type coerce issues

[zhouqingqing]

We have some type coerce code in place but needs strengthen it.

Tasks:

1. add more test cases covering type coerce;
   • operators like +/-/like
   • functions like year/Aggs
   • built in language like UNION, CASE
2. bug fixes based with above test cases.

Refs: SQL Server data types conversion rules PostgreSQL data types conversion algorithms

Known issues:

• sql = "select year(date('2011-01-01'));";