在Postgresql中如何计算表字段，表，数据库，以及索引等的大小？
需要介绍Postgresql的内置函数，在文档中就有，如下：
| pg_column_size(any) | int | Number of bytes used to store a particular value (possibly compressed) |
| pg_database_size(oid) | bigint | Disk space used by the database with the specified OID |
| pg_database_size(name) | bigint | Disk space used by the database with the specified name |
| pg_indexes_size(regclass) | bigint | Total disk space used by indexes attached to the specified table |
| pg_relation_size(relation regclass,fork text) | bigint | Disk space used by the specified fork ('main', 'fsm' or 'vm') of the specified table or index |
| pg_relation_size(relation regclass) | bigint | Shorthand for pg_relation_size(...,'main') |
| pg_size_pretty(bigint) | text | Converts a size in bytes expressed as a 64-bit integer into a human-readable format with size units |
| pg_size_pretty(numeric) | text | Converts a size in bytes expressed as a numeric value into a human-readable format with size units |
| pg_table_size(regclass) | bigint | Disk space used by the specified table,excluding indexes (but including TOAST,free space map,and visibility map) |
| pg_tablespace_size(oid) | bigint | Disk space used by the tablespace with the specified OID |
| pg_tablespace_size(name) | bigint | Disk space used by the tablespace with the specified name |
| pg_total_relation_size(regclass) | bigint | Total disk space used by the specified table,including all indexes and TOAST data |
第一个函数的参数是relation和fork，relation是一个对象应该是一个表(以我目前的对pg的了解)，fork指的是这个表在磁盘中存储的三个文件，文档说明也提到了，特指main，fsm，vm，那么这三种参数值表示说明意思呢？
这里涉及到Postgresql的存储方式，每一个数据库以其OID值会在磁盘中创建名称为OID的文件夹，文件夹里面存储的是这个数据库下的表的文件，也已OID为名称存储，其中就存在以fsm，vm为后缀的文件。fsm代表的是空闲空间映射表文件，vm表示可见性映射表文件main应该是没有后缀的那个文件(这个暂时是猜的)，如我有一个表emp，其oid=19467
并且其在oid=12029的database下。使用sql：select pg_relation_size('emp'::regclass,'vm') 结果为8kb，查看文件夹下的此表的vm文件也是8kb，emp表总共是40kb，正好是
原文链接：https://www.f2er.com/postgresql/195440.html