admin: fix admin check table compare bug

[crazycs520]

What problem does this PR solve?

SQL:

drop table if exists td1;
CREATE TABLE td1 (c2 INT NULL DEFAULT '70');
INSERT INTO td1 SET c2 = '5';
ALTER TABLE td1 ADD COLUMN c4 DECIMAL(12,8) NULL DEFAULT '213.41598062';
ALTER TABLE td1 ADD INDEX id2 (c4) ;
ADMIN CHECK TABLE td1;

root cause
decimal type is not reflect.DeepEqual between orginal datum and the datum after codec.encode and decode.
eg: 0.1234

origin:
&types.MyDecimal{digitsInt:1, digitsFrac:4, resultFrac:4, negative:false, wordBuf:[9]int32{0, 123400000, 0, 0, 0, 0, 0, 0, 0}} !=
after codec encode and decode:
&types.MyDecimal{digitsInt:0, digitsFrac:4, resultFrac:4, negative:false, wordBuf:[9]int32{123400000, 0, 0, 0, 0, 0, 0, 0, 0}}

What is changed and how it works?

Use Datum. CompareDatum() to replace reflect.DeepEqual in admin check table when datum type is decimal.

Check List

Tests

• Unit test

Code changes

Side effects

• Possible performance regression

Related changes

[crazycs520]

/run-all-tests

[shenli]

LGTM

[XuHuaiyu]

LGTM