> Say I have a simple table with an ordinary
> auto_increment column called id. Is MySQL smart
> enough to know that a query like

> select * from mytab where id=123

> will have only one match, or is it always a good
> idea to append limit 1 to such queries? If mysql
> knows (and my guess is that it probably does), then
> does adding limit 1 actually do anything to slow
> down the query?

> The same question would apply to a table with a
> column defined with the UNIQUE constraint.

> Unless this column is the table's primary key or has a unique index,
> even an autoincrement column can have duplicates.

> Autoincrement is only the default. You can still specify your own
> values on an insert or update statement.

You are right. But suppose it *is* a primary key or has
a unique index, is adding "limit 1" superfluous, i.e.,
does MySQL know to stop searching for matches after
finding one because it knows by the table definition
that there can't be any more?