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[BUGS] seems column(alias_name) is a valid syntax? I cant find anythingalike in docs - Mailing list pgsql-bugs

From Vladimir Svedov
Subject [BUGS] seems column(alias_name) is a valid syntax? I cant find anythingalike in docs
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Msg-id CADqDLE-+wsBAms6WcjwXsrzKDZpi3gKuN04xAwU-EiSiVXzeGA@mail.gmail.com
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Hi,
Sorry If bother you again for no reason. Was surprised by behaviour described in

https://stackoverflow.com/questions/45752412/count-column-with-name-count-returns-multiple-rows-why/45753374?noredirect=1#comment78464299_45753374

Quite soon found out that column_name(alias_name) seems a valid syntax, below a(c) works - is it a feature? I could not find it anywhere in docs. 
s=# with c(a,b) as (values(1,2),(2,3))
select a(c),(c).a from c;a | a 
---+---1 | 12 | 2
(2 rows)
Why would I use such syntax in first place? as described in SO original post, he has a column named "count" and thus 
select count(table_name.*) from table_name
returns column "count" instead of number of rows. I can't tell why would you name column with function name.

So please help finding the docs and understanding why it works.

Thank you

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