postgresql/lightdb和mysql、oracle不一样，通过一堆表函数提供查看各个对象的大小。
如查看每个表的物理位置、所属表空间和数据库：
| | 返回关系的物理路径，不支持TOAST，可以通过pg_class查询。 |
zjh@test=# select relname,reltablespace,current_database(),setting || '/' || pg_relation_filepath(relname::text) from pg_class,(select setting from pg_settings where name='data_directory') where relkind='r' and relisshared='f' limit 5;
relname | reltablespace | current_database | ?column?
-----------------------+---------------+------------------+--------------------------------------
t | 0 | test | /mnt/pmem1/zjh/data/base/16397/16398
pg_statistic | 0 | test | /mnt/pmem1/zjh/data/base/16397/2619
pg_type | 0 | test | /mnt/pmem1/zjh/data/base/16397/1247
pg_foreign_table | 0 | test | /mnt/pmem1/zjh/data/base/16397/3118
pg_statistic_ext_data | 0 | test | /mnt/pmem1/zjh/data/base/16397/3429
(5 rows)
数据库对象尺寸函数
函数名 | 返回类型 | 描述 |
pg_column_size(any) | int | 存储一个指定的数值需要的字节数（可能压缩过） |
pg_database_size(oid) | bigint | 指定OID的数据库使用的磁盘空间，不包括索引和TOAST |
pg_database_size(name) | bigint | 指定名称的数据库使用的磁盘空间，不包括索引和TOAST |
pg_indexes_size(regclass) | bigint | 关联指定表OID或表名的表索引的使用总磁盘空间 |
pg_relation_size(relation regclass, fork text) | bigint | 指定OID或名的表或索引，通过指定fork('main', 'fsm' 或'vm')所使用的磁盘空间 |
pg_relation_size(relation regclass) | bigint | pg_relation_size(..., 'main')的缩写 |
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 | 把以字节计算的数值转换成一个人类易读的尺寸单位 |
pg_table_size(regclass) | bigint | 指定表OID或表名的表使用的磁盘空间，除去索引（但是包含TOAST，自由空间映射和可视映射） |
pg_tablespace_size(oid) | bigint | 指定OID的表空间使用的磁盘空间 |
pg_tablespace_size(name) | bigint | 指定名称的表空间使用的磁盘空间 |
pg_total_relation_size(regclass) | bigint | 指定表OID或表名使用的总磁盘空间，包括所有索引和TOAST数据 |
public | bmsql_order_line | table | lightdb | permanent | 233 GB |
public | bmsql_stock | table | lightdb | permanent | 40 GB |
public | bmsql_warehouse | table | lightdb | permanent | 61 MB |
(175 rows)
lightdb@benchmarksql1000=# select pg_size_pretty(pg_total_relation_size('bmsql_order_line'));
pg_size_pretty
----------------
296 GB
(1 row)
查看数据库的事务ID
zjh@postgres=# select pg_current_xact_id();
pg_current_xact_id
--------------------
2594753
(1 row)
zjh@postgres=# select pg_current_xact_id_if_assigned();
pg_current_xact_id_if_assigned
--------------------------------
(1 row)
zjh@postgres=# select pg_xact_status(pg_current_xact_id()::xid8);
pg_xact_status
----------------
in progress
(1 row)
zjh@postgres=# select pg_current_snapshot();
pg_current_snapshot
---------------------
2594755:2594755:
(1 row)
查看当前LSN和最新的WAL文件
有些时候除了事务ID，还需要知道当前的最新LSN和WAL文件。如逻辑复制设置起始和结束点的时候。
zjh@postgres=# select * from pg_control_checkpoint();
checkpoint_lsn | redo_lsn | redo_wal_file | timeline_id | prev_timeline_id | full_page_writes | next_xid | next_oid | next_multixact_id | next_multi_offset | oldest_xid | olde
st_xid_dbid | oldest_active_xid | oldest_multi_xid | oldest_multi_dbid | oldest_commit_ts_xid | newest_commit_ts_xid | checkpoint_time
----------------+------------+--------------------------+-------------+------------------+------------------+------------+----------+-------------------+-------------------+------------+-----
------------+-------------------+------------------+-------------------+----------------------+----------------------+------------------------
B/CB7DAA48 | B/CB7DAA10 | 000000010000000B00000006 | 1 | 1 | t | 0:73587543 | 24700 | 1 | 0 | 479 |
1 | 73587543 | 1 | 1 | 0 | 0 | 2022-06-04 00:10:34+08
因为checkpoint除非在写检查点的时候，否则它是异步的。所以next_xid、redo_lsn会比pg_current_xact_id()以及pg_current_wal_lsn()要稍微滞后。
查看表的最后修改时间
zjh@postgres=# select pg_stat_file(setting || '/' || pg_relation_filepath(relname::text)) from pg_class,(select setting from pg_settings where name='data_directory') where relkind='r' limit 5;
pg_stat_file
--------------------------------------------------------------------------------------------
(0,"2022-05-28 22:30:15+08","2022-05-28 22:30:15+08","2022-05-28 22:30:15+08",,f)
(0,"2022-05-31 21:45:31+08","2022-05-31 21:45:31+08","2022-05-31 21:45:31+08",,f)
(172032,"2022-06-03 18:14:29+08","2022-06-03 18:40:33+08","2022-06-03 18:40:33+08",,f)
(81920,"2022-06-04 14:00:13+08","2022-06-03 18:40:32+08","2022-06-03 18:40:32+08",,f)
(2439987200,"2022-05-28 20:19:57+08","2022-05-28 20:38:36+08","2022-05-28 20:38:36+08",,f)
因为数据修改后，只保证WAL会刷新，数据文件不一定会更新，所以通过stat判断其实并不准确，只能作为参考。
标签：PostgreSQL,lightdb,LSN,08,current,relation,pg,2022,size From： https://blog.51cto.com/zhjh256/5927784