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Re: pgsql_fdw, FDW for PostgreSQL server

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
Cc: Shigeru Hanada <shigeru(dot)hanada(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp>
Subject: Re: pgsql_fdw, FDW for PostgreSQL server
Date: 2011-10-29 15:13:45
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com> writes:
> 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 have not looked at the code, but ISTM the way that this has to work is
that you set up a portal for each active scan.  Then you can fetch a few
rows at a time from any one of them.

If you're doing this through libpq, it'd be necessary to implement each
scan using a cursor.  I'm not sure whether it'd be worth our time to
add more functions to libpq to allow more-direct access to the protocol
portal feature.

			regards, tom lane

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