I figured out how to make the query faster. There should be a mailing list
set up for wasted questions since I always seem to figure out the problem
after I've bugged everyone for help.
In the query
select row_key, column1, column2, column3, column4, column5 from table1
where column6 = 1 order by column3 desc limit 21;
I changed the index to
message_index btree (column3, column6)
rather than
message_index btree (column6, column3, column7)
Since the data was being ordered by column3 it seems to have sped the query
up to 1 ms from 6000ms by making column 3 the first part of the index rather
than the second.
Kevin