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 one PGresult structure. If the submitted command string contains multiple SQL commands, all but the last PGresult are discarded by PQexec.
PQexec always collects the command's entire result, buffering it in a single PGresult. 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, PQsendDescribePortal, PQsendClosePrepared, and PQsendClosePortal, which can be used with PQgetResult to duplicate the functionality of PQexecParams, PQprepare, PQexecPrepared, PQdescribePrepared, PQdescribePortal, PQclosePrepared, and PQclosePortal 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, call PQgetResult one or more times to obtain the results. PQsendQuery cannot be called again (on the same connection) until PQgetResult has returned a null pointer, indicating that the command is done.
In pipeline mode, this function is 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 to PQexecParams. Like PQexecParams, 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, call PQgetResult to determine whether the server successfully created the prepared statement. The function's parameters are handled identically to PQprepare.
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 to PQexecPrepared.
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, call PQgetResult to obtain the results. The function's parameters are handled identically to PQdescribePrepared.
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, call PQgetResult to obtain the results. The function's parameters are handled identically to PQdescribePortal.
PQsendClosePrepared #Submits a request to close the specified prepared statement, without waiting for completion.
int PQsendClosePrepared(PGconn *conn, const char *stmtName);
This is an asynchronous version of PQclosePrepared: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, call PQgetResult to obtain the results. The function's parameters are handled identically to PQclosePrepared.
PQsendClosePortal #Submits a request to close specified portal, without waiting for completion.
int PQsendClosePortal(PGconn *conn, const char *portalName);
This is an asynchronous version of PQclosePortal: it returns 1 if it was able to dispatch the request, and 0 if not. After a successful call, call PQgetResult to obtain the results. The function's parameters are handled identically to PQclosePortal.
PQgetResult #Waits for the next result from a prior PQsendQuery, PQsendQueryParams, PQsendPrepare, PQsendQueryPrepared, PQsendDescribePrepared, PQsendDescribePortal, PQsendClosePrepared, PQsendClosePortal, PQsendPipelineSync, or PQpipelineSync 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 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 command is active and the necessary response data has not yet been read by PQconsumeInput .
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 type PGRES_PIPELINE_ABORTED will be returned, and a null pointer will be returned after it. When the pipeline synchronization point is reached, a result of type PGRES_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).
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 limited number of rows 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 case PQerrorMessage can be consulted). Note that the result does not say whether any input data was actually collected. After calling PQconsumeInput , the application can check PQisBusy and/or PQnotifies 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 a select() 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 1 if a command is busy, that is, PQgetResult would block waiting for input. A 0 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 server; therefore PQconsumeInput 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