Don't pay attention to uselessness of the provided query, it's just a simplified part of a complex one.
I run query:
SELECT elem FROM (SELECT id FROM data) AS elem;
It yields the result:
elem
------
(5)
(4)
(24)
(3)
(23)
Why does each value enclosed in parentheses?
So, to fix it I do:
SELECT trim(elem::text, ')(') FROM (SELECT id FROM data) AS elem;
I have a feeling it should not be that way...
https://www.postgresql.org/docs/9.6/static/rowtypes.html#ROWTYPES-USAGE
In PostgreSQL, a reference to a table name (or alias) in a query is effectively a reference to the composite value of the table's current row
and futher querying examples, also further regarding brackets:
The external text representation of a composite value consists of items that are interpreted according to the I/O conversion rules for the individual field types, plus decoration that indicates the composite structure. The decoration consists of parentheses (( and )) around the whole value, plus commas (,) between adjacent items.
also some relevant here: https://www.postgresql.org/docs/current/static/sql-expressions.html#SQL-SYNTAX-ROW-CONSTRUCTORS
You select rows. Select columns instead:
SELECT elem FROM (SELECT id FROM data) AS tbl(elem);