Postgresql表膨胀率计算

一、简介

PostgreSQL自带了pgstattuple模块，可用于精确计算表的膨胀率。譬如这里的tuple_percent字段就是元组实际字节占关系总大小的百分比，用1减去该值即为膨胀率。

二、示例一

#插入1000W数据

postgres=# insert into t select id,id from generate_series(1,10000000) as id;

INSERT 0 10000000

  

#表膨胀系数为0.097

postgres=# select *, 1.0 - tuple_len::numeric / table_len as bloat from pgstattuple('t');

 table_len | tuple_count | tuple_len | tuple_percent | dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space | free_percent |   bloat  

-----------+-------------+-----------+---------------+------------------+----------------+--------------------+------------+--------------+------------------------

 442818560 | 10000001 | 400000040 |   90.33 |    0 |    0 |     0 | 1304976 |   0.29 | 0.09669540499838127833

(1 row)

  

#占用54055个page

postgres=# select * from pg_relpages('t');

 pg_relpages

-------------

  54055

(1 row)

  

#删除数据

postgres=# delete from t where id<>10000000;

DELETE 9999999

  

#仍然占用54055个page

postgres=# select * from pg_relpages('t');

 pg_relpages

-------------

  54055

(1 row)

  

#膨胀率已经为0.999999

postgres=# select *, 1.0 - tuple_len::numeric / table_len as bloat from pgstattuple('t');

 table_len | tuple_count | tuple_len | tuple_percent | dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space | free_percent |   bloat  

-----------+-------------+-----------+---------------+------------------+----------------+--------------------+------------+--------------+----------------------------

 442818560 |   2 |  80 |    0 |   9999999 |  399999960 |    90.33 | 1304976 |   0.29 | 0.999999819339099065766349

  

#vacuum表

postgres=# vacuum (verbose,full,analyze) t;

INFO: vacuuming "public.t"

INFO: "t": found 5372225 removable, 2 nonremovable row versions in 54055 pages

DETAIL: 0 dead row versions cannot be removed yet.

CPU: user: 0.89 s, system: 0.00 s, elapsed: 0.89 s.

INFO: analyzing "public.t"

INFO: "t": scanned 1 of 1 pages, containing 2 live rows and 0 dead rows; 2 rows in sample, 2 estimated total rows

VACUUM

三、示例二

pg索引膨胀问题---重建索引

\di+ 索引名查看索引大小

发现数据库中很多表的索引大小超过数据大小。由于频繁更新就会带来索引膨胀。

对于大的索引可以采用重建的方式解决。以下两种方法推荐第一种。

方法一：停止应用（这个操作会锁表），重建索引（注：重建完索引名称不变）

sql：reindex index 索引名称

reindex table 表名

方法二：在线建新索引，再把旧索引删除