31.5. Retrieving Query Results Row-By-Row
Ordinarily, libpq collects a
SQL command's entire result and returns it to the application as
a single PGresult. This can be
unworkable for commands that return a large number of rows. For
such cases, applications can use PQsendQuery and PQgetResult in single-row
mode. In this mode, the result row(s) are returned to the
application one at a time, as they are received from the
server.
To enter single-row mode, call PQsetSingleRowMode immediately after a
successful call of PQsendQuery (or
a sibling function). This mode selection is effective only for
the currently executing query. Then call PQgetResult repeatedly, until it returns null,
as documented in Section 31.4. If
the query returns any rows, they are returned as individual
PGresult objects, which look like
normal query results except for having status code PGRES_SINGLE_TUPLE instead of PGRES_TUPLES_OK. After the last row, or
immediately if the query returns zero rows, a zero-row object
with status PGRES_TUPLES_OK is returned;
this is the signal that no more rows will arrive. (But note that
it is still necessary to continue calling PQgetResult until it returns null.) All of
these PGresult objects will contain
the same row description data (column names, types, etc) that an
ordinary PGresult object for the
query would have. Each object should be freed with PQclear as usual.
PQsetSingleRowMode-
Select single-row mode for the currently-executing query.
int PQsetSingleRowMode(PGconn *conn);
This function can only be called immediately after
PQsendQueryor one of its sibling functions, before any other operation on the connection such asPQconsumeInputorPQgetResult. If called at the correct time, the function activates single-row mode for the current query and returns 1. Otherwise the mode stays unchanged and the function returns 0. In any case, the mode reverts to normal after completion of the current query.
| Caution |
|
While processing a query, the server may return some rows and then encounter an error, causing the query to be aborted. Ordinarily, libpq discards any such rows and reports only the error. But in single-row mode, those rows will have already been returned to the application. Hence, the application will see some PGRES_SINGLE_TUPLE PGresult objects followed by a PGRES_FATAL_ERROR object. For proper transactional behavior, the application must be designed to discard or undo whatever has been done with the previously-processed rows, if the query ultimately fails. |