kimaidou <kimaidou(at)gmail(dot)com> writes:
> It seems PostgreSQL does not use the index parcelles_dep_idx on "dep" (text
> field), even if the corresponding number of lines for this WHERE clause is
> a smal subset of the entire data:
> approx 6M against 80M in total
6M out of 80M rows is not a "small subset". Typically I'd expect
the planner to use an index-based scan for up to 1 or 2 percent of
the table. Beyond that, you're going to be touching most pages
of the table anyway.
You can try reducing random_page_cost to favor indexscans, but
you might not find that the query gets any faster.
regards, tom lane