The result of `where <string>` is wrong because it will be converted to `int` type.

[lilinghai]

release-5.3

create table t(a float,b double);
alter table t set tiflash replica 2;
insert into t values(0.1,0.2),(0.2,0.2);
set @@tidb_enforce_mpp=on;
select count(*) from t group by a having case when count(*) then sum(b) else 'a' end; -- expect 1,1 ; but got empty result
/*
mysql> explain select count(*) from t group by a having case when count(*) then sum(b) else 'a' end;
+----------------------------------------+---------+-------------------+---------------+----------------------------------------------------------------------------------------------+
| id                                     | estRows | task              | access object | operator info                                                                                |
+----------------------------------------+---------+-------------------+---------------+----------------------------------------------------------------------------------------------+
| TableReader_48                         | 1.28    | root              |               | data:ExchangeSender_47                                                                       |
| └─ExchangeSender_47                    | 1.28    | batchCop[tiflash] |               | ExchangeType: PassThrough                                                                    |
|   └─Projection_7                       | 1.28    | batchCop[tiflash] |               | Column#4                                                                                     |
|     └─Selection_39                     | 1.28    | batchCop[tiflash] |               | case(Column#4, cast(Column#5, var_string(5)), "a")                                           |
|       └─Projection_43                  | 1.60    | batchCop[tiflash] |               | Column#4, Column#5                                                                           |
|         └─HashAgg_44                   | 1.60    | batchCop[tiflash] |               | group by:aggregation2.t.a, funcs:sum(Column#13)->Column#4, funcs:sum(Column#14)->Column#5    |
|           └─ExchangeReceiver_46        | 1.60    | batchCop[tiflash] |               |                                                                                              |
|             └─ExchangeSender_45        | 1.60    | batchCop[tiflash] |               | ExchangeType: HashPartition, Hash Cols: [name: aggregation2.t.a, collate: N/A]               |
|               └─HashAgg_41             | 1.60    | batchCop[tiflash] |               | group by:aggregation2.t.a, funcs:count(1)->Column#13, funcs:sum(aggregation2.t.b)->Column#14 |
|                 └─TableFullScan_29     | 2.00    | batchCop[tiflash] | table:t       | keep order:false, stats:pseudo                                                               |
+----------------------------------------+---------+-------------------+---------------+----------------------------------------------------------------------------------------------+
10 rows in set (0.04 sec)

[LittleFall]

minimal reproduce:

create table t(a char(5));
alter table t set tiflash replica 1;
insert into t values ('0.1');
mysql> select /*+ read_from_storage(tiflash[t]) */ * from t where a;
Empty set (0.01 sec)

[LittleFall]

root cause:

the semantics of where a is similar than where a != 0, which is done in function DAGExpressionAnalyzer::convertToUInt8. but when a is string type, this function will add a castToInt, which will convert 0.1 to 0, that's not compatible with tidb/tikv. we should add a castToDouble instead.