Even if IS itself is not supporting PostgrSQL this does not mean that the JDBC-Adapter is also not supporting it.
I would give it try with IS 9.6, JDBC-Adapter 6.5 and Dirver provided by PostgreSQL (as described in the AdapterRequirementsGuide for JDBC Adapter 6.5 on IS 8.2 SP2).
I think it will be working even if it is not stated in the AdapterRequirementsGuide.
As long as there is a JDBC-Type4 (Thin-Mode, pure Java) driver provided by the database vendor it should be possible to connect the database via JDBC-Adapter.
For JDBC-Adapter 9.0 a driver with JDBC 4.x Support might be needed.
Regards,
Holger
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