The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/14/git.html
Description:
We've spent days investigating a performance issue related to "explicit join
order"/"from_collapse_limit", where we would have expected a query using
"explicit join clauses" (we didn't know were named this way obviously, and
we are only using for ease-of-query-building purposes) to use the same plan
as a flat query.
During this time, we have naturally read the documentation at
https://www.postgresql.org/docs/current/planner-optimizer.html to see if it
would mention anything that would explain why these would behave
differently.
However, this documentation currently only says: "If the query uses fewer
than geqo_threshold relations, a near-exhaustive search is conducted to find
the best join sequence" (which we checked for), but does not mention the
explicit join behavior
(https://www.postgresql.org/docs/current/explicit-joins.html) nor the
from_collapse_limit or join_collapse_limit parameters.
(https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-FROM-COLLAPSE-LIMIT).
Mentioning these in a similar way as we explain geqo_threshold's possible
impact on query planning would be a huge improvement, and would have saved
us a lot of time.
Thanks,