1, b2 INT; SET diff =0; SET x = 0; WHILE (x&lt;1024 AND diff&lt;threshold) DO SET b1 = ASCII(hash1); --uses first character only!! SET b2 = ASCII(hash2); SET hash1=SUBSTRING(hash1, 2 ); SET hash2=SUBSTRING(hash2, 2 ); SET diff=diff+ ((b1-b2)*(b1-b2)); SET x=x+1; END WHILE; RETURN diff; END </code></pre> <p>If you not already read it from the code, I try to write a stored procedure to calculate the difference or distance between to hashes. The difference is the sum of the character-wise square distances (i.e. <code>hashDiff(AA,AC)=(65-65)²+(65-67)²=4</code>). The first major performance boost could be achieved by introducing a threshold to cancel the calculation if the hashes are already to different. But since mysql is not my "every day" language, I stuck at this point in finding other optimizations. For completeness two sample hashes: </p> <blockquote> <p>YAAAAAAYAAAYAAVAAQAARAOAAOAQASAQAMAKAKAJIAJAJIAHAHIAKJAIIAHHAHIIAIHGAGFFAGGFEAFEEEEAEDDDDDAEEEEDEEEFAFFFFFFEFFFEFFFFFGFEEFFEEEFFFJEFFEEEEEEELFFFFEEFJEEEEDIEEEEEIEEEEHEEEJEEFKFEFKGGFNHGOIIJTJKYONYNMTGHNHHQISJJQIKWLXJJSMYRQWJOGKDDFCCBBAAAAAAAAAAAAAAAAAAAAAAAYAAAAAAYAAAYAAWAARAASASAAQARAUAYAYATAOALKAJAJIAIAHHAHGAGFAFFAEFFAEFFAFFFAEEFFAFEEEDADEDDDDADDDCDDDDDAEEEFEEEEDDDEEEDEDDEEEFEFFGGFMFHGFFFGFFFLGHGGHGGNHHGGGOHGHGHMGGFGMFFFMFGFLFFFMGFFMGGMGGGNGGMGGLGGLGGMGGLEIEEHDCGCGCDGDGDCGDFCECCECECECECFCECFCFCFCFCFCGCJGYCYAAAAAAYAAAYAAUAATAAUAUAAUARARAQAPAPASARRAPARQAPAQQAQQAQSAKMATKKAIIHAIHGAGGGGAGHHGGAGGFGFFAFFGEFFFFFAFFGFGGGFFFEEFGFFGGFGGHIJJLKLWLKJJIJJJKJRLJKLKKKUKLLKKUMMKJIQIIIISKJJWKLLXMLMYMLNYMMYMLLWJIQIINFGKFFKEEIDHEDHDDFCECCFDECCFCFDGCDGCGCGEGCDCECECFDFCGDGCIEKEOAYNFBREUXKPQMMQTKTMMNJLPPVYYYTOUOPOLLJKKJJJIJIMJJJLIJJLLJIIHHIHHHIGHIHIHJHHHJHHIHGHGHFGHGFFEFEEEFEFEFFGGHIHIHGHGHHIIIIHIIJMNLONKLKKKKKKKMLKKLONMKOOOMLOPONMNMKKLLKKLMNKLMMMNMOPPOORPORSSVRTSSRTRRTSSTTXSTQRPONOKKLKLJMKJJIJIIHHHIIIJHIJIJJIJIKJIMWMYYDAAAAAAAAAAA</p> <p>AAAAAAAAAAABAABAACAACACAACADADAEADADADADDAEAEEAEAFEAEEAEFAFGAGGGAGGGAHHHAHIIIAIHIJHAIIHIHHAJIHIJIJKJAJJJIKJJJJKKJKJKKLKLKLLMMMNNMYOOOOOOPOONYOONONNPYNOOOPYOOPPPYNONNYMLLWLLKUJIISHIHOGGMFGFLFFMGGLFGLGFLFFKFKFFLEEKFLEFJFKFGNGNHLFHJFIEGDIEKGOIRFGBBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABABACACDACADDACADDADDAEDAFEAEEFAFFFAFFGAFGGGAIGHHIAHIHHHHAHIHIIJJIIAIIIIJIJKJIIIIJJHIIHIIIIJIIIIRJJJJKJJJJLVKLLKLLKXLMMKMXMLLLMWMMMMYMNLYMNNYNNMYMMNYMLYLMLXKJRIHPHIMGGMFEJEJEEIEEHDGCDFCFDCFCECECCEBEBECFDGCFDNGLDBAAAAAAAAAAAAAAAAAAAAAABAAAAABAABABAACACACACACACACADDADAEEAFAFGAFGAHGAGGAGGHAGGIAIHJAJJJJAJKKKKAMLMNNNANOMMNNMMNAONMNOOOMOOPOMNOMMNPOOPPPPRQQYPPRPPPPPNOYLLMMMMLYLMLMLYLMLMMYLNNMYNLLWMLKXLLLUKIKQIIQGHHPFHNGFLFFLGFJEEJEIDDIDCHDFCDGCFCCFCECECCECFCGDGDHDHDIFIDEBBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAABBBBBCBCCCCDCCCCCCCCDDDDEDEEEEFFDEGGHGHHHGHHHHHHIIJJJJJIJJJJJJIKJJKLKKMMNMMMMMMMNNNNNNLNNONPONNNOOOOPQQQRSSSSSSUTSTUUUVWVVXUYXWVXVXWYVYWYVYYUWVUTTSSPQPQOPOPONONOMONOOONNNMMNLJJKJIIJHHGGGFHFGFFFFEEEDDEEEEFGGIGJLRNEAAAAAAAAAAAAA</p> </blockquote> <p>Any help or hint would be appreciated.</p>
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    1. COyou might be able to handle your strings four bytes at a time with the HEX function.
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