Bit shift for BLOB type behaves incosistently with MySQL8 about tidb

[zanmato1984]

MySQL's right shift supports two types: bigint and binary [1]. So no casting is applied to t2.a >> 8 and it evaluates to a binary as well. Whereas TiDB so far only supports bigint for right shift [2], this is why an implicit cast to bigint is applied to t2.a and this cast evaluates to 0 (with warning saying truncation). Then t2.a >> 8 still evaluates to a bigint 0. Then it is compared with binary (t3.a), causing both sides being casted to double, and the comparison is true for 0 and 0.

[1] https://dev.mysql.com/doc/refman/8.4/en/bit-functions.html#operator_right-shift
[2]

tidb/pkg/expression/builtin_op.go

Line 420 in 29fc940

func (b *builtinRightShiftSig) evalInt(ctx EvalContext, row chunk.Row) (int64, bool, error) {

[wengsy150943]

We explain the execution plan. It seems that TiDB will cast blob into other types before right shift, which applies incorrect truncation.

explain select * from t2,t3 where (t2.a >> 8) = t3.a;
+------------------------------+---------+-----------+---------------+---------------------------------------------------------------------------------------------------------+
| id                           | estRows | task      | access object | operator info                                                                                           |
+------------------------------+---------+-----------+---------------+---------------------------------------------------------------------------------------------------------+
| HashJoin_12                  | 1.00    | root      |               | inner join, equal:[eq(Column#5, Column#6)]                                                              |
| ├─Projection_17(Build)       | 1.00    | root      |               | test.t3.a, cast(test.t3.a, double BINARY)->Column#6                                                     |
| │ └─TableReader_19           | 1.00    | root      |               | data:TableFullScan_18                                                                                   |
| │   └─TableFullScan_18       | 1.00    | cop[tikv] | table:t3      | keep order:false, stats:pseudo                                                                          |
| └─Projection_14(Probe)       | 1.00    | root      |               | test.t2.a, cast(rightshift(cast(test.t2.a, bigint(65535) BINARY), 8), double UNSIGNED BINARY)->Column#5 |
|   └─TableReader_16           | 1.00    | root      |               | data:TableFullScan_15                                                                                   |
|     └─TableFullScan_15       | 1.00    | cop[tikv] | table:t2      | keep order:false, stats:pseudo                                                                          |
+------------------------------+---------+-----------+---------------+---------------------------------------------------------------------------------------------------------+