[CH] Support convert Char type of HiveTable to FixedString in ClickHouse backend

[KevinyhZou]

Description

Now that the char/varchar type of hive table are always convert to string type in clickhouse backend. And the char type in hive table means the field has fixed bytes, which is suitable to convert to FixedString type in clickhouse.

And in some suitation, the fixed string type can have better performance that the string type in clickhouse. such as the querys contains order by.

Here we defined 2 tables, has the same data, the first one is defined as test_tbl1(id UInt64, d String), and the second is defined as test_tbl2(id UInt64, d FixedString(20)), and we do the query select * from test_tbl order by d limit 100 settings max_threads=1 to compare the performance

In the first table, the result show as below

In the second table, the result show as below

and we can see that abort 20% ~ 30% performance improve when the table field is defined as FixedString type.