optimizer: Improve column name of the operator

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dev=> create table t(c STRUCT<x INTEGER, y INTEGER>);
dev=> create materialized view mv as select t1.c from t t1 join t t2 on (t1.c).y = (t2.c).y;
CREATE_MATERIALIZED_VIEW
dev=> show internal tables;
                   Name                   
------------------------------------------
 __internal_mv_3_hashjoindegreeright_1006
 __internal_mv_3_hashjoinright_1005
 __internal_mv_5_chain_1008
 __internal_mv_3_hashjoindegreeleft_1004
 __internal_mv_3_hashjoinleft_1003
 __internal_mv_4_chain_1007
(6 rows)

dev=> select * from __internal_mv_3_hashjoindegreeright_1006;
 $expr2 | t__row_id | _degree 
--------+-----------+---------
(0 rows)

The $expr2 is an alias referring to an expression in the plan which can get in the explain. But the user can not get the reference when he query the internal table.

dev=> explain create materialized view mv as select t1.c from t t1 join t t2 on (t1.c).y = (t2.c).y;
                                                                                                   QUERY PLAN                                                                                                   
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 StreamMaterialize { columns: [c, t._row_id(hidden), $expr1(hidden), t._row_id#1(hidden)], stream_key: [t._row_id, t._row_id#1, $expr1], pk_columns: [t._row_id, t._row_id#1, $expr1], pk_conflict: "NoCheck" }
 └─StreamHashJoin { type: Inner, predicate: $expr1 = $expr2 }
   ├─StreamExchange { dist: HashShard($expr1) }
   | └─StreamProject { exprs: [t.c, Field(t.c, 1:Int32) as $expr1, t._row_id] }
   |   └─StreamTableScan { table: t, columns: [c, _row_id] }
   └─StreamExchange { dist: HashShard($expr2) }
     └─StreamProject { exprs: [Field(t.c, 1:Int32) as $expr2, t._row_id] }
       └─StreamTableScan { table: t, columns: [c, _row_id] }

I propose to change the column name of the schema() to the expression name back. And add a column_alias() -> Vec<Option<String>> on the PlanNode with the alias.

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c.c. @chenzl25 @fuyufjh

[fuyufjh]

I acknowledge this inconvenience, but using an expression as column name such as is more confusing to me 🥲

[fuyufjh]

If we can provide some way to let user inspect the streaming job (i.e. a running materialized view), it might be better?

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