Communicating with PostgreSQL Using LibPQ - PostgreSQL Developer's Guide - page 182

Using PQfnumber
The PQfnumber function returns the column number associated with the
column_name argument. The syntax for PQfnumber is as follows:
int PQfnumber(const PGresult *res,
const char *column_name);
The following is a code snippet to explain the PQfnumber function:
ntuples = PQntuples(result);
for (i = 0; i < ntuples; i++)
{
char *id, *name;
/* Get the column number of column "id" and "name" */
int id_col = PQfnumber(result, "id");
int name_col = PQfnumber(result, "name");
id = PQgetvalue(result, i, id_col);
name = PQgetvalue(result, i, name_col);
fprintf ("Data: id = %s, name = %s\n", id, name);
}
Using PQftable
In complex queries where multiple tables are referenced, we need to know the table
OID from which that column is fetched. The PQftable function gets the OID of table
of that particular column in the result set.
The syntax for PQftable is as follows:
Oid PQftable(const PGresult *res,
int column_number);
The following is a code snippet to explain the PQftable function:
ntuples = PQntuples(result);
for (i = 0; i < ntuples; i++)
{
char *id, *name;
int id_col = PQfnumber(result, "id");
int name_col = PQfnumber(result, "name");
/* Get Table oid of column "id" */
int oid = PQftable(result, id_col);
fprintf(stdout, "Table oid is = %d \n", oid);
}