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From:Reindl Harald Date:February 12 2014 2:02am
Subject:Re: LIKE sql optimization
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because a index is not just a dumb copy of the whole field
and you simply can't seek in the middle of it?

http://en.wikipedia.org/wiki/B-tree
http://mattfleming.com/node/192

Am 12.02.2014 02:48, schrieb Zhigang Zhang:
> I want to know the reason, in my opinion, to scan the smaller index data has
> better performance than to scan the whole table data. 
>
> From: Mathieu Desharnais [mailto:mdesharnais@stripped] 
> Sent: Wednesday, February 12, 2014 9:41 AM
> To: Zhigang Zhang; mysql@stripped
> Subject: Re: LIKE sql optimization
> 
> Sql database doesn't use index in like statement if it starts with % .. 
> 
> like 'abcd%' would work though...   
> 
> To use an index you can store your value using reverse function and index it
> .. then your like would use the index.
> 
> 2014-02-11 20:23 GMT-05:00 Zhigang Zhang <zzgang2008@stripped>:
> 
> For example:
> 
> Select * from T where col like ‘%abcd’;
> 
> The table T is myisam table and we created a index on col.
>
> As we known, this like sql does not use the index created on col, it confuse
> me, why?
> 
> I think in mysiam engine, the index data is smaller, it can use index link
> list to optimize it so as to reduce the disk scan than to the whole table
> scan.


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Thread
LIKE sql optimizationZhigang Zhang12 Feb 2014
  • Re: LIKE sql optimizationReindl Harald12 Feb 2014
  • Re: LIKE sql optimizationMathieu Desharnais12 Feb 2014
    • RE: LIKE sql optimizationZhigang Zhang12 Feb 2014
      • Re: LIKE sql optimizationkitlenv12 Feb 2014
        • Re: LIKE sql optimizationlouis liu12 Feb 2014
          • Re: LIKE sql optimizationMathieu Desharnais12 Feb 2014
      • Re: LIKE sql optimizationReindl Harald12 Feb 2014
        • RE: LIKE sql optimizationZhigang Zhang12 Feb 2014
      • Re: LIKE sql optimizationMorgan Tocker12 Feb 2014
        • RE: LIKE sql optimizationZhigang Zhang12 Feb 2014
        • Re: LIKE sql optimizationJesper Wisborg Krogh12 Feb 2014
          • RE: LIKE sql optimizationZhigang Zhang12 Feb 2014