The PQexec function is adequate for submitting queries in simple synchronous applications. It has a couple of major deficiencies however:
PQexec waits for the query to be completed. The application may 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 control is buried inside PQexec, it is hard for the frontend to decide it would like to try to cancel the ongoing query. (It can be done from a signal handler, but not otherwise.)
PQexec can return only one PGresult structure. If the submitted query string contains multiple SQL commands, all but the last PGresult are discarded by PQexec.
Applications that do not like these limitations can instead use the underlying functions that PQexec is built from: PQsendQuery and PQgetResult.
PQsendQuery Submit a query to Postgres without waiting for the result(s). TRUE is returned if the query was successfully dispatched, FALSE if not (in which case, use PQerrorMessage to get more information about the failure).
int PQsendQuery(PGconn *conn,
const char *query);
After successfully calling PQsendQuery, call PQgetResult one
or more times to obtain the query results. PQsendQuery may not be
called again (on the same connection) until PQgetResult has
returned NULL, indicating that the query is done.
PQgetResult Wait for the next result from a prior PQsendQuery, and return it. NULL is returned when the query is complete and there will be no more results.
PGresult *PQgetResult(PGconn *conn);PQgetResult must be called repeatedly until it returns NULL, indicating that the query is done. (If called when no query is active, PQgetResult will just return NULL at once.) Each non-null result from PQgetResult should be processed using the same PGresult accessor functions previously described. Don't forget to free each result object with PQclear when done with it. Note that PQgetResult will block only if a query is active and the necessary response data has not yet been read by PQconsumeInput.
Using PQsendQuery and PQgetResult solves one of PQexec's problems: if a query 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 frontend can be handling the results of one query while the backend is still working on later queries in the same query string.) However, calling PQgetResult will still cause the frontend to block until the backend completes the next SQL command. This can be avoided by proper use of three more functions:
PQconsumeInput If input is available from the backend, 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 case PQerrorMessage is set). Note that the result does not say whether any input data was actually collected. After calling PQconsumeInput, the application may check PQisBusy and/or PQnotifies to see if their state has changed. PQconsumeInput may be called even if the application is not prepared to deal with a result or notification just yet. The routine will read available data and save it in a buffer, thereby causing a select(2) read-ready indication to go away. The application can thus use PQconsumeInput to clear the select condition immediately, and then examine the results at leisure.
PQisBusy Returns TRUE if a query is busy, that is, PQgetResult would block waiting for input. A FALSE return indicates that PQgetResult can be called with assurance of not blocking.
int PQisBusy(PGconn *conn);PQisBusy will not itself attempt to read data from the backend; therefore PQconsumeInput must be invoked first, or the busy state will never end.
PQsocket Obtain the file descriptor number for the backend connection socket. A valid descriptor will be >= 0; a result of -1 indicates that no backend connection is currently open.
int PQsocket(PGconn *conn);PQsocket should be used to obtain the backend socket descriptor in preparation for executing select(2). This allows an application to wait for either backend responses or other conditions. If the result of select(2) indicates that data can be read from the backend socket, then PQconsumeInput should be called to read the data; after which, PQisBusy, PQgetResult, and/or PQnotifies can be used to process the response.
A typical frontend using these functions will have a main loop that uses select(2) to wait for all the conditions that it must respond to. One of the conditions will be input available from the backend, which in select's terms is 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. It can also call PQnotifies to detect NOTIFY messages (see "Asynchronous Notification", below).
A frontend that uses PQsendQuery/PQgetResult can also attempt to cancel a query that is still being processed by the backend.
PQrequestCancel Request that Postgres abandon processing of the current query.
int PQrequestCancel(PGconn *conn);The return value is TRUE if the cancel request was successfully dispatched, FALSE if not. (If not, PQerrorMessage tells why not.) Successful dispatch is no guarantee that the request will have any effect, however. Regardless of the return value of PQrequestCancel, the application must continue with the normal result-reading sequence using PQgetResult. If the cancellation is effective, the current query will terminate early and return an error result. If the cancellation fails (say because the backend was already done processing the query), then there will be no visible result at all.
Note that if the current query is part of a transaction, cancellation will abort the whole transaction.
PQrequestCancel can safely be invoked from a signal handler. So, it is also possible to use it in conjunction with plain PQexec, if the decision to cancel can be made in a signal handler. For example, psql invokes PQrequestCancel from a SIGINT signal handler, thus allowing interactive cancellation of queries that it issues through PQexec. Note that PQrequestCancel will have no effect if the connection is not currently open or the backend is not currently processing a query.