Re: pgsql_fdw, FDW for PostgreSQL server

2011/10/29 Hitoshi Harada <umi.tanuki@gmail.com>:
> I have a doubt here, on sharing connection for each server. What if
> there are simultaneous scan on the same plan? Say,
>
> -> Nested Loop
>  -> Foreign Scan to table T1 on server A
>  -> Foreign Scan to table T2 on server A
>
> Okay, you are thinking about Foreign Join, so example above is too
> simple. But it is always possible to execute such a query if foreign
> scan nodes are separated far, isn't it? As far as I see from your
> explanation, scan T1 and scan T2 share the same connection. Now join
> node scans one row from left (T1) while asking rows from right (T2)
> without fetching all the rows from left. If T2 requests to server A,
> the connection's result (of T1) is discarded. Am I understand
> correctly?

I think that sharing a connection doesn't cause any problem.

In cursor mode, using multiple cursors concurrently through one connection
is OK.  In SELECT mode, pgsql_fdw executes SELECT statement with
PQexecParams and retrieves whole result *inside* the first Iterate call for
an outer tuple.  So libpq connection is already available when another scan
needs to call Iterate function.

--
Shigeru Hanada