PostgreSQL 14 Bottom-up index tuple deletion 大大减少btree索引膨胀

Posted 2021-11-24 PostgreSQLChina

作者：金涛

准备PG12和PG14两个环境，主要参数保持一致，并执行如下SQL初始化测试表：

CREATE TABLE testtab (
   id        bigint
      CONSTRAINT testtab_pkey PRIMARY KEY,
   unchanged integer,
   changed   integer
);
INSERT INTO testtab
   SELECT i, i, 0
   FROM generate_series(1, 10000) AS i;
CREATE INDEX testtab_unchanged_idx ON testtab (unchanged);
CREATE INDEX testtab_changed_idx ON testtab (changed);
vacuum ANALYZE testtab;

PG12查看表索引大小：

postgres=# \\dt+ testtab
                     List of relations
 Schema |  Name   | Type  |  Owner   |  Size  | Description 
--------+---------+-------+----------+--------+-------------
 public | testtab | table | postgres | 576 kB | 
(1 row)
postgres=# \\di+ testtab*
                                 List of relations
 Schema |         Name          | Type  |  Owner   |  Table  |  Size  | Description 
--------+-----------------------+-------+----------+---------+--------+-------------
 public | testtab_changed_idx   | index | postgres | testtab | 288 kB | 
 public | testtab_pkey          | index | postgres | testtab | 288 kB | 
 public | testtab_unchanged_idx | index | postgres | testtab | 288 kB | 
(3 rows)

PG14查看表索引大小：

postgres=# \\di+ testtab*
                                                List of relations
 Schema |         Name          | Type  |  Owner   |  Table  | Persistence | Access method |  Size  | Description 
--------+-----------------------+-------+----------+---------+-------------+---------------+--------+-------------
 public | testtab_changed_idx   | index | postgres | testtab | permanent   | btree         | 128 kB | 
 public | testtab_pkey          | index | postgres | testtab | permanent   | btree         | 288 kB | 
 public | testtab_unchanged_idx | index | postgres | testtab | permanent   | btree         | 288 kB | 
(3 rows)

新建pgbench.sql文件并写入如下SQL:

\\set id random_gaussian(1, 10000, 10)
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;
UPDATE testtab SET changed = changed + 1 WHERE id = :id;

分别在PG12和PG14运行如下pgbench基准测试：

time pgbench -M prepared -n -c 6 -f pgbench.sql -t 10000  postgres

查看索引及表大小：

PG12

postgres=# \\di+ testtab*
                                  List of relations
 Schema |         Name          | Type  |  Owner   |  Table  |  Size   | Description 
--------+-----------------------+-------+----------+---------+---------+-------------
 public | testtab_changed_idx   | index | postgres | testtab | 14 MB   | 
 public | testtab_pkey          | index | postgres | testtab | 1472 kB | 
 public | testtab_unchanged_idx | index | postgres | testtab | 14 MB   | 
(3 rows)

PG14

postgres=# \\di+ testtab*
                                                 List of relations
 Schema |         Name          | Type  |  Owner   |  Table  | Persistence | Access method |  Size   | Description 
--------+-----------------------+-------+----------+---------+-------------+---------------+---------+-------------
 public | testtab_changed_idx   | index | postgres | testtab | permanent   | btree         | 5248 kB | 
 public | testtab_pkey          | index | postgres | testtab | permanent   | btree         | 288 kB  | 
 public | testtab_unchanged_idx | index | postgres | testtab | permanent   | btree         | 512 kB  | 
(3 rows)

可以看到明显PG14中的索引比PG12大小小的多得多。 升级！升级！升级！