Re: Should we update the random_page_cost default value?

On Tue, Oct 7, 2025 at 3:15 PM Greg Sabino Mullane <htamfids(at)gmail(dot)com> wrote:
> I've been doing this sort of thing for clients a long time, and I always test both directions when I come across a query that should be faster. For real-world queries, 99% of them have no change or improve with a lowered rpc, and 99% get worse via a raised rpc. So color me unconvinced.

Color me equally unconvinced. Tomas seems quite convinced that we
ought to be raising random_page_cost rather than lowering it, and that
absolutely does not correspond to my experience in any way. It's not
that I've actually tested raising the random page cost on very many
systems, mind you. It's that my experience with real-world systems is
that we tend to pick sequential scans when we should use an index, not
the other way around. And obviously that problem will never be fixed
by raising random_page_cost, since that will tend to favor sequential
scans even more.

Tomas's test involves scanning big tables that don't fit in RAM, and I
agree that if that's most of what you do, you might benefit from a
higher random_page_cost. However, even users who have some tables that
are a lot bigger than RAM also tend to have frequently-accessed tables
that are much smaller. For example, if you join a fact table to a
bunch of dimension tables, the fact table may not fit in RAM, but the
dimension tables probably do, and we're using the same
random_page_cost for all of those tables. Moreover, we're least likely
to make a wrong decision about what to do about the big table. It will
often be the case that the query will be phrased such that a
sequential scan on the big table is unavoidable, and if it is possible
to avoid a sequential scan, we're going to want to do so almost
regardless of random_page_cost because the number of pages accesses we
can save will tend to be large. On a smaller table, it's more likely
that both a full table scan and an index-based approach will be
competitive, so there the value of random_page_cost will matter more
to the final outcome. So, in this scenario, it's more important that
the random_page_cost is close to accurate for the smaller tables
rather than for the larger ones, and those are the ones that are most
likely to benefit from caching.

One of the planner changes that I think would be worth exploring is to
have the system try to estimate the percentage of a given table that
is likely to be in cache, and that could be configured via a reloption
or estimated based on the size of the table (or maybe even the
frequency of access, though that is fraught, since it can change
precipitously on short time scales and is thus not great to use for
planning). If we estimated that small tables are likely to be cached
and bigger ones are likely to be less-cached or, if very big,
completely uncached, then it would probably be right to raise
random_page_cost as well. But without that kind of a change, the
correct value of random_page_cost is the one that takes into account
both the possibility of caching and the possibility of nothing being
cached, and this test is rotated all the way toward one end of that
spectrum.

--
Robert Haas
EDB: http://www.enterprisedb.com