Partial Index with group by

I am trying to optimize a query that uses a group by on a varchar(255)
column. The column has a large enough cardinality that a 10 character
partial index uniquely covers over 99% of all values. I was hoping
that this partial index would be able to help with the group by
(though obviously not as much as a full index), but it seems that only
full indexes can be used with group by optimizations. Would anyone be
able confirm this one way or another? I am using 5.1.45 with InnoDB
Storage Engine.

I ended up just adding the full index for the performance, but would
like to know what's going on for my personal knowledge. Any insights
would be great. Thanks.

Les Fletcher

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