Search notes:

Column constraint_type (DBA_CONSTRAINTS, ALL_CONSTRAINTS, USER_CONSTRAINTS)

select
  -- count(*),
  constraint_type
from
  dba_constraints
group by
  constraint_type
order by
  count(*) desc;
-- C
-- P
-- R
-- O
-- U
-- F
-- V

Github repository oracle-patterns, path: /Installed/data-dictionary/constraints/constraint_type.sql

The constraint type is a one letter flag which is one of (as of 12cR1):

C: Check constraint on a table
F: Constraint that involves a REF column
H: Hash expression
O: With read only, on a view
P: Primary key
R: Referential integrity (aka foreign key)
S: Supplemental logging
U: Unique key
V: With check option, on a view

Column constraint_type (DBA_CONSTRAINTS, ALL_CONSTRAINTS, USER_CONSTRAINTS)

See also