expression: fix cast json to decimal bug.

[crazycs520]

What problem does this PR solve?

sql

create table test ( a decimal(60,2) as (JSON_EXTRACT(b,'$.c')), b json );
insert into test (b) values 
('{"c": "1267.1"}'),
('{"c": "1267.01"}'),
('{"c": "1267.1234"}'),
('{"c": "1267.3456"}'),
('{"c": "1234567890123456789012345678901234567890123456789012345"}'),
('{"c": "1234567890123456789012345678901234567890123456789012345.12345"}');
select * from test\G

Original TiDB return

***************************[ 1. row ]***************************
a | 1267.1
b | {"c": "1267.1"}
***************************[ 2. row ]***************************
a | 1267.01
b | {"c": "1267.01"}
***************************[ 3. row ]***************************
a | 1267.1234
b | {"c": "1267.1234"}
***************************[ 4. row ]***************************
a | 1267.3456
b | {"c": "1267.3456"}
***************************[ 5. row ]***************************
a | 1234567890123456800000000000000000000000000000000000000
b | {"c": "1234567890123456789012345678901234567890123456789012345"}
***************************[ 6. row ]***************************
a | 1234567890123456800000000000000000000000000000000000000
b | {"c": "1234567890123456789012345678901234567890123456789012345.12345"}

Now:

***************************[ 1. row ]***************************
a | 1267.10
b | {"c": "1267.1"}
***************************[ 2. row ]***************************
a | 1267.01
b | {"c": "1267.01"}
***************************[ 3. row ]***************************
a | 1267.12
b | {"c": "1267.1234"}
***************************[ 4. row ]***************************
a | 1267.35
b | {"c": "1267.3456"}
***************************[ 5. row ]***************************
a | 1234567890123456789012345678901234567890123456789012345.00
b | {"c": "1234567890123456789012345678901234567890123456789012345"}
***************************[ 6. row ]***************************
a | 1234567890123456789012345678901234567890123456789012345.12
b | {"c": "1234567890123456789012345678901234567890123456789012345.12345"}

As you can see, original TiDB will lose precision and can‘t resolving big decimal number.

What is changed and how it works?

Add ConvertJSONToDecimal function, Use MyDecimal.FromStrin to parse json when json type is string.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

[XuHuaiyu]

Please add some integration test cases

[winoros]

What about create two new function NewDecFromFloat and NewDecFromString and call it here?

[crazycs520]

MyDecimal already have FromFloat64 and FromString methods. What is the difference between NewDecFromFloat and FromFloat64?

[crazycs520]

@XuHuaiyu done. PTAL

[zz-jason]

LGTM

[zz-jason]

@crazycs520 Should we cherrypick this commit to release-2.1 and release-2.0?

[crazycs520]

@zz-jason OK, I will do cherrypick after this merge.

[crazycs520]

/run-all-tests

[zz-jason]

@winoros @lamxTyler PTAL

[alivxxx]

LGTM