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Re: pgsql_fdw, FDW for PostgreSQL server

From: "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at>
To: "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at>,"Shigeru Hanada *EXTERN*" <shigeru(dot)hanada(at)gmail(dot)com>
Cc: "Kohei KaiGai" <kaigai(at)kaigai(dot)gr(dot)jp>,"Etsuro Fujita" <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"Robert Haas" <robertmhaas(at)gmail(dot)com>,"Hitoshi Harada" <umi(dot)tanuki(at)gmail(dot)com>,"Martijn van Oosterhout" <kleptog(at)svana(dot)org>,"PostgreSQL-development" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pgsql_fdw, FDW for PostgreSQL server
Date: 2012-02-20 09:18:33
Message-ID: D960CB61B694CF459DCFB4B0128514C2077EC73C@exadv11.host.magwien.gv.at (view raw or flat)
Thread:
Lists: pgsql-hackers
I wrote:
> Shigeru Hanada wrote:
>>> - Since a rescan is done by rewinding the cursor, is it necessary
>>>    to have any other remote isolation level than READ COMMITED?
>>>    There is only one query issued per transaction.
>>
>> If multiple foreign tables on a foreign server is used in a local
query,
>> multiple queries are executed in a remote transaction.  So IMO
isolation
>> levels are useful even if remote query is executed only once.
> 
> Oh, I see. You are right.

I thought some more about this and changed my mind.

If your query involves foreign scans on two foreign tables on the same
foreign server, these should always see the same snapshot, because
that's how it works with two scans in one query on local tables.

So I think it should be REPEATABLE READ in all cases - SERIALIZABLE
is not necessary as long as all you do is read.

Yours,
Laurenz Albe

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