Reformat code in ATPostAlterTypeParse.

The code in ATPostAlterTypeParse was very deeply indented, mostly because
there were two nested switch-case statements, which add a lot of
indentation. Use if-else blocks instead, to make the code less indented
and more readable.

This is in preparation for next patch that makes some actualy changes to
the function. These cosmetic parts have been separated to make it easier
to see the real changes in the other patch.

src/backend/commands/tablecmds.c

@@ -8645,69 +8645,67 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 		Node	   *stm = (Node *) lfirst(list_item);
 		AlteredTableInfo *tab;
 
-		switch (nodeTag(stm))
+		tab = ATGetQueueEntry(wqueue, rel);
+
+		if (IsA(stm, IndexStmt))
+		{
+			IndexStmt  *stmt = (IndexStmt *) stm;
+			AlterTableCmd *newcmd;
+
+			if (!rewrite)
+				TryReuseIndex(oldId, stmt);
+
+			newcmd = makeNode(AlterTableCmd);
+			newcmd->subtype = AT_ReAddIndex;
+			newcmd->def = (Node *) stmt;
+			tab->subcmds[AT_PASS_OLD_INDEX] =
+				lappend(tab->subcmds[AT_PASS_OLD_INDEX], newcmd);
+		}
+		else if (IsA(stm, AlterTableStmt))
 		{
-			case T_IndexStmt:
+			AlterTableStmt *stmt = (AlterTableStmt *) stm;
+			ListCell   *lcmd;
+
+			foreach(lcmd, stmt->cmds)
+			{
+				AlterTableCmd *cmd = (AlterTableCmd *) lfirst(lcmd);
+
+				if (cmd->subtype == AT_AddIndex)
 				{
-					IndexStmt  *stmt = (IndexStmt *) stm;
-					AlterTableCmd *newcmd;
+					Assert(IsA(cmd->def, IndexStmt));
 
 					if (!rewrite)
-						TryReuseIndex(oldId, stmt);
+						TryReuseIndex(get_constraint_index(oldId),
+									  (IndexStmt *) cmd->def);
 
-					tab = ATGetQueueEntry(wqueue, rel);
-					newcmd = makeNode(AlterTableCmd);
-					newcmd->subtype = AT_ReAddIndex;
-					newcmd->def = (Node *) stmt;
+					cmd->subtype = AT_ReAddIndex;
 					tab->subcmds[AT_PASS_OLD_INDEX] =
-						lappend(tab->subcmds[AT_PASS_OLD_INDEX], newcmd);
-					break;
+						lappend(tab->subcmds[AT_PASS_OLD_INDEX], cmd);
 				}
-			case T_AlterTableStmt:
+				else if (cmd->subtype == AT_AddConstraint)
 				{
-					AlterTableStmt *stmt = (AlterTableStmt *) stm;
-					ListCell   *lcmd;
-
-					tab = ATGetQueueEntry(wqueue, rel);
-					foreach(lcmd, stmt->cmds)
-					{
-						AlterTableCmd *cmd = (AlterTableCmd *) lfirst(lcmd);
-						Constraint *con;
-
-						switch (cmd->subtype)
-						{
-							case AT_AddIndex:
-								Assert(IsA(cmd->def, IndexStmt));
-								if (!rewrite)
-									TryReuseIndex(get_constraint_index(oldId),
-												  (IndexStmt *) cmd->def);
-								cmd->subtype = AT_ReAddIndex;
-								tab->subcmds[AT_PASS_OLD_INDEX] =
-									lappend(tab->subcmds[AT_PASS_OLD_INDEX], cmd);
-								break;
-							case AT_AddConstraint:
-								Assert(IsA(cmd->def, Constraint));
-								con = (Constraint *) cmd->def;
-								con->old_pktable_oid = refRelId;
-								/* rewriting neither side of a FK */
-								if (con->contype == CONSTR_FOREIGN &&
-									!rewrite && tab->rewrite == 0)
-									TryReuseForeignKey(oldId, con);
-								cmd->subtype = AT_ReAddConstraint;
-								tab->subcmds[AT_PASS_OLD_CONSTR] =
-									lappend(tab->subcmds[AT_PASS_OLD_CONSTR], cmd);
-								break;
-							default:
-								elog(ERROR, "unexpected statement type: %d",
-									 (int) cmd->subtype);
-						}
-					}
-					break;
+					Constraint *con;
+
+					Assert(IsA(cmd->def, Constraint));
+
+					con = (Constraint *) cmd->def;
+					con->old_pktable_oid = refRelId;
+					/* rewriting neither side of a FK */
+					if (con->contype == CONSTR_FOREIGN &&
+						!rewrite && tab->rewrite == 0)
+						TryReuseForeignKey(oldId, con);
+					cmd->subtype = AT_ReAddConstraint;
+					tab->subcmds[AT_PASS_OLD_CONSTR] =
+						lappend(tab->subcmds[AT_PASS_OLD_CONSTR], cmd);
 				}
-			default:
-				elog(ERROR, "unexpected statement type: %d",
-					 (int) nodeTag(stm));
+				else
+					elog(ERROR, "unexpected statement type: %d",
+						 (int) cmd->subtype);
+			}
 		}
+		else
+			elog(ERROR, "unexpected statement type: %d",
+				 (int) nodeTag(stm));
 	}
 
 	relation_close(rel, NoLock);