RE: [Pypgsql-users] Re: How to stop query execution from Python/p yPgSQL

[Murthy K. <mur...@go...>]

Gerhard, thanks. I think I can use this for what I need. At a more
fundamental level though, if you submit a synchronous query through =
libpq,
and then kill the process from which the connection was opened/query =
was
submitted, shouldn't libpq assume and implicit requestCancel, and clean =
up?
I'll ask on pgsql-admin admin also; there was a recent thread on =
canceling
queries which I'll pick up.

Thanks again,
	Murthy

-----Original Message-----
From: Gerhard H=E4ring [mailto:ger...@op...]
Sent: Friday, December 13, 2002 06:37
To: pyp...@li...
Subject: [Pypgsql-users] Re: How to stop query execution from
Python/pyPgS QL


Billy G. Allie <bga=3DN...@pu...> wrote:
> [cancelling a query]
> Since PgSQL is built upon the libpq module, you can access the
> PGrequestCancel function as follows (assuming 'cx' is the connection
> running the request you wish to cancel):
>=20
> 	cx.conn.requestCancel()
>=20
> This would have to be issued by a different thread of execution or =
from a
> signal handler since the execute method does not return until the =
query
> has finished processing at the backend.

Can this work? I was under the impression that this was one of the =
functions
that's supposed to be used under the asynchronous API.

Incidentally, I've tried to use the async functions in pyPgSQL.libpq a =
few
days back (and cancelling a query), but didn't succeed.  Today, I've =
found
out why and committed a change to pgconnection.c. I believe you'll have =
to
use the CVS version of pyPgSQL to cancel queries for now.

Here's a toy example with which I tested this functionality:

#v+
# Experiment with asynchronous query processing and cancelling queries.

from pyPgSQL import libpq
import select

conn =3D libpq.PQconnectdb("host=3Dmyhost dbname=3Dmydb")

class BreakOut(Exception): pass

counter =3D 0

for i in range(10):
    try:
        print "-" * 50
        print "Starting query."
        conn.sendQuery("SELECT NAME FROM TEST") # 100000 rows
        while True:
            counter +=3D 1
            if counter =3D=3D 300:
                raise BreakOut

            fds =3D select.select([conn.socket], [], [])
            if len(fds) !=3D 0:
                conn.consumeInput()
                if not conn.isBusy:
                    break

        res =3D conn.getResult()
        # here we have the entire resultset

        res =3D conn.getResult()
        assert res =3D=3D None
        print "Query successfully processed."
    except BreakOut:
        # Cancel currently running query.
        print "Cancelling query ..."
        conn.requestCancel()
        while True:
            fds =3D select.select([conn.socket], [], [])
            if len(fds) !=3D 0:
                conn.consumeInput()
                if not conn.isBusy:
                    break
        try:
            res =3D conn.getResult()
        except libpq.OperationalError, e:
            assert str(e).strip() =3D=3D "ERROR:  Query was cancelled."
            res =3D conn.getResult()
            assert res =3D=3D None
        print "Query sucessfully cancelled."

conn.finish()
#v-

ciao,

Gerhard
--=20
Gerhard H=E4ring
OPUS GmbH M=FCnchen
Tel.: +49 89 - 889 49 7 - 32
http://www.opus-gmbh.net/




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