Re: Cancelling Requests Frontend/Backend Protocol TCP/IP

Craig Ringer <craig(at)postnewspapers(dot)com(dot)au> writes:
> On 2/11/2009 5:15 PM, Raimon Fernandez wrote:
>> If for example I send a SELECT * from myTable, it has 20000 rows, and
>> postgre starts sending the rows, how I can cancel this operation ?

> Assuming you're asking "is there any way to cancel a running query using
> the postgresql tcp/ip protocol" then, as you've noted, you can cancel
> the request until you start getting data.

> After that, you can still cancel the query by establishing another
> connection to the server and calling pg_cancel_backend( ) at the SQL
> level. This does, unfortunately, involve the overhead of setting up and
> tearing down a connection and the associated backend.

The above distinction is nonsense. Query cancel works the same way
whether you have started receiving data or not --- it will open a
transient connection in any case. Otherwise there would be race
condition problems if the backend is just about to start sending data.

regards, tom lane