例行维护表

为了保证数据库的有效运行，数据库必须在插入/删除操作后，基于客户场景，定期做VACUUM FULL和ANALYZE，更新统计信息，以便获得更优的性能。

相关概念

使用VACUUM、VACUUM FULL和ANALYZE命令定期对每个表进行维护，主要有以下原因:

VACUUM FULL可回收已更新或已删除的数据所占据的磁盘空间，同时将小数据文件合并。
VACUUM对每个表维护了一个可视化映射来跟踪包含对别的活动事务可见的数组的页。一个普通的索引扫描首先通过可视化映射来获取对应的数组，来检查是否对当前事务可见。若无法获取，再通过堆数组抓取的方式来检查。因此更新表的可视化映射，可加速唯一索引扫描。
VACUUM可避免执行的事务数超过数据库阈值时，事务ID重叠造成的原有数据丢失。
ANALYZE可收集与数据库中表内容相关的统计信息。统计结果存储在系统表PG_STATISTIC中。查询优化器会使用这些统计数据，生成最有效的执行计划。

操作步骤

使用VACUUM或VACUUM FULL命令，进行磁盘空间回收。
- VACUUM:
  
  对表执行VACUUM操作
```
postgres=# VACUUM customer;
```
```
VACUUM
```
  可以与数据库操作命令并行运行。（执行期间，可正常使用的语句: SELECT、INSERT、UPDATE和DELETE。不可正常使用的语句: ALTER TABLE）。
  
  对表分区执行VACUUM操作
```
postgres=# VACUUM customer_par PARTITION ( P1 );
```
```
VACUUM
```
- VACUUM FULL:
```
postgres=# VACUUM FULL customer;
```
```
VACUUM
```
  需要向正在执行的表增加排他锁，且需要停止其他所有数据库操作。
使用ANALYZE语句更新统计信息。
```
postgres=# ANALYZE customer;
```
```
ANALYZE
```
使用ANALYZE VERBOSE语句更新统计信息，并输出表的相关信息。
```
postgres=# ANALYZE VERBOSE customer;
```
```
ANALYZE
```
也可以同时执行VACUUM ANALYZE命令进行查询优化。
```
postgres=# VACUUM ANALYZE customer;
```
```
VACUUM
```
说明: VACUUM和ANALYZE会导致I/O流量的大幅增加，这可能会影响其他活动会话的性能。因此，建议通过"vacuum_cost_delay"参数设置。

删除表。

postgres=# DROP TABLE customer;
postgres=# DROP TABLE customer_par;
postgres=# DROP TABLE part;

当结果显示为如下信息，则表示删除成功。

DROP TABLE

维护建议

定期对部分大表做VACUUM FULL，在性能下降后为全库做VACUUM FULL，目前暂定每月做一次VACUUM FULL。
定期对系统表做VACUUM FULL，主要是PG_ATTRIBUTE。
启用系统自动清理线程（AUTOVACUUM）自动执行VACUUM和ANALYZE，回收被标识为删除状态的记录空间，并更新表的统计数据。

MogDB空闲空间回收机制简介

VACUUM的作用

空闲空间回收

由于MogDB的MVCC机制，表中会同时存放一行记录的多个历史版本，对于经常更新的表来说，会造成表空间的膨胀。为了解决这个问题，引入了空闲空间回收机制，并提供三种不同力度的空闲空间回收方式，用于不同场景下的空闲空间回收。根据触发时机和清理方式的不同，空闲空间回收分为三类：

轻量级：轻量级空间回收在访问数据页时顺带清理页面中的过期元组，不清理索引，磁盘上的物理空间不会回收
中量级：中量级空间回收有两种触发场景，一是后台线程自动发起的autovacuum，另一种是用户手动执行的VACUUM操作，会对索引和数据页面进行清理，特殊场景下会回收磁盘上的物理空间
重量级：重量级空间回收主要由VACUUM FULL操作触发，会对指定的表进行彻底重建，回收磁盘上的物理空间

更新统计信息

中量级的空间回收会更新系统的统计信息，使planner能够计算出更准确的执行计划。

更新vm

在MogDB中，通过visibility map来标记数据页面的可见性，被vm标记的数据页面中没有过期元组，下一次VACUUM时会跳过该页面。此外，在进行索引扫描时，会首先检查页面的vm标记，以判断是否可进行indexonlyscan。

VACUUM的分类

轻量级

当查询扫描到某个数据页面时，会顺带清理页面中的过期元组。由于是顺带清理页面内容，因此只会删除过期元组本身，在页面内释放过期元组占用的空间，行指针则不做处理，避免在索引侧造成空指针或空引用。一个特殊情况是HOT场景，HOT场景是指对于表上的所有的索引，更新前后的索引键值均没有发生变化，因此对于更新后的元组只需要在数据页面中插入一条新记录，而不需要插入一条新的索引记录，通过HOT链来维护数据元组的新老版本。在轻量级空间回收时，只保留第一个版本的行指针，并将其重定向到第一个需要保留的元组版本的行指针。

中量级

MogDB提供VACUUM语句来让用户主动执行对某个astore表及其表上的索引进行空间回收，在astore中，新老版本的数据混合存储，进行VACUUM时会会首先对数据页面进行顺序扫描，判断哪些元组需要被清理，对于需要被清理的元组，会首先清理其对应的索引，然后再清理数据页面中的元组，从而避免索侧的空指针/空引用问题。

除了用户手动执行VACUUM操作外，MogDB还提供了自动的空闲空间回收功能，通过autovacuum周期性对表中的过期元组进行清理。与手动的VACUUM不同的是，autovacuum清理表和表的toast表是分开处理的，而手动vacuum是连续处理（该差异在MogDB 3.0.8最新补丁版本提供参数handle_toast_in_autovac供用户选择）。此外，autovacuum会被DDL操作中断，而手动的VACUUM操作则会阻塞DDL操作。

重量级

轻量级和中量级的空间回收都无法彻底释放磁盘上的物理空间，只能将数据页内的过期元组回收，释放其占用的空间，但是这些空间只能被当前表复用，无法将磁盘空间归还给操作系统。因此，MogDB提供了重量级的空闲空间回收VACUUM FULL操作。VACUUM FULL操作会将一个表中所有未过期元组从原数据页面中读取出来，然后重新紧密地插入到新的数据文件中，对于有索引的表，会在新的数据文件上重建索引，以彻底回收表中过期元组占用的空间。重量级空间回收操作的主体流程中只允许执行查询操作，而在提交流程，只读查询也会被阻塞。

示例及说明

自动空闲空间回收autovacuum

-- 1. update后等待连续的两次autovacuum
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# update tb set id = 1 ;
UPDATE 1
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)

MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  17095 |  17096 |       16386 |        258 | (0,2)
  2 |  17096 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup |        last_autovacuum        | autovacuum_count 
---------+-----------+-----------+------------+------------+-------------------------------+------------------
 tb      |         1 |         0 |          1 |          1 | 2023-12-24 15:36:23.018133+08 |                1
(1 row)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup |        last_autovacuum        | autovacuum_count 
---------+-----------+-----------+------------+------------+-------------------------------+------------------
 tb      |         1 |         0 |          1 |          0 | 2023-12-24 15:37:23.079814+08 |                2
(1 row)


-- 2. update后等待一次autovacuum，然后insert一条记录，查看xid是否被推进
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# update tb set id = 1 ;
UPDATE 1
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)

MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  17088 |  17089 |       16386 |        258 | (0,2)
  2 |  17089 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  17088 |  17089 |       16386 |       1282 | (0,2)
  2 |  17089 |      0 |       32770 |      10498 | (0,2)
(2 rows)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup |        last_autovacuum        | autovacuum_count 
---------+-----------+-----------+------------+------------+-------------------------------+------------------
 tb      |         1 |         0 |          1 |          1 | 2023-12-24 15:33:22.873957+08 |                1
(1 row)

MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  17088 |  17089 |       16386 |       1282 | (0,2)
  2 |  17089 |      0 |       32770 |      10498 | (0,2)
  3 |  17091 |      0 |           2 |       2050 | (0,3)
(3 rows)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup |        last_autovacuum        | autovacuum_count 
---------+-----------+-----------+------------+------------+-------------------------------+------------------
 tb      |         2 |         0 |          2 |          0 | 2023-12-24 15:34:22.897349+08 |                2
(1 row)

MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |        |        |             |            | 
  2 |  17089 |      0 |       32770 |      10498 | (0,2)
  3 |  17091 |      0 |           2 |       2306 | (0,3)
(3 rows)

从第一个示例可以看到，update后，第二次对表执行autovacuum，表中的过期元组将被清理掉
在第二个示例中，第一次insert的xid是17088，update的xid是17089，等待一次autovacuum之后再次进行insert的xid是17091，也就是autovacuum将xid从17089推进到了17090

手动空闲空间回收VACUUM

-- 1. update后直接vacuum
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16702 |      0 |           2 |       2050 | (0,1)
(1 row)
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          0 |                 |                0
(1 row)

MogDB=# update tb set id = 1;
UPDATE 1
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16702 |  16703 |       16386 |        258 | (0,2)
  2 |  16703 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# vacuum tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16702 |  16703 |       16386 |       1282 | (0,2)
  2 |  16703 |      0 |       32770 |      10498 | (0,2)
(2 rows)
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)

MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16702 |  16703 |       16386 |       1282 | (0,2)
  2 |  16703 |      0 |       32770 |      10498 | (0,2)
  3 |  16704 |      0 |           2 |       2050 | (0,3)
(3 rows)

MogDB=# vacuum tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |        |        |             |            | 
  2 |  16703 |      0 |       32770 |      10498 | (0,2)
  3 |  16704 |      0 |           2 |       2306 | (0,3)
(3 rows)
MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         2 |         0 |          2 |          0 |                 |                0
(1 row)

-- 2. update后执行其他操作推进一下xid，然后再vacuum
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# update tb set id = 1;
UPDATE 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16719 |  16720 |       16386 |        258 | (0,2)
  2 |  16720 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# create table tbb(id int);
CREATE TABLE
MogDB=# vacuum tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |        |        |             |            | 
  2 |  16720 |      0 |       32770 |      10498 | (0,2)
(2 rows)

MogDB=# insert into tb values (0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |        |        |             |            | 
  2 |  16720 |      0 |       32770 |      10498 | (0,2)
  3 |  16722 |      0 |           2 |       2050 | (0,3)
(3 rows)

示例1中，update的xid是16702，update的xid是16703，update之后进行vacuum，然后进行了第二次insert，第二次的insert的xid是16704，也就是vacuum操作没有推进xid

表重建VACUUM FULL

-- 1. update后直接vacuum full
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16707 |      0 |           2 |       2050 | (0,1)
(1 row)

MogDB=# update tb set id = 1;
UPDATE 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16707 |  16708 |       16386 |        258 | (0,2)
  2 |  16708 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)
MogDB=# vacuum full tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16708 |      0 |           2 |      10498 | (0,1)
  2 |      2 |  16708 |           2 |       1282 | (0,1)
(2 rows)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          1 |                 |                0
(1 row)

MogDB=# vacuum full tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |      2 |      0 |           2 |      10498 | (0,1)
(1 row)

MogDB=# select relname, n_tup_ins, n_tup_del, n_live_tup, n_dead_tup, last_autovacuum, autovacuum_count from pg_stat_user_tables where relname = 'tb';
 relname | n_tup_ins | n_tup_del | n_live_tup | n_dead_tup | last_autovacuum | autovacuum_count 
---------+-----------+-----------+------------+------------+-----------------+------------------
 tb      |         1 |         0 |          1 |          0 |                 |                0
(1 row)

-- 2. update后先执行一次insert，再vacuum full
MogDB=# create table tb(id int, name text);
CREATE TABLE
MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16713 |      0 |           2 |       2050 | (0,1)
(1 row)

MogDB=# update tb set id = 1;
UPDATE 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16713 |  16714 |       16386 |        258 | (0,2)
  2 |  16714 |      0 |       32770 |      10242 | (0,2)
(2 rows)

MogDB=# insert into tb values(0, 'asdf');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |  16713 |  16714 |       16386 |        258 | (0,2)
  2 |  16714 |      0 |       32770 |      10242 | (0,2)
  3 |  16715 |      0 |           2 |       2050 | (0,3)
(3 rows)

MogDB=# vacuum full tb;
VACUUM
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |      2 |      0 |           2 |      10498 | (0,1)
  2 |  16715 |      0 |           2 |       2306 | (0,2)
(2 rows)

--  3. 测试vacuum full是否会推进xid
MogDB=# insert into tb values(0, 'asef');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |        |        |             |            | 
  2 |  17096 |      0 |       32770 |      10498 | (0,2)
  3 |  17100 |      0 |           2 |       2050 | (0,3)
(3 rows)

MogDB=# vacuum full tb;
VACUUM
MogDB=# insert into tb values(0, 'asef');
INSERT 0 1
MogDB=# select lp, t_xmin, t_xmax, t_infomask2, t_infomask, t_ctid from heap_page_items(get_raw_page('tb', 0));
 lp | t_xmin | t_xmax | t_infomask2 | t_infomask | t_ctid 
----+--------+--------+-------------+------------+--------
  1 |      2 |      0 |           2 |      10498 | (0,1)
  2 |  17100 |      0 |           2 |       2306 | (0,2)
  3 |  17102 |      0 |           2 |       2050 | (0,3)
(3 rows)

示例1中，第一次vacuum full将过期元组的xmax设置为无效的xmax，将更新后的xmin设置为2，xmax设置为更新的xid，并推进xid，第二次vacuum full的时候将过期的元组清理掉，将更新后的元组xmin设置为2
示例2中，update后通过insert将xid推进，然后进行vacuum full，所有过期元组均被清理掉
示例3连续的两次insert之间进行了一次vacuum full，可以看到vacuum full将xid进行了推进

例行维护表

MogDB空闲空间回收机制简介

VACUUM的作用

空闲空间回收

更新统计信息

更新vm

VACUUM的分类

轻量级

中量级

重量级

示例及说明

自动空闲空间回收autovacuum

手动空闲空间回收VACUUM

表重建VACUUM FULL

相关日志

autovacuum日志

手动vacuum日志

vacuum full日志

相关结论