Are you saying your indexed field has only 50 distinct values? Seems a horrible candidate for an index. Might be good to partition on those fifty values but ten million records probably doesn't warrant that.
i have a table of about 10 millions of records, with the index on a string field. Actually is alphabetical; since queries are about 100/200 per seconds, i was looking for a better way to improve performance and reduce workload.
The unique values, of that fields, are about the 50 (category name), and we could create a second table to codify, with numerical integer values, the 50 recurring names.
Is index are integer and not characteral, performance are better and workload reduces?