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Subject [GitHub] hequn8128 edited a comment on issue #6519: [FLINK-9559] [table] The type of a union of CHAR columns of different lengths should be VARCHAR
Date Tue, 28 Aug 2018 09:25:39 GMT
hequn8128 edited a comment on issue #6519: [FLINK-9559] [table] The type of a union of CHAR
columns of different lengths should be VARCHAR
URL: https://github.com/apache/flink/pull/6519#issuecomment-416501305
 
 
   @pnowojski  Thanks for looking into it.
    I think there are bugs in all your 4 sqls, so we can't get the conclusions from them.
   - In mysql, trailing spaces are removed when CHAR values are retrieved from a table. 
   - You use literal of length 3('aa ') to compare to CHAR(4) and VARCHAR(4).
   - Except for mysql, other database will not remove trailing spaces so that the result of
`case when` contains blanks.
   
   What result type of `case when`? Should we change case when result type to varchar? I think
we can get the answer form the following sqls.
   1. Mysql
   There are no doubt, since mysql Reference Manual says the return type is VARCHAR.
   
   2. Oracle
   [http://sqlfiddle.com/#!4/cd577/2/0](http://sqlfiddle.com/#!4/cd577/2/0)
   
   3. PostgreSql
   [http://sqlfiddle.com/#!17/c20bd/3/0](http://sqlfiddle.com/#!17/c20bd/3/0)
   
   4. SqlServer
   [http://sqlfiddle.com/#!18/c20bd/10/0](http://sqlfiddle.com/#!18/c20bd/10/0)
   
   From the results, we can see that
   - There are no trailing spaces 
   - The result type length should not be 1, i.e., can not be char(1) or varchar(1). It should
be char(3) or varchar(3)
   - Result of `concat(CAST('a' AS char(3)), '|')` contains blanks while `concat(CAST('a'
AS varchar(3)), '|')`  not, so the result type of `case when` should be varchar.
   
   In Calcite, character constant, for example 'Hello, world!', '', _N'Bonjour', _ISO-8859-1'It''s
superman!' COLLATE SHIFT_JIS$ja_JP$2. These are always CHAR, never VARCHAR. So it seems not
possible or acceptable to change all literals to VARCHAR in Calcite.
   I think it is meaningful to turn the result type of `case when` to VARCHAR. What do you
think?
   
   Thanks, Hequn
   

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