Calculating the size of a column type in Postgresql

Question

I am trying to figure out how to determine the size of a specific column in database for instance I have two columns called sourceip, destinationip that are both 16 byte fie

Answer 1

only few types in pg has fixed length - almost all types are varlena type - it has dynamic length. You can check queries like

 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)

When result is not -1, then type has not fixed length

for varlena types use pg_column_size function:

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