Re: consequent PQsendQueryPrepared() failed: another command is already in progress

Anton Maksimenkov wrote:
> I'm using libpq C Library. I prepared some query and trying to call it
> many times.
> But it success only at first time, and then fail with error:
>
> ... "another command is already in progress"
>
You are using the asynchronous queries while the testbed example program
does not have a structure where an asynchronous loop is useful, and
while doing that made an error probably with the consumeinput/isbusy
duo. The problem will probably be gone when you switch to the
synchronous libpq functions (for execute a prepared command)

If you really want the asynchronous api, I found the PQtrace facility
very useful, to debug what's actually going on (or not).

regards,
Yeb Havinga

> Here is my testbed:
> int
> main (register int const argc, register char *const argv[])
> {
> PGconn *conn;
> PGresult *res;
>
> conn = PQsetdbLogin(PGHOST, PGPORT, PGOPTIONS, PGTTY, PGDBNAME,
> PGLOGIN, PGPWD);
> if (PQstatus(conn) == CONNECTION_BAD) {
> fprintf(stderr, "PQstatus(): %s", PQerrorMessage(conn));
> PQfinish(conn);
> exit(1);
> }
> if ((res = PQprepare(conn, "GET_USER", "SELECT uid FROM users WHERE
> uid = $1::INT LIMIT 1", 1, NULL)) == NULL) {
> fprintf(stderr, "PQprepare() res == NULL");
> PQfinish(conn);
> exit(1);
> }
> if (PQresultStatus(res) != PGRES_COMMAND_OK) {
> fprintf(stderr, "PQprepare() failed: %s", PQerrorMessage(conn));
> PQclear(res);
> PQfinish(conn);
> exit(1);
> }
>
> fprintf(stderr, "FIRST: ");
> query(conn);
>
> fprintf(stderr, "SECOND: ");
> query(conn);
>
> exit(0);
> }
>
> int
> query(PGconn *conn)
> {
> const char *paramValues[1];
> int paramLengths[1];
> int paramFormats[1];
> uint32_t binaryIntVal;
> PGresult *res;
>
> binaryIntVal = htonl((uint32_t) 15);
> paramValues[0] = (char *) &binaryIntVal;
> paramLengths[0] = sizeof(binaryIntVal);
> paramFormats[0] = 1;
>
> if (PQsendQueryPrepared(conn, "GET_USER", 1, paramValues,
> paramLengths, paramFormats, 1) == 0) {
> fprintf(stderr, "PQsendQueryPrepared() failed: %s", PQerrorMessage(conn));
> return -1;
> }
> while (PQisBusy(conn))
> if (PQconsumeInput(conn) == 0) {
> fprintf(stderr, "PQconsumeInput() failed: %s", PQerrorMessage(conn));
> return -1;
> }
>
> if ((res = PQgetResult(conn)) == NULL) {
> fprintf(stderr, "PQgetResult() res == NULL");
> PQfinish(conn);
> return -1;
> }
> if (PQresultStatus(res) != PGRES_TUPLES_OK) {
> fprintf(stderr, "PQgetResult() failed: %s", PQerrorMessage(conn));
> PQclear(res);
> PQfinish(conn);
> return -1;
> }
>
> int i, uidFN;
> char *uidPTR;
> int uid;
>
> uidFN = PQfnumber(res, "uid");
> printf("tuples %d\n", PQntuples(res));
> for (i = 0; i < PQntuples(res); i++) {
> uidPTR = PQgetvalue(res, i, uidFN);
> uid = ntohl(*((uint32_t *) uidPTR));
> printf("tuple %d: uid[%d]\n", i, uid);
> }
> PQclear(res);
>
> return 0;
> }
>
> $ ./test
> FIRST: tuples 1
> tuple 0: uid[15]
> SECOND: PQsendQueryPrepared() failed: another command is already in progress
>
> Where I was wrong?
>
>
> And another question. Is it possible to simultaneously keep a number
> of prepared queries and run any of them from time to time?
> Something like this:
> {
> PQprepare("Q1");
> PQprepare("Q2");
> PQprepare("Q3");
> ...
> query(Q1);
> ...
> query(Q2);
> ...
> query(Q1);
> ...
> query(Q3);
> ...
> query(Q2);
> ...
> query(Q3);
> ...
> query(Q1);
> ...
> }
>