planner: costs of Selection operators from DataSources are not accumulated

[qw4990]

Enhancement

mysql> create table t (a int);

mysql> insert into mysql.expr_pushdown_blacklist values ('>','tikv','');
Query OK, 1 row affected (0.00 sec)

mysql> admin reload expr_pushdown_blacklist;
Query OK, 0 rows affected (0.00 sec)

mysql> explain format=verbose select * from tx where a>0;
+-------------------------+----------+-----------+-----------+---------------+--------------------------------+
| id                      | estRows  | estCost   | task      | access object | operator info                  |
+-------------------------+----------+-----------+-----------+---------------+--------------------------------+
| Selection_7             | 3333.33  | 34418.00  | root      |               | gt(test.tx.a, 0)               |
| └─TableReader_6         | 10000.00 | 34418.00  | root      |               | data:TableFullScan_5           |
|   └─TableFullScan_5     | 10000.00 | 435000.00 | cop[tikv] | table:tx      | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+-----------+---------------+--------------------------------+
3 rows in set, 1 warning (0.01 sec)

As you can see above, Selection_7 has the same cost with TableReader_6, which is not correct.

[qw4990]

For safety, I'll just fix it in cost model ver2 and keep it unchanged in ver1.