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List:       mysql-bugs
Subject:    Re: Optimization Suggestion
From:       "Holyfoot" <holyfoot () mysql ! com>
Date:       2002-07-31 8:22:15
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> the index is only used when the number of scanned rows is less than some
> value (the value depends on the total number of rows in the table), and
when
> it is greater than the value, the index is not used. I guess this is done
> consciously, but I don't get why.
Row-by-row table scanning works much faster than reading through the index.
If you have more than 30% of table records in your selection reading using
index is slower.

> explain select * from Words Order By Id Limit 16555
>
> outputs that now the index is used (16555=the number of rows in the
table-1)
I think it's natural - usind index you don't have to sort selection.

Regards.
A.


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