Trabalhando com Constraints - PostgreSQL

create-drop.sql

SELECT 
  'ALTER TABLE ' || 
  schema.nspname || '.' || class.relname
  || ' DROP CONSTRAINT IF EXISTS ' || 
  cons.conname || ';'  
FROM pg_constraint AS cons
JOIN pg_class AS class on class.oid = cons.conrelid
JOIN pg_namespace AS schema on schema.oid = cons.connamespace
WHERE schema.nspname = 'schema'
  AND class.relname = 'tabela';