On Mon, 2022-07-11 at 17:38 +0530, aditya desai wrote: > I have one Oracle fdw table which is giving performance issue when joined > local temp table gives performance issue. > > select * from oracle_fdw_table where transaction_id in ( select transaction_id from temp_table) > ---- 54 seconds. Seeing HASH SEMI JOIN in EXPLAIN PLAN. temp_table has only 74 records. > > select * from from oracle_fdw_table where transaction_id in ( 1,2,3,.....,75)--- 23ms. > > Could you please help me understand this drastic behaviour change?
The first query joins a local table with a remote Oracle table. The only way for such a join to avoid fetching the whole Oracle table would be to have the foreign scan on the inner side of a nested loop join. But that would incur many round trips to Oracle and is therefore perhaps not a great plan either.
In the second case, the whole IN list is shipped to the remote side.
In short, the queries are quite different, and I don't think it is possible to get the first query to perform as well as the second.
