It is PostgreSQL only as it right now.
There is some work going on updating the documentation on tuning the Directory that is coming - but I do not know exactly what will be included in the updates.
In most cases it is fine with the default setting - but if you are working with huge deployments (+100K identities) you may want to connect to IBM specialists to get some help.
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Franz Wolfhagen
WW IAM Solution Architect - Certified Consulting IT Specialist
IBM Expert Labs
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