Hi, A small patch to the documentation about how to reduce the number of parameterized paths, because it kept me searching for a long time :-) (The code this documents is in add_paths_to_joinrel(), the loop foreach(lc, root->join_info_list).)
diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README index 41c120e0cd..79e270188d 100644 --- a/src/backend/optimizer/README +++ b/src/backend/optimizer/README @@ -863,6 +863,19 @@ that. An exception occurs for parameterized paths for the RHS relation of a SEMI or ANTI join: in those cases, we can stop the inner scan after the first match, so it's primarily startup not total cost that we care about. +Furthermore, join trees involving parameterized paths are kept as left-deep +as possible; for nested loops consisting of inner joins only, bushy plans +are equivalent to left-deep ones, so keeping bushy plans is only a waste. +This is enforced by refusing to join parameterized paths together unless +the parameterization is resolved, *or* the remaining parameterization is +one that must cannot be delayed right away (because of outer join +restrictions). This ensures that we do not keep around large subplans that +are parameterized on a whole host of external relations, without losing +any plans. An exception is that we are allowed to keep a parameterization +around if we *partially* resolve it, i.e., we had a multi-part index and +resolved only one table from it. This is known as the "star-schema" +exception. + LATERAL subqueries ------------------ /* Steinar */ -- Homepage: https://www.sesse.net/
