concurrent queries in a connection

Hi

What happens when I submit two sequential queries:

client.query("SELECT ...", function() {} )
client.query("SELECT ...", function() {} )

I understand there is a queue and second query will execute only after first is complete. But what does it mean “complete” - will pg send the second query to the server only after all results from the first query came back? Is it the driver behavior or does postgres mandate that only one query should be sent to the server by the client at a given moment?

Thanks, Yaron

[yaronn]

thanks @brianc ! So to finally sum it up see those 3 snippets:

1:

pg.connect(conString, function(err, client, done) {
  client.query("SELECT ...", function(err, result) {})
});
pg.connect(conString, function(err, client, done) {
  client.query("SELECT ...", function(err, result) {})
});

2:

pg.connect(conString, function(err, client, done) {
  client.query("SELECT ...", function(err, result) {})
  client.query("SELECT ...", function(err, result) {})
});

3:

pg.connect(conString, function(err, client, done) {
  client.query("SELECT ...", function(err, result) {
    client.query("SELECT ...", function(err, result) {})
  })
});

snippet 1 is faster than the others (assuming there is an available connection) since the second query will not wait for the first to finish. snippets 2, 3 are the same since the second query will not start until first finishes, which is implicit in 2 and explicit in 3.

right again?

[brianc]

node-postgres maintains an internal queue of queries and sends them serially to the PostgreSQL server backend one at a time because the protocol & server mandate a maximum of 1 in-flight query per connected client. If you don’t care about order execution you can use something like pg-query and async just be aware transactions don’t work at all across multiple clients.