store log of update,insert operation of postgresql user
Have a look at the Postgresql wiki page where there are comprehensive examples of audit mechanisms for both Postgresql 9.0 and 9.1+


PG 9.0: https://wiki.postgresql.org/wiki/Audit_trigger
PG 9.1+:
https://wiki.postgresql.org/wiki/Audit_trigger_91plus, with more code from Craig Ringer at https://github.com/2ndQuadrant/audit-trigger