Re: Protocol Question - Mailing list pgsql-interfaces
| From | Thomas Heller |
|---|---|
| Subject | Re: Protocol Question |
| Date | |
| Msg-id | CAGTxmOuW94eHab4==QxzPFxVkBB16jQ2UXpYH7CRjH3r+vimNg@mail.gmail.com Whole thread Raw |
| In response to | Re: Protocol Question (Tom Lane <tgl@sss.pgh.pa.us>) |
| Responses |
Re: Protocol Question
(Tom Lane <tgl@sss.pgh.pa.us>)
|
| List | pgsql-interfaces |
Thanks, explicit test included.
Another quick Question about the request flow.
In an Extended Query Lifecycle, in order to prepare a query I send the Commands
Parse('P') / Describe('D') / Sync('S')
read 1/t/T/Z then to execute
Bind('B') / Execute('E') / Flush('H')
then read until either CommandComplete('C') or PortalSuspended('s')
on 's' I send another 'E' (only happens when I executed with a limit before)
once I arrive at 'C' I send the final 'S'.
That all works perfectly fine, I am unsure however on the 'H' (Flush) part.
I looked at a tcpdump for a ruby and pgjdbc query flow but couldn't figure out how to execute with a limit. Anyways they send:
P/B/D/E/S
"The Flush message does not cause any specific output to be generated, but forces the backend to deliver any data pending in its output buffers. A Flush must be sent after any extended-query command except Sync, if the frontend wishes to examine the results of that command before issuing more commands. Without Flush, messages returned by the backend will be combined into the minimum possible number of packets to minimize network overhead."
If I skip the Flush after Execute I receive no data, if I Execute and Sync I receive the the Limit of rows and a ReadyForQuery('Z').
Is it ok to send a Flush or should I Sync always? I'm leaning towards Sync, but I don't really need the ReadyForQuery cause I'm still reading. (Only related to Execute with a Limit).
Hope that wasn't too confusing, just trying to cover an edge case.
Regards,
/thomas
On Mon, Aug 11, 2014 at 5:06 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Thomas Heller <info@zilence.net> writes:A quick look into libpq says that it doesn't support them being different:
> The first 16 bits (well skip the first 40) in every DataRow ('D') packet
> refer to the number of columns following. Is there any way that this number
> is different from the number received in the RowDescription ('T')?
/* Get the field count and make sure it's what we expect */
if (pqGetInt(&tupnfields, 2, conn))
{
/* We should not run out of data here, so complain */
errmsg = libpq_gettext("insufficient data in \"D\" message");
goto advance_and_error;
}
if (tupnfields != nfields)
{
errmsg = libpq_gettext("unexpected field count in \"D\" message");
goto advance_and_error;}An assert seems overly optimistic. libpq has an explicit test with an
> Currently I use an assert since I assume it always is the same, however I
> only have tested with a very limited dataset so far.
error recovery path, you should too.
regards, tom lane
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