admin: fix admin check table bug of byte compare

[crazycs520]

What problem does this PR solve?

execute below sql will return:

drop table if exists t1;
CREATE TABLE t1 (c1 int);
INSERT INTO t1 SET c1 = 1;
ALTER TABLE t1 ADD COLUMN cc1 CHAR(36) NULL DEFAULT '';
ALTER TABLE t1 ADD INDEX idx1 (cc1);
admin check table t1;

(8003, u't1 err:[admin:1]index:&admin.RecordData{Handle:1, Values:[]types.Datum{types.Datum{k:0x6, collation:0x0, decimal:0x0, length:0x0, i:0, b:[]uint8{}, x:interface {}(nil)}}} != record:&admin.RecordData{Handle:1, Values:[]types.Datum{types.Datum{k:0x6, collation:0x0, decimal:0x0, length:0x0, i:0, b:[]uint8(nil), x:interface {}(nil)}}}')

The root cause is hack.Slice() function, here is a demo:

b = []byte("")
b2 := hack.Slice("")

fmt.Println(b == nil)
fmt.Println(b2 == nil)
fmt.Println(bytes.Equal(b, b2))
fmt.Println(reflect.DeepEqual(b, b2))

/* out put is 
false
true
true
false
*/

What is changed and how it works?

Add benchmark about compare Datum:

goos: darwin
goarch: amd64
pkg: github.com/pingcap/tidb/types
BenchmarkCompareDatum           30000000               120 ns/op
BenchmarkCompareDatumByReflect   1000000              5387 ns/op

admin check uses Datum.Compare() instead of reflect.DeepEqual()

Check List

Tests

• Integration test

Side effects

Related changes

• Need to cherry-pick to the release branch ?

[crazycs520]

/run-all-tests

[jackysp]

I'm afraid that there may be some other kinds need to CompareDatum instead of DeepEqual.

[crazycs520]

done.
I used to think that using CompareDatum instead of reflect.DeepEqual may cause poor performance. I felt I was wrong after the benchmark test.

[crazycs520]

/run-all-tests

[jackysp]

LGTM

[winkyao]

LGTM