Thread: libpq: why only one PQsendQuery per connection at a time?

Hello!
Question generally to (libpq) developers.

According to http://www.postgresql.org/docs/9.4/static/libpq-async.html:
"PQsendQuery cannot be called again (on the same connection) until PQgetResult
has returned a null pointer, indicating that the command is done."

As I guess this is also true for all PQsend* functions, like PQsendQueryPrepared,
although this is not explicitly stated in the documentation.

So question is why this limitaion exists?

Why PQgetResult(PGconn *conn) operates on connection,
and not on some unique handler that each "PQsend*" could return.

Is it limitaion of libpq or architecture of postgresql backend or backend-frontend protocol?

sftf-2 wrote
> So question is why this limitaion exists?
>
> Why PQgetResult(PGconn *conn) operates on connection,
> and not on some unique handler that each "PQsend*" could return.
>
> Is it limitaion of libpq or architecture of postgresql backend or
> backend-frontend protocol?

IIUC this is a backend Postgres limitation - it was designed in a least
complex way where each process acts in a serial fashion.  A single process
has no way to maintain concurrent state for multiple active queries.

David J.



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