> On Aug 19, 2025, at 05:37, Tom Lane <[email protected]> wrote: >> > > Yeah, we could make multi-batch PHJ do this differently from the other > cases, but I don't want to go there: too much complication and risk of > bugs for what is a purely hypothetical performance issue. Besides > which, if the join is large enough to be worth worrying over, it's > most likely taking that code path anyhow. > > >> We can simply added a new flag to HashTable, say named skip_building_hash. >> Upon right join (join to the hash side), and outer table is empty, set the >> flag to true, then in the MultiExecPrivateHash(), if skip_building_hash is >> true, directly put all tuples into node->null_tuple_store without building a >> hash table. >> Then in ExecHashJoinImpl(), after "(void) MultiExecProcNode()" is called, if >> hashtable->skip_building_hash is true, directly set node->hj_JoinState = >> HJ_FILL_INNER_NULL_TUPLES. > > I'm not excited about this idea either. It's completely abusing the > data structure, because the "null_tuple_store" is now being used for > tuples that (probably) don't have null join keys. The fact that you > could cram it in with not very many lines of code does not mean that > the result will be understandable or maintainable --- and certainly, > hash join is on the hairy edge of being too complicated already. > > regards, tom lane
Thanks for the explanation. Then these two comments are resolved. -- Chao Li (Evan) HighGo Software Co., Ltd. https://www.highgo.com/
