At 01:17 AM 3/20/2002, you wrote:
>An Oracle DB programmer reviewed a query that I wrote and told me that
>putting constants at the beginning of the query would make it slower. I
>thought I'd go to the experts on MySQL and be told the truth one way of
>the other.
>
>Example query:
>
>SELECT
> TABLE1.COL,
> TABLE2.COL
>FROM
> TABLE1,
> TABLE2
>WHERE
> TABLE1.COL='CONSTANT'
>AND
> TABLE1.COL=TABLE2.COL
>
> [ OR ]
>
>SELECT
> TABLE1.COL,
> TABLE2.COL
>FROM
> TABLE1,
> TABLE2
>WHERE
> TABLE1.COL=TABLE2.COL
>AND
> TABLE1.COL='CONSTANT'
>
>I know that indexes are important and such, the above is just an example
>to give some visual.
>
>Thanks for any help you can offer.
>
>Rob.
Rob,
The only thing to watch out for is that the constant should be the
same data type as the column. Fox example, don't compare a Char to an
Integer (or vice-versa) otherwise MySQL will have to convert the column on
a row by row basis thus eliminating the use of the index.. Also if you
perform a function on a column name, then it also can't use the index. So
if you need to use functions it is best that you move the function to the
constant side of the comparison if possible.
Brent
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