Function PQsendQueryStart

src/interfaces/libpq/fe-exec.c:1660–1732

 */

bool
PQsendQueryStart(PGconn *conn, bool newQuery)
{
	if (!conn)
		return false;

	/*
	 * If this is the beginning of a query cycle, reset the error buffer.
	 */
	if (newQuery)
		resetPQExpBuffer(&conn->errorMessage);

	/* Don't try to send if we know there's no live connection. */
	if (conn->status != CONNECTION_OK)
	{
		appendPQExpBufferStr(&conn->errorMessage,
							 libpq_gettext("no connection to the server\n"));
		return false;
	}

	/* Can't send while already busy, either, unless enqueuing for later */
	if (conn->asyncStatus != PGASYNC_IDLE &&
		conn->pipelineStatus == PQ_PIPELINE_OFF)
	{
		appendPQExpBufferStr(&conn->errorMessage,
							 libpq_gettext("another command is already in progress\n"));
		return false;
	}

	if (conn->pipelineStatus != PQ_PIPELINE_OFF)
	{
		/*
		 * When enqueuing commands we don't change much of the connection
		 * state since it's already in use for the current command. The
		 * connection state will get updated when pqPipelineProcessQueue()
		 * advances to start processing the queued message.
		 *
		 * Just make sure we can safely enqueue given the current connection
		 * state. We can enqueue behind another queue item, or behind a
		 * non-queue command (one that sends its own sync), but we can't
		 * enqueue if the connection is in a copy state.
		 */
		switch (conn->asyncStatus)
		{
			case PGASYNC_IDLE:
			case PGASYNC_READY:
			case PGASYNC_READY_MORE:
			case PGASYNC_BUSY:
				/* ok to queue */
				break;
			case PGASYNC_COPY_IN:
			case PGASYNC_COPY_OUT:
			case PGASYNC_COPY_BOTH:
				appendPQExpBufferStr(&conn->errorMessage,
									 libpq_gettext("cannot queue commands during COPY\n"));
				return false;
		}
	}