withRollup: The design doc for withRollup clause. (#4250)

[mamiao]

ret #4250

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/3d2d4d0631e9e6c9f58354313ed9bb384ce222d6

[Mini256]

It is better to put it in the code block for easy reviewing.

mysql> explain select a from t group by a; 
+--------------------------+---------+-----------+---------------+-------------------------------------------------------+
| id                       | estRows | task      | access object | operator info                                         |
+--------------------------+---------+-----------+---------------+-------------------------------------------------------+
| HashAgg_7                | 1.00    | root      |               | group by:test.t.a, funcs:firstrow(test.t.a)->test.t.a |
| └─TableReader_12         | 1.00    | root      |               | data:TableFullScan_11                                 |
|   └─TableFullScan_11     | 1.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo                        |
+--------------------------+---------+-----------+---------------+-------------------------------------------------------+
3 rows in set (0.01 sec)