Re: consequent PQsendQueryPrepared() failed: another command is already in progress - Mailing list pgsql-general

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);
>  ...
> }
>