32.4. Asynchronous Command Processing
The PQexec
function is adequate for submitting commands in normal, synchronous applications. It has a few deficiencies, however, that can be of importance to some users:
PQexec
waits for the command to be completed. The application might have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response.Since the execution of the client application is suspended while it waits for the result, it is hard for the application to decide that it would like to try to cancel the ongoing command. (It can be done from a signal handler, but not otherwise.)
PQexec
can return only onePGresult
structure. If the submitted command string contains multiple SQL commands, all but the lastPGresult
are discarded byPQexec
.PQexec
always collects the command's entire result, buffering it in a singlePGresult
. While this simplifies error-handling logic for the application, it can be impractical for results containing many rows.
Applications that do not like these limitations can instead use the underlying functions that PQexec
is built from: PQsendQuery
and PQgetResult
. There are also PQsendQueryParams
, PQsendPrepare
, PQsendQueryPrepared
, PQsendDescribePrepared
, and PQsendDescribePortal
, which can be used with PQgetResult
to duplicate the functionality of PQexecParams
, PQprepare
, PQexecPrepared
, PQdescribePrepared
, and PQdescribePortal
respectively.
PQsendQuery
Submits a command to the server without waiting for the result(s). 1 is returned if the command was successfully dispatched and 0 if not (in which case, use
PQerrorMessage
to get more information about the failure).int PQsendQuery(PGconn *conn, const char *command);
After successfully calling
PQsendQuery
, callPQgetResult
one or more times to obtain the results.PQsendQuery
cannot be called again (on the same connection) untilPQgetResult
has returned a null pointer, indicating that the command is done.In pipeline mode, command strings containing more than one SQL command are disallowed.
PQsendQueryParams
Submits a command and separate parameters to the server without waiting for the result(s).
int PQsendQueryParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat);
This is equivalent to
PQsendQuery
except that query parameters can be specified separately from the query string. The function's parameters are handled identically toPQexecParams
. LikePQexecParams
, it allows only one command in the query string.PQsendPrepare
Sends a request to create a prepared statement with the given parameters, without waiting for completion.
int PQsendPrepare(PGconn *conn, const char *stmtName, const char *query, int nParams, const Oid *paramTypes);
This is an asynchronous version of
PQprepare
: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, callPQgetResult
to determine whether the server successfully created the prepared statement. The function's parameters are handled identically toPQprepare
.PQsendQueryPrepared
Sends a request to execute a prepared statement with given parameters, without waiting for the result(s).
int PQsendQueryPrepared(PGconn *conn, const char *stmtName, int nParams, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat);
This is similar to
PQsendQueryParams
, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. The function's parameters are handled identically toPQexecPrepared
.PQsendDescribePrepared
Submits a request to obtain information about the specified prepared statement, without waiting for completion.
int PQsendDescribePrepared(PGconn *conn, const char *stmtName);
This is an asynchronous version of
PQdescribePrepared
: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, callPQgetResult
to obtain the results. The function's parameters are handled identically toPQdescribePrepared
.PQsendDescribePortal
Submits a request to obtain information about the specified portal, without waiting for completion.
int PQsendDescribePortal(PGconn *conn, const char *portalName);
This is an asynchronous version of
PQdescribePortal
: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, callPQgetResult
to obtain the results. The function's parameters are handled identically toPQdescribePortal
.PQgetResult
Waits for the next result from a prior
PQsendQuery
,PQsendQueryParams
,PQsendPrepare
,PQsendQueryPrepared
,PQsendDescribePrepared
,PQsendDescribePortal
, orPQpipelineSync
call, and returns it. A null pointer is returned when the command is complete and there will be no more results.PGresult *PQgetResult(PGconn *conn);
PQgetResult
must be called repeatedly until it returns a null pointer, indicating that the command is done. (If called when no command is active,PQgetResult
will just return a null pointer at once.) Each non-null result fromPQgetResult
should be processed using the samePGresult
accessor functions previously described. Don't forget to free each result object withPQclear
when done with it. Note thatPQgetResult
will block only if a command is active and the necessary response data has not yet been read byPQconsumeInput
.In pipeline mode,
PQgetResult
will return normally unless an error occurs; for any subsequent query sent after the one that caused the error until (and excluding) the next synchronization point, a special result of typePGRES_PIPELINE_ABORTED
will be returned, and a null pointer will be returned after it. When the pipeline synchronization point is reached, a result of typePGRES_PIPELINE_SYNC
will be returned. The result of the next query after the synchronization point follows immediately (that is, no null pointer is returned after the synchronization point.)Note
Even when
PQresultStatus
indicates a fatal error,PQgetResult
should be called until it returns a null pointer, to allow libpq to process the error information completely.
Using PQsendQuery
and PQgetResult
solves one of PQexec
's problems: If a command string contains multiple SQL commands, the results of those commands can be obtained individually. (This allows a simple form of overlapped processing, by the way: the client can be handling the results of one command while the server is still working on later queries in the same command string.)
Another frequently-desired feature that can be obtained with PQsendQuery
and PQgetResult
is retrieving large query results a row at a time. This is discussed in Section 32.6.
By itself, calling PQgetResult
will still cause the client to block until the server completes the next SQL command. This can be avoided by proper use of two more functions:
PQconsumeInput
If input is available from the server, consume it.
int PQconsumeInput(PGconn *conn);
PQconsumeInput
normally returns 1 indicating “no error”, but returns 0 if there was some kind of trouble (in which casePQerrorMessage
can be consulted). Note that the result does not say whether any input data was actually collected. After callingPQconsumeInput
, the application can checkPQisBusy
and/orPQnotifies
to see if their state has changed.PQconsumeInput
can be called even if the application is not prepared to deal with a result or notification just yet. The function will read available data and save it in a buffer, thereby causing aselect()
read-ready indication to go away. The application can thus usePQconsumeInput
to clear theselect()
condition immediately, and then examine the results at leisure.PQisBusy
Returns 1 if a command is busy, that is,
PQgetResult
would block waiting for input. A 0 return indicates thatPQgetResult
can be called with assurance of not blocking.int PQisBusy(PGconn *conn);
PQisBusy
will not itself attempt to read data from the server; thereforePQconsumeInput
must be invoked first, or the busy state will never end.
A typical application using these functions will have a main loop that uses select()
or poll()
to wait for all the conditions that it must respond to. One of the conditions will be input available from the server, which in terms of select()
means readable data on the file descriptor identified by PQsocket
. When the main loop detects input ready, it should call PQconsumeInput
to read the input. It can then call PQisBusy
, followed by PQgetResult
if PQisBusy
returns false (0). It can also call PQnotifies
to detect NOTIFY
messages (see Section 32.9).
A client that uses PQsendQuery
/PQgetResult
can also attempt to cancel a command that is still being processed by the server; see Section 32.7. But regardless of the return value of PQcancel
, the application must continue with the normal result-reading sequence using PQgetResult
. A successful cancellation will simply cause the command to terminate sooner than it would have otherwise.
By using the functions described above, it is possible to avoid blocking while waiting for input from the database server. However, it is still possible that the application will block waiting to send output to the server. This is relatively uncommon but can happen if very long SQL commands or data values are sent. (It is much more probable if the application sends data via COPY IN
, however.) To prevent this possibility and achieve completely nonblocking database operation, the following additional functions can be used.
PQsetnonblocking
Sets the nonblocking status of the connection.
int PQsetnonblocking(PGconn *conn, int arg);
Sets the state of the connection to nonblocking if
arg
is 1, or blocking ifarg
is 0. Returns 0 if OK, -1 if error.In the nonblocking state, successful calls to
PQsendQuery
,PQputline
,PQputnbytes
,PQputCopyData
, andPQendcopy
will not block; their changes are stored in the local output buffer until they are flushed. Unsuccessful calls will return an error and must be retried.Note that
PQexec
does not honor nonblocking mode; if it is called, it will act in blocking fashion anyway.PQisnonblocking
Returns the blocking status of the database connection.
int PQisnonblocking(const PGconn *conn);
Returns 1 if the connection is set to nonblocking mode and 0 if blocking.
PQflush
Attempts to flush any queued output data to the server. Returns 0 if successful (or if the send queue is empty), -1 if it failed for some reason, or 1 if it was unable to send all the data in the send queue yet (this case can only occur if the connection is nonblocking).
int PQflush(PGconn *conn);
After sending any command or data on a nonblocking connection, call PQflush
. If it returns 1, wait for the socket to become read- or write-ready. If it becomes write-ready, call PQflush
again. If it becomes read-ready, call PQconsumeInput
, then call PQflush
again. Repeat until PQflush
returns 0. (It is necessary to check for read-ready and drain the input with PQconsumeInput
, because the server can block trying to send us data, e.g., NOTICE messages, and won't read our data until we read its.) Once PQflush
returns 0, wait for the socket to be read-ready and then read the response as described above.