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MySQL > Index question

Using MySQL + an ETL tool to parse large amounts of XML.  I end up with a table that holds variables and values of those variables that are found in the XML.  I already have an index on the Variable column, but wondering if there is a way to optimize queries on the VariableValue column.  The problem is that based on the 'Variable' name, the 'VariableValue' value may or may not contain a value that would otherwise be good to index on.  For example:

[Variable]               [VariableValue]
State                      TX
CustomerType       New
CustomerType       Existing
State                      FL
Address                 123 Whatever Street
OrderNumber       13456789

In the table above, I think it would make sense to want to index State (will hold 1 of 50 values) and Customer Type (will hold 1 of 2 values), but Address values, and Order Number values will always hold 1 of infinite values and it wouldn't make much sense to index those.

Is there a good way to tackle these besides building new tables/fields and doing some kind of weird mapping?  I wonder if indexing on the first X bytes of VariableValue would make sense.  I'd love your advice.
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ducky801
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ducky801
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3 Solutions
 
lwadwellCommented:
Curious structure ... how is the data accessed?  Indexes are used to speed the retrieval of data - are you searching on just [VariableValue] or both [VariableValue] and [Variable]?  I would guess both as the value has little meaning without [Variable] value to give it context.  Do you ever lookup based on just [Variable]?

Can you index on certain rows ... no, it is the entire table or nothing.
Can you index only a portion (substring) of a column ... yes; but I am not sure that would be of much benefit ... I am assuming [VariableValue] is varchar.

My best guess would be to index both columns ... I am not sure which should be first in the index however.  I need to understand more about the data access.
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ZberteocCommented:
You either index the column or not, you can't make indexes on N characters unless you create a third column and populate it with the first N characters of the Value column but I don't think is worth the effort.

If you create index on the second column it should be part of a composite index with the variable column and the second in position. If you will make queries based on the value only then you need 2 indexes:

[Variable] + [VariableValue]
[VariableValue]

This way you're covered regardless of how you make the searches and for sure will help with the retrieval speed. I wouldn't be worried of the values themselves as log as the column is varchar anyway.
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ZberteocCommented:
If you are worried by the size of the index on the Value column, but i don't think you should, you can create a third column and populate it with the SOUNDEX([VariableValue]). Later you will do queries based on the soundex of a value but it will return all the values that "sound" the same. The index will be considerably less in size but not as exact.

Here about soundex function:

http://www.tutorialspoint.com/mysql/mysql-string-functions.htm#function_soundex
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ducky801Author Commented:
Thanks guys.  You've given me some ideas. I think what I'll end up doing is adding a column called VarValueIndexed and have that value be the same as VariableValue in cases where we're dealing with a limited number of values (State, CustomerType, etc), and null for the rest.

To answer your question about access, lwadwell:  "Lots and lots of subqueries"

The challenge is that I'm re-structuring this data, not always knowing what I'll end up using it for, and the variable names could change pretty easily without me knowing.  Thanks again for your help!
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