Postgresqlで列タイプのサイズを計算する

pgのいくつかのタイプだけが固定長を持っています-ほとんどすべてのタイプはvarlenaタイプです-それは動的な長さを持っています。

 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)

結果が-1でない場合、タイプの長さは固定されていません

varlenaタイプの場合は、pg_column_size関数を使用します：

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