BUG #12625: operation 'union' confirms the type of the column with 2 unknown types.

The following bug has been logged on the website:

Bug reference:      12625
Logged by:          Chongwei Yuan
Email address:      yuanchongwei007@163.com
PostgreSQL version: 9.3.5
Operating system:   Ubuntu 14.04.1 LTS
Description:

1. exce sql [ select 'abc' as col ] ; result : column 'col' has unknow
type.
2. exce sql [ select 'def' as col ] ; result : column 'col' has unknow
type.
But
3. exce sql [ select 'abc' as col union select 'def' as col ] ; result :
colum 'col' has text type.