Description of problem:
An entry containing membership attribute is usually a group and the attribute may contains many values.
When changing the valueset, substring index is very expensive even for a minor change like deleting a single value.

When adding such index type, the administrator should be warn of the cost and double check that the index is required.

Version-Release number of selected component (if applicable):

How reproducible:
See https://bugzilla.redhat.com/show_bug.cgi?id=1868348 and the discussion in the related jira ticket Actual results:

Expected results:
Warn the admin of possible high cost of substing index on membership attributes

Additional info: