>> Point above about spread still applies, but if you
>> can join index to index, the join goes a lot faster.  (A primary key in
>> MySQL
>> is always indexed.)
>>
> How much is the *a lot*? Thanks. :)

If it's a unique (including primary) key then orders of magnitude for
millions of rows.

If it's a non-unique key, it depends on how many distinct values there are.

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