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
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