From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Watkins <noahwatkins@gmail.com>
cc: pgsql-interfaces@postgresql.org
Subject: Re: Using threads in FDW for read-ahead
In-reply-to: 
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Comments: In-reply-to Noah Watkins <noahwatkins@gmail.com>
 message dated "Tue, 10 Jun 2014 12:10:22 -0700"
Date: Tue, 10 Jun 2014 15:21:01 -0400
Message-ID: <3383.1402428061@sss.pgh.pa.us>
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Sender: pgsql-interfaces-owner@postgresql.org

Noah Watkins <noahwatkins@gmail.com> writes:
> I have created a FDW for a storage backend and it is working well, and now
> I would like to overlap processing with I/O by performing read-ahead. I
> started by using a thread to do background I/O and this worked, but
> problems started to arise when I tried to do predicate filtering in the
> thread.

> In particular, it seems as though `check_stack_depth` is built to assume a
> single threaded environment (`stack_base_ptr` is global).

That is not even the tip of the iceberg of what will break if you try
to use multiple threads in a Postgres backend.  It's not supported.
You might possibly manage to not break things if you keep the extra
threads sufficiently narrowly scoped --- which for starters would include
no use of palloc nor elog.  Executing query predicates is right out.

			regards, tom lane


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