Function DescribeQuery

src/bin/psql/common.c:1468–1571

* DescribeQuery: describe the result columns of a query, without executing it * * Returns true if the operation executed successfully, false otherwise. * * If pset.timing is on, total query time (exclusive of result-printing) is * stored into *elapsed_msec. */

 * stored into *elapsed_msec.
 */
static bool
DescribeQuery(const char *query, double *elapsed_msec)
{
	PGresult   *results;
	bool		OK;
	instr_time	before,
				after;

	*elapsed_msec = 0;

	if (pset.timing)
		INSTR_TIME_SET_CURRENT(before);

	/*
	 * To parse the query but not execute it, we prepare it, using the unnamed
	 * prepared statement.  This is invisible to psql users, since there's no
	 * way to access the unnamed prepared statement from psql user space. The
	 * next Parse or Query protocol message would overwrite the statement
	 * anyway.  (So there's no great need to clear it when done, which is a
	 * good thing because libpq provides no easy way to do that.)
	 */
	results = PQprepare(pset.db, "", query, 0, NULL);
	if (PQresultStatus(results) != PGRES_COMMAND_OK)
	{
		pg_log_info("%s", PQerrorMessage(pset.db));
		SetResultVariables(results, false);
		ClearOrSaveResult(results);
		return false;
	}
	PQclear(results);

	results = PQdescribePrepared(pset.db, "");
	OK = AcceptResult(results) &&
		(PQresultStatus(results) == PGRES_COMMAND_OK);
	if (OK && results)
	{
		if (PQnfields(results) > 0)
		{
			PQExpBufferData buf;
			int			i;

			initPQExpBuffer(&buf);

			printfPQExpBuffer(&buf,
							  "SELECT name AS \"%s\", pg_catalog.format_type(tp, tpm) AS \"%s\"\n"
							  "FROM (VALUES ",
							  gettext_noop("Column"),
							  gettext_noop("Type"));

			for (i = 0; i < PQnfields(results); i++)
			{
				const char *name;
				char	   *escname;

				if (i > 0)
					appendPQExpBufferStr(&buf, ",");

				name = PQfname(results, i);