Apologies, I should've clarified. I just meant postgres doesn't necessarily have to scan the actual table to get the count in some cases if an index is available, not that it caches the actual count.
So that's not the greatest quote but the article shows postgres having to fall back on a sequential scan for a naive COUNT(*). The author goes on to mention that as of 9.2 Postgres can do an index scan (in the context of SELECT DISTINCT):
But now we come to a quirk, SELECT COUNT(DISTINCT n) FROM items will not use
the index even though SELECT DISTINCT n does. As many blog posts mention (“one
weird trick to make postgres 50x faster!”) you can guide the planner by rewriting
count distinct as the count of a subquery
To me it seems like you can trick the query planner into doing an index scan sometimes, but that it's a bit brittle. Has this improved much since?
It says that index only scans can be used with predicates and less often without predicates, though in my experience I've seen the index often used even without predicates.
Index-only scans are opportunistic, in that they take advantage of a pre-existing
state of affairs where it happens to be possible to elide heap access. However,
the server doesn't make any particular effort to facilitate index-only scans,
and it is difficult to recommend a course of action to make index-only scans
occur more frequently, except to define covering indexes in response to a
measured need
And:
Index-only scans are only used when the planner surmises that that will reduce the
total amount of I/O required, according to its imperfect cost-based modelling. This all
heavily depends on visibility of tuples, if an index would be used anyway (i.e. how
selective a predicate is, etc), and if there is actually an index available that could
be used by an index-only scan in principle.
So yeah, I don't know if your use case is exceptionally lucky or if the documentation is just pessimistic (or both). Good to know you can coerce the query planner into doing an index scan though.
