sql - 计算Postgresql中列类型的大小

Question

我想弄清楚如何确定数据库中特定列的大小，例如我有两列分别称为 sourceip 和 destinationip，它们都是 16 字节字段。

我认为这会在 information_schema 或\d+ 中的某处，但我找不到特定的命令来隔离每个列类型的大小。

Can you calculate column type size in database or do you just have to reference the byte size for each type in the Postgresql documentation?

Answer 1

pg 中只有少数类型具有固定长度——几乎所有类型都是 varlena 类型——它具有动态长度。您可以检查类似

的查询

 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)

当结果不为-1时，类型不定长

对于 varlena 类型使用 pg_column_size 函数:

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