index buckets bound couldn't decode itself out with corresponding index column type.

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use test
create table t(a datetime, b int, index ia(a,b));
insert into t value('2023-12-27',1),(null, 2),('2023-12-28',3),(null,4);
analyze table t with 0 topn;
explain select * from t where a > 1;
show stats_buckets where db_name = 'test';

AND mock the code in v6.5.4 showExec

// ValueToString converts a possible encoded value to a formatted string. If the value is encoded, then
// idxCols equals to number of origin values, else idxCols is 0.
func ValueToString(vars *variable.SessionVars, value *types.Datum, idxCols int, idxColumnTypes []byte) (string, error) {
	if idxCols == 0 {
		return value.ToString()
	}
	var loc *time.Location
	if vars != nil {
		loc = vars.Location()
	}
	// Ignore the error and treat remaining part that cannot decode successfully as bytes.
	decodedVals, remained, err := codec.DecodeRange(value.GetBytes(), idxCols, idxColumnTypes, loc)
	// Ignore err explicit to pass errcheck.
        // mock the code here, rather than ignore it!!!
	if err != nil {
		panic(err.Error())
	}
	if len(remained) > 0 {
		decodedVals = append(decodedVals, types.NewBytesDatum(remained))
	}
	str, err := types.DatumsToString(decodedVals, true)
	return str, err
}

2. What did you expect to see? (Required)

show stats_buckets where table_name = 'xxx' and column_name = 'xxx'\G

3. What did you see instead (Required)

show stats_buckets successfully

4. What is your TiDB version? (Required)

panic with recovery with msg: invalid encoded key flag 0

[AilinKid]

from @time-and-fate the minumun reproduce step

use test
create table t(a datetime, b int, index ia(a,b));
insert into t value('2023-12-27',1),(null, 2),('2023-12-28',3),(null,4);
analyze table t with 0 topn;
explain select * from t where a > 1;
show stats_buckets where db_name = 'test';

basic reason is that:

after fix this, show stats buckets can show correctness result

[tiancaiamao]

B.T.W, it's better to avoid the ONCALL id in github @AilinKid