From: Martin Kleppmann <martin@kleppmann.com>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable
Subject: Access to receive/output functions from libpq client
Message-Id: <D7E62929-32EC-4664-BBB2-7EE3595379A1@kleppmann.com>
Date: Mon, 29 Dec 2014 21:36:46 +0100
To: pgsql-interfaces@postgresql.org
Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\))
Precedence: bulk
Sender: pgsql-interfaces-owner@postgresql.org

Hi,

I'm trying to export the contents of a PG database to an external format, u=
sing 9.4's exciting new logical decoding facility. I am currently writing a=
n output plugin for logical replication (using PGXS). To capture a snapshot=
 of the data before the replication slot was created, I am using libpq to s=
et up a transaction with the replication slot's exported snapshot, and doin=
g a "select * from table".

In this context, my question is about decoding datatypes. An output plugin =
sees the changed tuples in binary format, it can find the the OID of a datu=
m, and use PG's functions to decode the datum -- for example, it can use Da=
tumGetNumeric() to get a Numeric struct, and it can use OidOutputFunctionCa=
ll() to get the text representation of a datum. That's very nice.

In the external client, when doing PQsendQueryParams() I can choose between=
 binary and text result format. I'd like to be able to request the binary f=
ormat and use the same conversion functions as in the output plugin. This w=
ould allow the data to be exported in a structured representation, and only=
 falling back to using a datatype's output function to get a string represe=
ntation if we don't know what to do with a particular OID.

My problem is that the libpq client doesn't have access to the same library=
 functions as the output plugin. The linker complains about undefined symbo=
ls _OidOutputFunctionCall, _getTypeOutputInfo and _pg_detoast_datum, for ex=
ample.

Is there a library that I could link into the libpq client to provide the f=
unctions for decoding binary datatypes? Or is there some other recommended =
way of decoding binary datums in a client?

Thanks,
Martin


--=20
Sent via pgsql-interfaces mailing list (pgsql-interfaces@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-interfaces