Retrieving Query Results Row-By-Row
| PostgreSQL 9.6.10 Documentation | |||
|---|---|---|---|
| Prev | Up | Chapter 32. libpq - C Library | Next |
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 32.4
. 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. |