duplicate CastValue when executing import into from select statement

[crazycs520]

Enhancement

duplicate CastValue when executing import into from select statement, it can be optimized to save CPU resource.

[lance6716]

Apart from the "duplication", I think the cast logic should have a fast path if the source and target type are the same type. In above CPU profile, the SQL statement is

IMPORT INTO `bdcp`.`result_tmp` FROM (
  SELECT 
    RPAD(`AC`, 25, _UTF8MB4' ') AS `AC`,
    RPAD(`CCY`, 3, _UTF8MB4' ') AS `CCY`
...

while the bdcp.result_tmp is

CREATE TABLE `result_tmp` (
  `AC` varchar(50) NOT NULL DEFAULT '',
  `CCY` varchar(50) NOT NULL DEFAULT '',
...

There's no need to run the checking logic in type cast