On 1/25/2012 11:42 PM, MB Software Solutions, LLC wrote:
> I'm dealing with very large datasets (millions of rows) and so my SQL
> needs to be "on" more than ever! I'm using MySQL and I'm using EXPLAIN
> <sql> to help me make sure my SQL is optimized. I've got several
> fields, two of which are in my WHERE clause for several UPDATE SQLs.
> The EXPLAIN didn't work for UPDATE so I did it for a SELECT with my
> WHERE clause being the same as in the UPDATE SQL. I thought if I had a
> compound index on the two fields (one being VARCHAR(10) and the other
> being CHAR(10)) that it would use that for best optimization (as opposed
> to a separate index on each field). However, many of my UPDATE SQLs are
> like this:
>
> UPDATE MyTable SET MyFactor = 1.23456 WHERE Field1 = 'VALUE' AND Field2
> IN ("VALUE2","VALUE3")
>
> The EXPLAIN says it won't use the combined index. But for simpler
> UPDATEs where it's just one value for Field2 and using an =, it says it
> WILL use the index. For example, this SQL would utilize the compound
> index:
>
> UPDATE MyTable SET MyFactor = 1.23456 WHERE Field1 = 'VALUE' AND Field2
> = "VALUE2"
>
> So by using the IN clause, that nullifies any chance of optimizable tag
> use????
>
> (On a separate note, I'm not sure why the 2nd field was a fixed CHAR
> instead of a VARCHAR? I didn't define these formats; the scripts from
> the vendor defined them. Wonder if changing it would make a difference?)
This is helping:
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html
http://dev.mysql.com/doc/refman/5.0/en/index-merge-optimization.html
--
Mike Babcock, MCP
MB Software Solutions, LLC
President, Chief Software Architect
http://mbsoftwaresolutions.com
http://fabmate.com
http://twitter.com/mbabcock16
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