PostgreSQL中怎么监控VACUUM的处理过程

这篇文章主要讲解了“postgresql中怎么监控VACUUM的处理过程”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习“Postgresql中怎么监控VACUUM的处理过程”吧！

概览
PG的mvcC要求“过期”的数据不能马上被物理清除，而是标记为dead rows，这些dead rows后续会通过vacuuming过程清理。
vacuuming通过以下方式让数据库保持健康：
1.标记dead rows可用于存储新数据，这样可以避免不必要的磁盘浪费以及可以跳过dead rows以提升顺序扫描的性能；
2.更新vm（用于跟踪过期或已废弃的数据，反应在pages上）。这可以提升index-only scans的性能；
3.避免出现事务ID回卷失败。
PG提供了autovacuum机制，通过周期性的运行ANALYZE来收集最近频繁更新的数据表统计信息。

监控指标
为了让VACUUMs平滑运行，应该监控以下几个指标：
1.dead rows
2.table disk usage
3.VACUUM/AUTOVACUUM最近执行的时间
4.监控vacuum full

dead rows
PG提供了pg_stat_user_tables视图用于监控dead rows

[local:/data/run/pg12]:5120 pg12@testdb=# \d pg_stat_user_tables                      View "pg_catalog.pg_stat_user_tables"       Column        |           Type           | Collation | Nullable | Default ---------------------+--------------------------+-----------+----------+--------- relid               | oid                      |           |          |  schemaname          | name                     |           |          |  relname             | name                     |           |          |  seq_scan            | bigint                   |           |          |  seq_tup_read        | bigint                   |           |          |  idx_scan            | bigint                   |           |          |  idx_tup_fetch       | bigint                   |           |          |  n_tup_ins           | bigint                   |           |          |  n_tup_upd           | bigint                   |           |          |  n_tup_del           | bigint                   |           |          |  n_tup_hot_upd       | bigint                   |           |          |  n_live_tup          | bigint                   |           |          |  n_dead_tup          | bigint                   |           |          |  n_mod_since_analyze | bigint                   |           |          |  last_vacuum         | timestamp with time zone |           |          |  last_autovacuum     | timestamp with time zone |           |          |  last_analyze        | timestamp with time zone |           |          |  last_autoanalyze    | timestamp with time zone |           |          |  vacuum_count        | bigint                   |           |          |  autovacuum_count    | bigint                   |           |          |  analyze_count       | bigint                   |           |          |  autoanalyze_count   | bigint                   |           |          | [local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 2;UPDATE 20000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname,relname, n_dead_tup FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup ------------+--------------------+------------ public     | tbl                |          0 public     | t2                 |          0 public     | b                  |          0 public     | a                  |          0 public     | rel                |          0 public     | t_count            |          0 public     | t_big_autovacuum_1 |          0 public     | t_autovacuum_1     |          0 public     | t1                 |      20000(9 rows)

监控每张表的dead rows，特别是监控频繁更新的表上，这样有助于DBA确定VACUUM进程是否已有效的周期性的清除这些dead rows。

Table disk usage
在出现dead rows时，磁盘空间会逐步增大，vacuuming执行后可标记dead rows为空闲空间，通过监控空间的变化

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT        relname AS "table_name",        pg_size_pretty(pg_table_size(C.oid)) AS "table_size" FROM        pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) WHERE nspname NOT IN ('pg_catalog', 'infORMation_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') ORDER BY pg_table_size(C.oid) DESC;     table_name     | table_size --------------------+------------ rel                | 845 MB t_big_autovacuum_1 | 498 MB tbl                | 100 MB a                  | 65 MB b                  | 65 MB t1                 | 1456 kB t_autovacuum_1     | 504 kB t2                 | 360 kB t_count            | 64 kB(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 4;UPDATE 20000[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 5;UPDATE 20000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT        relname AS "table_name",        pg_size_pretty(pg_table_size(C.oid)) AS "table_size" FROM        pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') ORDER BY pg_table_size(C.oid) DESC;     table_name     | table_size --------------------+------------ rel                | 845 MB t_big_autovacuum_1 | 498 MB tbl                | 100 MB a                  | 65 MB b                  | 65 MB t1                 | 2864 kB t_autovacuum_1     | 504 kB t2                 | 360 kB t_count            | 64 kB(9 rows)

对t1执行全量更新，然后执行vacuum t1后再次插入等量的数据

[local:/data/run/pg12]:5120 pg12@testdb=# vacuum t1;VACUUM[local:/data/run/pg12]:5120 pg12@testdb=# select count(*) from t1; count ------- 20000(1 row)[local:/data/run/pg12]:5120 pg12@testdb=# \d t1                 Table "public.t1" Column |  Type   | Collation | Nullable | Default --------+---------+-----------+----------+--------- id     | integer |           |          | [local:/data/run/pg12]:5120 pg12@testdb=# insert into t1 select generate_series(1,20000);INSERT 0 20000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT        relname AS "table_name",        pg_size_pretty(pg_table_size(C.oid)) AS "table_size" FROM        pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') ORDER BY pg_table_size(C.oid) DESC;     table_name     | table_size --------------------+------------ rel                | 845 MB t_big_autovacuum_1 | 498 MB tbl                | 100 MB a                  | 65 MB b                  | 65 MB t1                 | 2864 kB t_autovacuum_1     | 504 kB t2                 | 360 kB t_count            | 64 kB(9 rows)

可以看到table占用的空间并没有出现变化，原因是新的rows使用了dead rows的空间。
如不执行vacuum直接插入，则明显可以看到table size的变化。

[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 10;UPDATE 60000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT                       relname AS "table_name",        pg_size_pretty(pg_table_size(C.oid)) AS "table_size" FROM        pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') ORDER BY pg_table_size(C.oid) DESC;     table_name     | table_size --------------------+------------ rel                | 845 MB t_big_autovacuum_1 | 498 MB tbl                | 100 MB a                  | 65 MB b                  | 65 MB t1                 | 4288 kB -->这是原占用空间 t_autovacuum_1     | 504 kB t2                 | 360 kB t_count            | 64 kB(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=# insert into t1 select generate_series(1,20000);INSERT 0 20000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT                                                relname AS "table_name",        pg_size_pretty(pg_table_size(C.oid)) AS "table_size" FROM        pg_class C LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') ORDER BY pg_table_size(C.oid) DESC;     table_name     | table_size --------------------+------------ rel                | 845 MB t_big_autovacuum_1 | 498 MB tbl                | 100 MB a                  | 65 MB b                  | 65 MB t1                 | 4992 kB  --> 新增占用空间 t_autovacuum_1     | 504 kB t2                 | 360 kB t_count            | 64 kB(9 rows)

Last time (auto)vacuum ran
PG提供了pg_stat_user_tables 视图用于监控最近一次vacuum运行的时间。

[local:/data/run/pg12]:5120 pg12@testdb=# \d pg_stat_user_tables                      View "pg_catalog.pg_stat_user_tables"       Column        |           Type           | Collation | Nullable | Default ---------------------+--------------------------+-----------+----------+--------- relid               | oid                      |           |          |  schemaname          | name                     |           |          |  relname             | name                     |           |          |  seq_scan            | bigint                   |           |          |  seq_tup_read        | bigint                   |           |          |  idx_scan            | bigint                   |           |          |  idx_tup_fetch       | bigint                   |           |          |  n_tup_ins           | bigint                   |           |          |  n_tup_upd           | bigint                   |           |          |  n_tup_del           | bigint                   |           |          |  n_tup_hot_upd       | bigint                   |           |          |  n_live_tup          | bigint                   |           |          |  n_dead_tup          | bigint                   |           |          |  n_mod_since_analyze | bigint                   |           |          |  last_vacuum         | timestamp with time zone |           |          |  last_autovacuum     | timestamp with time zone |           |          |  last_analyze        | timestamp with time zone |           |          |  last_autoanalyze    | timestamp with time zone |           |          |  vacuum_count        | bigint                   |           |          |  autovacuum_count    | bigint                   |           |          |  analyze_count       | bigint                   |           |          |  autoanalyze_count   | bigint                   |           |          | [local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       |          last_vacuum          |        last_autovacuum        ------------+--------------------+-------------------------------+------------------------------- public     | tbl                |                               |  public     | t2                 |                               |  public     | b                  |                               |  public     | a                  |                               |  public     | rel                |                               |  public     | t_count            |                               |  public     | t_big_autovacuum_1 |                               |  public     | t_autovacuum_1     |                               |  public     | t1                 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:32:02.639873+08(9 rows)

可以看到最近一次的vacumm是14:29:31，而最近一次的autovacuum是14:32:02，autovacuum默认60s执行一次，其他没有变化的表PG不会执行autovacuum。

[local:/data/run/pg12]:5120 pg12@testdb=# select name,setting from pg_settings where name like '%autovacuum%';                name                 |  setting  -------------------------------------+----------- autovacuum                          | on autovacuum_analyze_scale_factor     | 0.1 autovacuum_analyze_threshold        | 50 autovacuum_freeze_max_age           | 200000000 autovacuum_max_workers              | 3 autovacuum_multixact_freeze_max_age | 400000000 autovacuum_naptime                  | 60 --> 60s autovacuum_vacuum_cost_delay        | 2 autovacuum_vacuum_cost_limit        | -1 autovacuum_vacuum_scale_factor      | 0.2 autovacuum_vacuum_threshold         | 50 autovacuum_work_mem                 | -1 log_autovacuum_min_duration         | -1(13 rows)

执行update操作，60s后再次查询，发现last_autovacuum已更新。

[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 1;UPDATE 80000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       |          last_vacuum          |        last_autovacuum        ------------+--------------------+-------------------------------+------------------------------- public     | tbl                |                               |  public     | t2                 |                               |  public     | b                  |                               |  public     | a                  |                               |  public     | rel                |                               |  public     | t_count            |                               |  public     | t_big_autovacuum_1 |                               |  public     | t_autovacuum_1     |                               |  public     | t1                 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:38:02.771566+08(9 rows)

监控vacuum full
通过视图pg_stat_progress_vacuum可监控vacuum full的进度

[local:/data/run/pg12]:5120 pg12@testdb=# \d pg_stat_progress_vacuum           View "pg_catalog.pg_stat_progress_vacuum"       Column       |  Type   | Collation | Nullable | Default --------------------+---------+-----------+----------+--------- pid                | integer |           |          |  datid              | oid     |           |          |  datname            | name    |           |          |  relid              | oid     |           |          |  phase              | text    |           |          |  heap_blks_total    | bigint  |           |          |  heap_blks_scanned  | bigint  |           |          |  heap_blks_vacuumed | bigint  |           |          |  index_vacuum_count | bigint  |           |          |  max_dead_tuples    | bigint  |           |          |  num_dead_tuples    | bigint  |           |          | [local:/data/run/pg12]:5120 pg12@testdb=#

VACUUM的相关主题
如果上述指标提示VACUUMs没有正常执行，可以通过查询设置可发现问题所在，包括：
1.The autovacuum process is disabled on your database
2.The autovacuum process is disabled on one or more tables
3.Autovacuuming settings aren’t keeping pace with updates
4.Lock conflicts
5.Long-running open transactions

1.The autovacuum process is disabled on your database
通过执行ps -axww | grep autovacuum命令可监控autovacuum是否正在运行

[root@localhost ~]# ps -axww | grep autovacuum55958 ?        Ss     0:00 postgres: autovacuum launcher  56057 pts/4    S+     0:00 grep --color=auto autovacuum[root@localhost ~]#

同时亦可通过查询pg_settings获得

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT name, setting FROM pg_settings WHERE name='autovacuum';    name    | setting ------------+--------- autovacuum | on(1 row)

如autovacuum已开启，但结果没有如我们预期，那么问题可能出现在statistics collector上面，autovacuum依赖statistics collector用于确定何时以及间隔多少时间应该运行。通常来说，statistics collector应启用，但如果禁用此项，对autovacuum的正常运行会有较大影响。通过检查track_counts配置项来检查statistics collector是否启用。

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT name, setting FROM pg_settings WHERE name='track_counts';     name     | setting --------------+--------- track_counts | on(1 row)[local:/data/run/pg12]:5120 pg12@testdb=#

如track_counts为OFF，则statistics collector不会更新dead rows信息，而该项是autovacuum所依赖的信息。

[local:/data/run/pg12]:5120 pg12@testdb=# set track_counts=off;SET[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 2;UPDATE 80000[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 3;UPDATE 80000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | t2                 |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:38:02.771566+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=#

手工把track_counts设置为off，更新t1，查询pg_stat_user_tables发现n_dead_tup没有统计dead rows，导致autovacuum并没有对t1表进行“vacuum”。
手工设置track_counts为on，但没有触发统计信息的更新，退出psql重新登录，更新数据表后才会出现新的统计信息

[local:/data/run/pg12]:5120 pg12@testdb=# set track_counts=on;SET[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | t2                 |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:38:02.771566+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=# \q[pg12@localhost ~]$ psqlExpanded display is used automatically.psql (12.1)Type "help" for help.[local:/data/run/pg12]:5120 pg12@testdb=# update t1 set id = 100;UPDATE 80000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | t2                 |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |      79868 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:38:02.771566+08(9 rows)

2.The autovacuum process is disabled on one or more tables
PG可在表级别上设置autovacuum是否生效

[local:/data/run/pg12]:5120 pg12@testdb=# create table t2(id int);CREATE TABLE[local:/data/run/pg12]:5120 pg12@testdb=# alter table t2 SET (autovacuum_enabled = false);ALTER TABLE[local:/data/run/pg12]:5120 pg12@testdb=# \d t2                 Table "public.t2" Column |  Type   | Collation | Nullable | Default --------+---------+-----------+----------+--------- id     | integer |           |          | [local:/data/run/pg12]:5120 pg12@testdb=# SELECT reloptions FROM pg_class WHERE relname='t2';         reloptions         ---------------------------- {autovacuum_enabled=false}(1 row)[local:/data/run/pg12]:5120 pg12@testdb=#

在t2上插入数据并更新

[local:/data/run/pg12]:5120 pg12@testdb=# insert into t2 select generate_series(1,100000);INSERT 0 100000[local:/data/run/pg12]:5120 pg12@testdb=# update t2 set id = 1;UPDATE 100000[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |     100000 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)

t2的dead rows为100000，但60s超时后，autovacuum并没有对该表进行vacuum处理。

[local:/data/run/pg12]:5120 pg12@testdb=# \! dateTue Dec 10 15:06:54 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# \! dateTue Dec 10 15:08:28 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |     100000 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=#

设置数据表autovacuum_enabled为true，等待60s，这时候发现t2已被vacuum

[local:/data/run/pg12]:5120 pg12@testdb=# alter table t2 SET (autovacuum_enabled = true);ALTER TABLE[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |     100000 |                               |  public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=# \! dateTue Dec 10 15:09:05 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# [local:/data/run/pg12]:5120 pg12@testdb=# \! dateTue Dec 10 15:10:26 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |          0 |                               | 2019-12-10 15:09:57.621123+08 public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=#

3.Autovacuuming settings aren’t keeping pace with updates
如果autovacuum已启用，但没有我们想象中那么频繁的执行，这时候需要调整默认的配置选项。

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT name,setting,boot_val,pending_restart from pg_settings where cateGory like 'Autovacuum';                name                 |  setting  | boot_val  | pending_restart -------------------------------------+-----------+-----------+----------------- autovacuum                          | on        | on        | f autovacuum_analyze_scale_factor     | 0.1       | 0.1       | f autovacuum_analyze_threshold        | 50        | 50        | f autovacuum_freeze_max_age           | 200000000 | 200000000 | f autovacuum_max_workers              | 3         | 3         | f autovacuum_multixact_freeze_max_age | 400000000 | 400000000 | f autovacuum_naptime                  | 60        | 60        | f autovacuum_vacuum_cost_delay        | 2         | 2         | f autovacuum_vacuum_cost_limit        | -1        | -1        | f autovacuum_vacuum_scale_factor      | 0.2       | 0.2       | f autovacuum_vacuum_threshold         | 50        | 50        | f(11 rows)

查询pg_settings，其中setting为当前配置的值，boot_val是默认值，可以看到当前库的配置与默认值一样。
确定autovacuum运行频度的参数有：
1.autovacuum_vacuum_threshold，触发阈值，默认为50
2.autovacuum_vacuum_scale_factor，触发dead rows率，默认为0.2，即20%
3.表的估算行数，存储在pg_class.reltuples中
PG结合上述3个参数来确定autovacuum是否需要执行，计算公式如下：

autovacuuming threshold = autovacuum_vacuum_threshold + (autovacuum_vacuum_scale_factor * estimated number of rows in the table)

通过调整参数，如减少autovacuum_vacuum_scale_factor可触发VACUUMs运行得更频繁。
PG还提供了log_autovacuum_min_duration参数来诊断autovacuum的运行间隔时间，如超过该时间设置则会记录在日志中，这样有助于诊断autovacuum的设定是否合理。

4.Lock conflicts
vacuum的执行需要持有SHARE UPDATE EXCLUSIVE lock，如有session持有的锁（SHARE UPDATE EXCLUSIVE，SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE）与其冲突，则无法执行vacuum。
更新t2

[local:/data/run/pg12]:5120 pg12@testdb=#  begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#*  update t2 set id = 10;UPDATE 100000[local:/data/run/pg12]:5120 pg12@testdb=#* commit;COMMIT[local:/data/run/pg12]:5120 pg12@testdb=# [local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |     100000 |                               | 2019-12-10 15:09:57.621123+08 public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)

开另外一个窗口，lock表

[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* lock t2 in SHARE UPDATE EXCLUSIVE mode;LOCK TABLE[local:/data/run/pg12]:5120 pg12@testdb=#*

autovacuum由于无法获取锁，因此无法对表进行vacuum

[local:/data/run/pg12]:5120 pg12@testdb=# select pid,locktype,relation::reGClass,mode,granted from pg_locks where pid <> pg_backend_pid();  pid  |  locktype  | relation |           mode           | granted -------+------------+----------+--------------------------+--------- 58050 | virtualxid |          | ExclusiveLock            | t 58050 | relation   | t2       | ShareUpdateExclusiveLock | t(2 rows)[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |     100000 |                               | 2019-12-10 15:09:57.621123+08 public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)

释放锁

[local:/data/run/pg12]:5120 pg12@testdb=#* commit;COMMIT[local:/data/run/pg12]:5120 pg12@testdb=#

autovacuum可正常执行，last_autovacuum已更新

[local:/data/run/pg12]:5120 pg12@testdb=# \! date Tue Dec 10 15:33:01 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# \! date Tue Dec 10 15:33:40 CST 2019[local:/data/run/pg12]:5120 pg12@testdb=# SELECT schemaname, relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables; schemaname |      relname       | n_dead_tup |          last_vacuum          |        last_autovacuum        ------------+--------------------+------------+-------------------------------+------------------------------- public     | tbl                |          0 |                               |  public     | b                  |          0 |                               |  public     | a                  |          0 |                               |  public     | rel                |          0 |                               |  public     | t2                 |          0 |                               | 2019-12-10 15:32:58.743764+08 public     | t_count            |          0 |                               |  public     | t_big_autovacuum_1 |          0 |                               |  public     | t_autovacuum_1     |          0 |                               |  public     | t1                 |          0 | 2019-12-10 14:29:31.420908+08 | 2019-12-10 14:59:57.269249+08(9 rows)[local:/data/run/pg12]:5120 pg12@testdb=#

通过进程状态亦可诊断

-- session 1[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* lock t2 in SHARE UPDATE EXCLUSIVE mode;LOCK TABLE[local:/data/run/pg12]:5120 pg12@testdb=#* -- session 2[local:/data/run/pg12]:5120 pg12@testdb=# vacuum t2;-- console[pg12@localhost ~]$ ps -ef|grep 'waiting'pg12     56540 55944  0 14:59 ?        00:00:01 postgres: pg12 testdb [local] VACUUM waitingpg12     58502 53760  0 15:36 pts/2    00:00:00 grep --color=auto waiting[pg12@localhost ~]$

进程显示为VACUUM waiting

5.Long-running open transactions
MVCC的一个副作用是vacuum不能清理那些其他事务还需要访问的过期dead rows。因此，如无必要确保事务正常完结。
通过视图pg_stat_activity可监控事务的状态

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT pid,xact_start, state, usename FROM pg_stat_activity;  pid  |          xact_start           | state  | usename -------+-------------------------------+--------+--------- 55958 |                               |        |  55960 |                               |        | pg12 56540 | 2019-12-10 15:42:47.210597+08 | active | pg12 58050 |                               | idle   | pg12 55956 |                               |        |  55955 |                               |        |  55957 |                               |        | (7 rows)

如state列显示为disabled，则需检查系统参数track_activities

[local:/data/run/pg12]:5120 pg12@testdb=# show track_activities; track_activities ------------------ on(1 row)[local:/data/run/pg12]:5120 pg12@testdb=# set track_activities=off;SET[local:/data/run/pg12]:5120 pg12@testdb=# SELECT pid,xact_start, state, usename FROM pg_stat_activity;  pid  | xact_start |  state   | usename -------+------------+----------+--------- 55958 |            |          |  55960 |            |          | pg12 56540 |            | disabled | pg12 58050 |            | idle     | pg12 55956 |            |          |  55955 |            |          |  55957 |            |          | (7 rows)[local:/data/run/pg12]:5120 pg12@testdb=# [local:/data/run/pg12]:5120 pg12@testdb=# set track_activities=on;SET[local:/data/run/pg12]:5120 pg12@testdb=# SELECT pid,xact_start, state, usename FROM pg_stat_activity;  pid  |          xact_start           | state  | usename -------+-------------------------------+--------+--------- 55958 |                               |        |  55960 |                               |        | pg12 56540 | 2019-12-10 15:52:19.500017+08 | active | pg12 58050 |                               | idle   | pg12 55956 |                               |        |  55955 |                               |        |  55957 |                               |        | (7 rows)

对于长时间闲置的session，PG提供了参数idle_in_transaction_session_timeout 用于控制这些session，超过该参数配置的时间（以ms为单位），PG会自动终止这些session。

感谢各位的阅读，以上就是“PostgreSQL中怎么监控VACUUM的处理过程”的内容了，经过本文的学习后，相信大家对PostgreSQL中怎么监控VACUUM的处理过程这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是编程网，小编将为大家推送更多相关知识点的文章，欢迎关注！