solo unos pocos tipos en pg tienen una longitud fija - casi todos los tipos son de tipo varlena - tiene una longitud dinámica. Puede consultar consultas como
postgres=# select typlen from pg_type where oid = 'int'::regtype::oid;
typlen
--------
4
(1 row)
postgres=# select attlen from pg_attribute where attrelid = 'x'::regclass and attname = 'a';
attlen
--------
4
(1 row)
Cuando el resultado no es -1, entonces el tipo no tiene una longitud fija
para los tipos de varlena use la función pg_column_size:
postgres=# \df *size*
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+------------------------+------------------+---------------------+--------
pg_catalog | pg_column_size | integer | "any" | normal
pg_catalog | pg_database_size | bigint | name | normal
pg_catalog | pg_database_size | bigint | oid | normal
pg_catalog | pg_indexes_size | bigint | regclass | normal
pg_catalog | pg_relation_size | bigint | regclass | normal
pg_catalog | pg_relation_size | bigint | regclass, text | normal
pg_catalog | pg_size_pretty | text | bigint | normal
pg_catalog | pg_size_pretty | text | numeric | normal
pg_catalog | pg_table_size | bigint | regclass | normal
pg_catalog | pg_tablespace_size | bigint | name | normal
pg_catalog | pg_tablespace_size | bigint | oid | normal
pg_catalog | pg_total_relation_size | bigint | regclass | normal
(12 rows)
postgres=# select pg_column_size('Hello');
pg_column_size
----------------
6
(1 row)
postgres=# select pg_column_size(10);
pg_column_size
----------------
4
(1 row)
postgres=# select pg_column_size(now());
pg_column_size
----------------
8
