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Re: pgsql_fdw, FDW for PostgreSQL server

From: "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at>
To: "Kevin Grittner *EXTERN*" <Kevin(dot)Grittner(at)wicourts(dot)gov>,"Shigeru Hanada *EXTERN*" <shigeru(dot)hanada(at)gmail(dot)com>
Cc: "Robert Haas" <robertmhaas(at)gmail(dot)com>,"Hitoshi Harada" <umi(dot)tanuki(at)gmail(dot)com>,"Kohei KaiGai" <kaigai(at)kaigai(dot)gr(dot)jp>,"Etsuro Fujita" <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>,"PostgreSQL-development" <pgsql-hackers(at)postgresql(dot)org>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"Martijn van Oosterhout" <kleptog(at)svana(dot)org>
Subject: Re: pgsql_fdw, FDW for PostgreSQL server
Date: 2012-02-20 15:51:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Kevin Grittner wrote:
> > 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.
> That makes sense.

> > So I think it should be REPEATABLE READ in all cases -
> > SERIALIZABLE is not necessary as long as all you do is read.
> That depends on whether you only want to see states of the database
> which are consistent with later states of the database and any
> invariants enforced by triggers or other software.  See this example
> of how a read-only transaction can see a bogus state at REPEATABLE
> READ or less strict transaction isolation:
> Perhaps if the transaction using the pgsql_fdw is running at the
> SERIALIZABLE transaction isolation level, it should run the queries
> at the that level, otherwise at REPEATABLE READ.

I read the example carefully, and it seems to me that it is necessary
for the read-only transaction (T3) to be SERIALIZABLE so that
T1 is aborted and the state that T3 saw remains valid.

If I understand right, I agree with your correction.

Laurenz Albe

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