Re: pgsql_fdw, FDW for PostgreSQL server

On Sat, Oct 29, 2011 at 8:13 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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.

Hmm, true. Looking back at the original proposal (neither did I look
at the code,) there seems to be a cursor mode. ISTM it is hard for fdw
to know how the whole plan tree looks, so consequently do we always
cursor regardless of estimated row numbers? I haven't had much
experiences around cursor myself, but is it as efficient as
non-cursor?

Regards,
--
Hitoshi Harada