Re: Oracle_FDW table performance issue

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.

Yours,
Laurenz Albe
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