Minor thing. Our implementation of clustering keys do not ensure
uniqueness. So in the examples below in the mapping tables, the key b
does not have uniqueness enforced.
So, if one does "create table foo (a int, b int, c int, ..., primary
key (a), clustering key (b));", 'b' may have multiple values that are
the same.
> A simple mapping table illustrates it:
> CREATE TABLE map ( a int ..., b int..., # just the 2 fields
> PRIMARY KEY (a),
> UNIQUE (b) # (unique is usually desired for a 'mapping')
> ) ENGINE = InnoDB; # effectively 2 clustered indexes in InnoDB
>
> To get the equivalent in MyISAM takes:
> CREATE TABLE map ( a int ..., b int..., # just the 2 fields
> PRIMARY KEY (a), # This exists only as a constraint
> UNIQUE (b), # This exists only as a constraint
> INDEX (a, b), # The index is "clustered" on a (actually a+b)
> INDEX (b, a) # clustered on b
> ) ENGINE = MyISAM; # had to add extra indexes to get same effect in
> MyISAM
>
> Which is a prime example of the need for
> UNIQUE (a ; b)
> (or some other syntax) meaning that UNIQUE applies only to (a), but the
> index contains (a,b) to facilitate "Using index".
>
>> Regards,
>> Monty
>>
>
