DROP命令（Commands）实现：

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/*
 * Drop one or more objects.
 *
 * We don't currently handle all object types here.  Relations, for example,
 * require special handling, because (for example) indexes have additional
 * locking requirements.
 *
 * We look up all the objects first, and then delete them in a single
 * performMultipleDeletions() call.  This avoids unnecessary DROP RESTRICT
 * errors if there are dependencies between them.
 */
void
RemoveObjects(DropStmt *stmt)




/*
 * Dispatch function for DropStmt
 */
static void
ExecDropStmt(DropStmt *stmt, bool isTopLevel)
{
	switch (stmt->removeType)
	{
		case OBJECT_INDEX:
			if (stmt->concurrent)
				PreventTransactionChain(isTopLevel,
										"DROP INDEX CONCURRENTLY");
			/* fall through */


		case OBJECT_TABLE:
		case OBJECT_SEQUENCE:
		case OBJECT_VIEW:
		case OBJECT_MATVIEW:
		case OBJECT_FOREIGN_TABLE:
			RemoveRelations(stmt);
			break;
		default:
			RemoveObjects(stmt);
			break;
	}
}


/*
 * standard_ProcessUtility itself deals only with utility commands for
 * which we do not provide event trigger support.  Commands that do have
 * such support are passed down to ProcessUtilitySlow, which contains the
 * necessary infrastructure for such triggers.
 *
 * This division is not just for performance: it's critical that the
 * event trigger code not be invoked when doing START TRANSACTION for
 * example, because we might need to refresh the event trigger cache,
 * which requires being in a valid transaction.
 */
void
standard_ProcessUtility(Node *parsetree,
						const char *queryString,
						ProcessUtilityContext context,
						ParamListInfo params,
						DestReceiver *dest,
						char *completionTag)
/*
 * ProcessUtility
 *		general utility function invoker
 *
 *	parsetree: the parse tree for the utility statement
 *	queryString: original source text of command
 *	context: identifies source of statement (toplevel client command,
 *		non-toplevel client command, subcommand of a larger utility command)
 *	params: parameters to use during execution
 *	dest: where to send results
 *	completionTag: points to a buffer of size COMPLETION_TAG_BUFSIZE
 *		in which to store a command completion status string.
 *
 * Notes: as of PG 8.4, caller MUST supply a queryString; it is not
 * allowed anymore to pass NULL.  (If you really don't have source text,
 * you can pass a constant string, perhaps "(query not available)".)
 *
 * completionTag is only set nonempty if we want to return a nondefault status.
 *
 * completionTag may be NULL if caller doesn't want a status string.
 */
void
ProcessUtility(Node *parsetree,
			   const char *queryString,
			   ProcessUtilityContext context,
			   ParamListInfo params,
			   DestReceiver *dest,
			   char *completionTag)

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详细调用栈如下：

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