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Re: pgsql_fdw, FDW for PostgreSQL server

From: Shigeru Hanada <shigeru(dot)hanada(at)gmail(dot)com>
To: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
Cc: 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: 2011-10-30 01:22:23
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
2011/10/29 Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)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

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