[fabian2k]

I think the big difference is that PostgreSQL doesn't cache query plans at all by default. Only if you use prepared statements. My understanding is that MSSQL does heavily cache them.

It sounds plausible to me that caching would often lead to significant performance improvements overall, but trigger bad plans much more often since the plans are not re-evaluated based on the statistics of each single query. So in Postgres you'd get individual queries with pathological performance when the statistics are off, in MSSQL all executions of that query have bad performance until the plan is re-evaluated again.