On Wed, Jan 25, 2012 at 10:54 PM, MB Software Solutions, LLC
<mbsoftwaresoluti...@mbsoftwaresolutions.com> wrote:
> 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"
---------------------------------
Would it be better to to run separate updates?
UPDATE MyTable SET MyFactor = 1.23456 WHERE Field1 = 'VALUE'
AND Field2 = "VALUE2" ,"VALUE3"
UPDATE MyTable SET MyFactor = 1.23456 WHERE Field1 = 'VALUE'
AND Field2 = "VALUE3"
I read in your first url that the size of the param may be your biggest issue.
--
Stephen Russell
901.246-0159 cell
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