planner: Unexpected TableScan

[qw4990]

Enhancement

create table t (a int, b int, key(a));
insert into t values (1, 1);
analyze table t;
explain select a from t;
+-----------------------+---------+-----------+---------------+----------------------+
| id                    | estRows | task      | access object | operator info        |
+-----------------------+---------+-----------+---------------+----------------------+
| TableReader_5         | 1.00    | root      |               | data:TableFullScan_4 |
| └─TableFullScan_4     | 1.00    | cop[tikv] | table:t       | keep order:false     |
+-----------------------+---------+-----------+---------------+----------------------+
2 rows in set (0.01 sec)

The above case should use an IndexScan instead of a TableScan since IndexScan has a smaller row-size.

[qw4990]

The root cause is that the current way to calculate row-size is too complex and not robust.
This can be solved by #35240.