Postgresql, libpq, C

Question

I have such program which works from "C" OK. Creates a table if they dont exists.

PGresult *result;
conn = PG_connect();
if (conn)
{
    if (!PG_begin(conn))
    {
        char strtable[512] = {0};
        sprintf(strtable, "%s", "CREATE TABLE IF NOT EXISTS mytable");
        strcat(strtable, " (setting TEXT, value TEXT, rez1 TEXT, rez2 TEXT)");
        result = PQexec(conn, strtable);
        if (PQresultStatus(result) != PGRES_COMMAND_OK)
        {
            printf("CREATE TABLE failed: %s\n", PQerrorMessage(conn));
            PQclear(result);
            exit_nicely(conn);
        }

        PQclear(result);
        PG_end(conn);
    }
}
PQfinish(conn);

That mean I am connected properly.
But for why (on earth) in the same code and situation this query don't work?

sprintf(strtable, "%s%s", "SELECT 1 FROM pg_tables WHERE tablename=", "\'invli\'");

I allways get PQresultStatus(result) = 2 and PQerrorMessage(conn) without any text!

All of that I uses from npgsql without problems. Additional question, how is best to get relsults from such simple queries, or "COUNT" which have only one information? In npgsql I uses "ExecuteScalar" function.

Craig Ringer · Accepted Answer · 2013-06-02 13:41:56Z

4

According to postgresql header file, status code 2 is PGRES_TUPLES_OK which means "a query command that returns tuples was executed properly by the backend, PGresult contains the result tuples". So no error here. On the contrary, returning PGRES_COMMAND_OK for a SELECT query would be strange.

See also status codes in documentation (http://www.postgresql.org/docs/current/static/libpq-exec.html):

PGRES_COMMAND_OK Successful completion of a command returning no data.

PGRES_TUPLES_OK Successful completion of a command returning data (such as a SELECT or SHOW).

edited Jun 2, 2013 at 13:41

Craig Ringer

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answered Jun 2, 2013 at 13:08

nullptr

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2 Comments

Wine Too Over a year ago

Sounds logical, thank you. Pitty, for that I lost almost all the day! And for read simple values from query result?

nullptr Over a year ago

I am not aware of a single libpq function that retrieves a scalar value from query result. But I suppose reading the value via traditional multi-tuple interface won't be a big performance problem (anyway, a query to a database takes much more time than parsing a single-row result and locating fields even if the fields are addressed by names instead of indexes).

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