Jeff,
> Does anyone see effective_cache_size make a difference anyway? If so,
> in what circumstances?
E_C_S, together with random_page_cost, the table and index sizes, the
row estimates and the cpu_* costs, form an equation which estimates the
cost of doing various kinds of scans, particularly index scan vs. table
scan. If you have an extremely small database (< shared_buffers) or a
very large database ( > 50X RAM ), the setting for E_C_S probably
doesn't matter, but in the fairly common case where some tables and
indexes fit in RAM and some don't, it matters.
> In my hands, queries for which effective_cache_size might come into
> play (for deciding between seq scan and index scan) are instead
> planned as bitmap scans.
You have a very unusual workload, or a very small database.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com