Function ATController

src/backend/commands/tablecmds.c:5252–5351

* ATController provides top level control over the phases. * * parsetree is passed in to allow it to be passed to event triggers * when requested. */

 * when requested.
 */
static void
ATController(AlterTableStmt *parsetree,
			 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode,
			 AlterTableUtilityContext *context)
{
	List	   *wqueue = NIL;
	ListCell   *lcmd;

	cdb_sync_oid_to_segments();

	/*
	 * If the parsetree is dispatched from QD, we use the wqueue generated in QD
	 * and ignore all subcmds that will be generated in QEs. Because the parsetree
	 * and its subcmds in `AlteredTableInfo` contain some info that must sync
	 * between QD and QEs, e.g. index name, sequence name. It's not allowed to
	 * choose the relation/index/sequence/etc. names on QEs
	 *
	 * The following phase 1 should also be executed in QEs, which is different to
	 * the previous version. When ALTER a column type with IDENTITY, the generated
	 * beforeStmts are also executed. In other words, the phase 1 does not only
	 * preparation step, but also execute some utilities, which can't be omitted.
	 * We only run the phase 1, but discard all generated subcmds by QE itself.
	 *
	 * GPDB_13_MERGE_FIXME:
	 * ATController() is called in two different situations:
	 * 1. By AlterTable(), parsetree and context are not NULL, and parsetree->wqueue
	 *     is dispatched from QD, we ignore all the subcmds generated in QE side.
	 * 2. By AlterTableInternal(), which is used in internal utility commands.
	 *    The parsetree and context are NULL, so the QE needs to build their own
	 *    subcmds normally.
	 *
	 * However, some internal functions doesn't know the difference. Currently,
	 * we assume that AlterTableInternal() always passes `context` as NULL,
	 * and the cmd->subtype isn't complicated, i.e. it doesn't call
	 * transformAlterTableStmt() internally, could not be AlterColumnType.
	 * If any of the assumptions is broken, we must refactor the logic to
	 * adapter GPDB.
	 */
	if (parsetree && parsetree->wqueue)
	{
		ListCell   *lc;

		Assert(Gp_role == GP_ROLE_EXECUTE);
		wqueue = parsetree->wqueue;

		foreach (lc, wqueue)
		{
			/*
			 * The old tuple descriptors are not dispatched, so fetch
			 * them here.
			 */
			AlteredTableInfo *tab = (AlteredTableInfo *) lfirst(lc);
			Relation rel;

			rel = relation_open(tab->relid, lockmode);
			tab->oldDesc = CreateTupleDescCopyConstr(RelationGetDescr(rel));
			relation_close(rel, NoLock);
		}