expression: support decode more types through tidb_decode_key()

[tangenta]

What problem does this PR solve?

Problem Summary: Before this PR, it is not possible to use tidb_decode_key() to decode some column types like datetime or decimal because of the lack of table information.

create table t (a varchar(255), b int, c datetime, primary key(a, b, c));
split table t by ('bbbb', 10, '2020-01-01');
select start_key, table_id from information_schema.tikv_region_status s, information_schema.tables t where s.table_id = t.tidb_table_id and t.table_name = 't' and is_index = 0 order by start_key;

+--------------------------------------------------------------------------------------------+----------+
| start_key                                                                                  | table_id |
+--------------------------------------------------------------------------------------------+----------+
| 7480000000000000FF3400000000000000F8                                                       | 52       |
| 7480000000000000FF345F720162626262FF00000000FB038000FF00000000000A0419FFA542000000000000FE | 52       |
+--------------------------------------------------------------------------------------------+----------+

select tidb_decode_key('7480000000000000FF345F720162626262FF00000000FB038000FF00000000000A0419FFA542000000000000FE');   

+---------------------------------------------------------------------------------------------------------------+
| tidb_decode_key('7480000000000000FF345F720162626262FF00000000FB038000FF00000000000A0419FFA542000000000000FE') |
+---------------------------------------------------------------------------------------------------------------+
| {"handle":"{bbbb, 10, 1847955789872889856}","table_id":52}                                                    |
+---------------------------------------------------------------------------------------------------------------+

We can see that '2020-01-01' is decoded to 1847955789872889856, which is not expected.

What is changed and how it works?

What's Changed:

• For the convenience of parsing, the format of tidb_decode_key() is changed to JSON.
• tidb_decode_key() may not idempotent anymore:
  • If the corresponding table exists, TiDB tries to fetch the column information to decode the original value.
  • If no such table found, the old decoding method is used(cannot restore the original value).

select tidb_decode_key('7480000000000000FF345F720162626262FF00000000FB038000FF00000000000A0419FFA542000000000000FE');                   

+---------------------------------------------------------------------------------------------------------------+
| tidb_decode_key('7480000000000000FF345F720162626262FF00000000FB038000FF00000000000A0419FFA542000000000000FE') |
+---------------------------------------------------------------------------------------------------------------+
| {"handle":{"a":"bbbb","b":"10","c":"2020-01-01 00:00:00"},"table_id":52}                                      |
+---------------------------------------------------------------------------------------------------------------+

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test

Side effects

N/A

Release note

• No release note

[crazycs520]

lgtm

[lzmhhh123]

LGTM

[lzmhhh123]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@tangenta merge failed.

[coocood]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@tangenta merge failed.

[coocood]

/merge

[ti-srebot]

/run-all-tests

[coocood]

LGTM

[coocood]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@tangenta merge failed.

[coocood]

/run-unit-test