We have faced with some lack of sharing resources. So in our test memory usage per session: Oracle: about 5M MSSqlServer: about 4M postgreSql: about 160М
Using shared resources also has significant problems, so care must be taken.
I think its clear that threading is out, but it is far from being the only solution to reducing the memory overhead of sharing.
Analysing the overhead and suggesting practical ways forward may help.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services