pg_freespacemap模块提供一种检查自由空间映射（FSM）的手段。它提供一个名为pg_freespace的函数，或精确的说是两个重载函数。该函数在一个给定的页面或关系中的所有页面的自由空间映射内显示记录的值。缺省的公共访问在该函数中取消了，只是因为潜藏的安全问题。
1，创建扩展
jinli=# create extension pg_freespacemap; CREATE EXTENSION jinli=# \dxS+ pg_freespacemap Objects in extension "pg_freespacemap" Object Description ---------------------------------------- function pg_freespace(regclass) function pg_freespace(regclass,bigint) (2 rows)
可以发现pg_freespacemap扩展模块被创建出来后，多了两个重载函数。
2，示例输出
jinli=# select * from pg_freespace(‘public.log_level_problem‘); blkno | avail
-------+-------
0 | 73921 | 66562 | 35523 | 3204 | 45765 | 49286 | 59527 | 80648 | 79369 | 793610 | 6944 (11 rows) select * from pg_freespace(‘public.log_level_problem‘,10); pg_freespace
--------------
6944 (1 row) jinli=# --查看表的平均空间空闲率 jinli=# select count(1) as "number of pages",pg_size_pretty(cast(avg(avail) as bigint)) as "freespace size",round(100 * avg(avail)/8192,2)||‘%‘ as "freespace tatio" from pg_freespace(‘public.log_level_problem‘); number of pages | freespace size | freespace tatio
-----------------+---------------------+----------------------
11 | 5841 bytes | 71.31%(1 row) jinli=# vacuum public.log_level_problem; VACUUM jinli=# select count(1) as "number of pages",pg_size_pretty(cast(avg(avail) as bigint)) as "freespace size",round(100 * avg(avail)/8192,2)||‘%‘ as "freespace tatio" from pg_freespace(‘public.log_level_problem‘); number of pages |freespace size |freespace tatio
-----------------+---------------------+----------------------
11 | 5690 bytes | 69.46% (1 row) vacuum full public.log_level_problem; VACUUM jinli=# select count(1) as "number of pages",pg_size_pretty(cast(avg(avail) as bigint)) as "freespace size",round(100 * avg(avail)/8192,2)||‘%‘ as "freespace tatio" from pg_freespace(‘public.log_level_problem‘); number of pages | freespace size | freespace tatio
-----------------+---------------------+----------------------
4 | 0 bytes | 0.00%
(1 row)
jinli=# select * from pg_freespace(‘public.log_level_problem‘);
blkno | avail
-------+-------
0 | 01 | 02 | 03 | 0 (4 rows)
j该示例可以看出来执行完vacuum并不能全部释放空间，需执行vacuum full之后才能彻底“压实”页面。
原文：https://www.cnblogs.com/jinli1771/p/14998195.html