hot to determine calculated fields (views)

the lights are beginning to glow. :)

those pg_* tables are really something! but how can i tell if an
attribute of a relation (field of a table) is calculated via a
view, or actual data from a table?

i'm trying to use the pg_* system tables in some views that'll
help my app determine various aspects of fields in the database,
such as how much of a limit to impose on data entry (varchar(20)
should get <input type="text" maxlength="20" ...> for example).

create view sys_field_size as
SELECT
c.relname as class,
a.attname as field,
a.attlen as storage,
a.attnum as field_no,
a.atttypmod as field_mod,
CASE
WHEN a.atttypmod<=0
THEN CASE
WHEN a.attlen<0
THEN -1
ELSE
NULL
END
ELSE
a.atttypmod - 4
END
AS SIZE
FROM
pg_attribute a
JOIN
pg_class c
ON c.oid = a.attrelid
WHERE
a.attnum > 0 -- only user-defined fields, thanks
AND
c.relname !~ '^pg_' -- not a postgres system table
AND
c.relkind IN ('v','r') -- view or relation/table
;

well, it's a start.

i've got tables (_name) and related views (name) where the views
do some munging to the fields for display, or they add new fields
based on the actual data in the table. for html-generation, i
don't want the calculated fields looking like data entry is
possible, so i need to distinguish them from the editable ones.

QUESTION:

how can i tell whether i'm looking at a computed field (from a
view) as opposed to actual data (brought in directly from a
table)? something in pg_attribute, i hope... :)

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