[HACKERS] New CORRESPONDING clause design

I am new here and I really want to contribute, I have read same resource that help understanding database system and postgresql. I would like to start implementing sql syntax corresponding by clause because I believe implementing sql syntax gives an opportunity to familiarize  many part of postgresql source code. Previous implementation is here and have an issue on explain query and break cases on unlabeled NULLs

To repeat what a corresponding by clause means

Corresponding clause either contains a BY(...) clause or not. If it

doesn't have a BY(...) clause the usage is as follows.

 

SELECT 1 a, 2 b, 3 c UNION CORRESPONDING SELECT 4 b, 5 d, 6 c, 7 f;

 

with output:

b c

-----

2 3

4 6

 

i.e. matching column names are filtered and are only output from the

whole set operation clause.

 

If we introduce a BY(...) clause, then column names are further

intersected with that BY clause:

 

SELECT 1 a, 2 b, 3 c UNION CORRESPONDING BY(b) SELECT 4 b, 5 d, 6 c, 7 f;

 

with output:

 

b

--

2

4

 My design is

 In parsing stage

1. Check at least one common column name appear in queries  

2. If corresponding column list is not specified, then make corresponding list from common column name in queries target lists in the order 

    that those column names appear in first query

3. If corresponding column list is specified, then check that every column name 

    in The corresponding column list appears in column names of both queries

    In planner

1. Take projection columen name from corresponding list

2. Reorder left and right queries target lists according to corresponding list

3. For each query, project columens as many as number of corresponding columen.