Illegal column for DataTypeNullable:Nullable(String)

[lilinghai]

create table t(a varchar(10));
alter table t set tiflash replica 2;
insert into t values("a");
select count(*) from t where coalesce(a,null)="a"; -- expect result is 1
-- but got ERROR 1105 (HY000): other error for mpp stream: DB::Exception: Illegal column for DataTypeNullable:Nullable(String) [column_name=coalesce(a, NULL_Nullable(String))_collator_46 ] [created=1] [nullable=0] [const=0]
/*
desc select count(*) from t where coalesce(a,null)="a";
+----------------------------+---------+--------------+---------------+---------------------------------+
| id                         | estRows | task         | access object | operator info                   |
+----------------------------+---------+--------------+---------------+---------------------------------+
| StreamAgg_10               | 1.00    | root         |               | funcs:count(1)->Column#3        |
| └─TableReader_13           | 0.80    | root         |               | data:Selection_12               |
|   └─Selection_12           | 0.80    | cop[tiflash] |               | eq(coalesce(rs.t.a, NULL), "a") |
|     └─TableFullScan_11     | 1.00    | cop[tiflash] | table:t       | keep order:false, stats:pseudo  |
+----------------------------+---------+--------------+---------------+---------------------------------+
*/

master nightly UTC Build Time: 2021-11-04 06:29:34
TiDB dcbfda914ad0b2647edc39788e0a6e8b5d90717d
tiflash 89c472f

[fuzhe1989]

Reproduced in unittest:

    const String & func_name = "coalesce";

    ASSERT_COLUMN_EQ(
        createColumn<Nullable<String>>({"a"}),
        executeFunction(
            func_name,
            createColumn<Nullable<String>>({"a"}),
            createConstColumn<Nullable<String>>(1, std::nullopt)));

Value of: DB::tests::columnEqual((createColumn<Nullable<String>>({"a"})), (executeFunction( func_name, createColumn<Nullable<String>>({"a"}), createConstColumn<Nullable<String>>(1, std::nullopt))))
  Actual: false (Column name mismatch
expected->getName(): Nullable(String)
actual->getName(): String)
Expected: true