Function exec_describe_statement_message

src/backend/tcop/postgres.c:3405–3493

* exec_describe_statement_message * * Process a "Describe" message for a prepared statement */

 * Process a "Describe" message for a prepared statement
 */
static void
exec_describe_statement_message(const char *stmt_name)
{
	CachedPlanSource *psrc;

	/*
	 * Start up a transaction command. (Note that this will normally change
	 * current memory context.) Nothing happens if we are already in one.
	 */
	start_xact_command();

	/* Switch back to message context */
	MemoryContextSwitchTo(MessageContext);

	/* Find prepared statement */
	if (stmt_name[0] != '\0')
	{
		PreparedStatement *pstmt;

		pstmt = FetchPreparedStatement(stmt_name, true);
		psrc = pstmt->plansource;
	}
	else
	{
		/* special-case the unnamed statement */
		psrc = unnamed_stmt_psrc;
		if (!psrc)
			ereport(ERROR,
					(errcode(ERRCODE_UNDEFINED_PSTATEMENT),
					 errmsg("unnamed prepared statement does not exist")));
	}

	/* Prepared statements shouldn't have changeable result descs */
	Assert(psrc->fixed_result);

	/*
	 * If we are in aborted transaction state, we can't run
	 * SendRowDescriptionMessage(), because that needs catalog accesses.
	 * Hence, refuse to Describe statements that return data.  (We shouldn't
	 * just refuse all Describes, since that might break the ability of some
	 * clients to issue COMMIT or ROLLBACK commands, if they use code that
	 * blindly Describes whatever it does.)  We can Describe parameters
	 * without doing anything dangerous, so we don't restrict that.
	 */
	if (IsAbortedTransactionBlockState() &&
		psrc->resultDesc)
		ereport(ERROR,
				(errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION),
				 errmsg("current transaction is aborted, "
						"commands ignored until end of transaction block"),
				 errdetail_abort()));

	if (whereToSendOutput != DestRemote)
		return;					/* can't actually do anything... */

	/*
	 * First describe the parameters...
	 */