how is the way for cancel query?

[emilioplatzer]

Can I cancel a query inside query.on('row'...?

I want to do something like:

query.on('row',function(row, result){
    if(row.field1=='ok'){
        result.addRow(row);
    }else{
        result.cancel(row, 'bla bla bla'); 
    }
});

The need is to not continue fetching rows if I detect an error condition.

[emilioplatzer]

1. Is this the way?
2. Why I need to pass client.connectionParameters?

query.on('row',function(row, result){
    if(row.field1=='ok'){
        result.addRow(row);
    }else{
        pg.cancel(client.connectionParameters, client, query);
    }
});

https://github.com/brianc/node-postgres/wiki/pg#cancelconfig-client-query

[emilioplatzer]

In #753 there is an example with client.cancel(query); but cancel isn't in the documentation.

[joskuijpers]

The cancel code for the native client seems to be completely different from the non-native client. The native simply cancels the query if it is the active one (and removes from queue if not active). The non-native code does some weird stuff with making a connection just to cancel the connection...
See also #753

[brianc]

When the PostgreSQL back-end is handling a query on a client it doesn't process any other incoming messages from the socket, so, you can't actually cancel a query from the client that's running the query. Hence having to pass connection parameters & the client you'd like to cancel in.

1. The api for canceling aquery here in node-postgres is still very weird though, I agree. It should probably be made easier to reason about...instead of auto-connecting the best thing is probably have something like this...

var badClient = new Client()
badClient.connect()
badClient.query('pg_sleep(10000000000)')

var goodClient = new Client()
goodClient.cancel(badClient)

Something like that - I'll need to check the client/server protocol documentation to see the details - I know you need the client id of the client you'd like to cancel. Anyways...that might be a bit more explicit and a bit less magical - still cumbersome having to keep around a reference to the old client...but that's how it goes.

2. The native driver doesn't need this because libpq internally does a bunch of this management for you. I just call pqcancel which internally likely opens a new client & sends a cancel request. Not 100% sure there.

[robhaswell]

You should be able to cancel queries using pg_cancel_backend(). I think the pattern looks like this:

const pool = pg.Pool()

/* Check out a client */
pool.connect()
.then((c) => {
  /* Get my PID */
  c.query('SELECT pg_backend_pid()')
  .then((result) => {
    const pid = result.rows[0][0]

    const badResult = c.query('SELECT pg_sleep(10000000000)')

    /* Kill this after 1 second */
    setTimeout(() => {
      /* NOTE this query is executed by the pool */
      const cancelResult = pool.query('SELECT pg_cancel_backend($1)', [ pid ])
    }, 1000)
  })
})

[akihikodaki]

Is it OK to use the existing APIs? I'm considering to add type definitions to https://github.com/Microsoft/DefinitelyTyped and would like to confirm the intention before doing that.

interface ClientBase {
  cancel(client: ClientBase, query: Query): void;
}

interface NativeClientBase {
  cancel(query: Query): void;
}

interface Connection {
  cancel(processId: number, secretKey: number): void;
}

[charmander]

@akihikodaki The existing APIs don’t actually work right now. See #1392.

[akihikodaki]

@charmander Thanks for your reply. But now I have been realized ClientBase.cancel works, and NativeClientBase.cancel may also work. I wonder if those low level interfaces can be used for now.

[pankleks]

I'm also looking for a way to cancel statement, is there any way in 2019? Is the Client.cancel method supported? If so, there seems to be no types for it.

[gajus]

@pankleks I have shared here what such an implementation would look like.

https://github.com/gajus/slonik/issues/38

There is no one readily available, though in node-postgres.

[felixfbecker]

Is there any way in 2020? 😄 😭