postgresql vacuum 垃圾整理

ameimeng

　　简单的说，这个命令就是垃圾整理，另外一个是vacuumdb
　　具体中文说明
　　http://www.kuqin.com/postgreSQL8.1_doc/sql-vacuum.html
写道
VACUUMName
VACUUM -- 垃圾收集以及可选地分析一个数据库
Synopsis
VACUUM [ FULL | FREEZE ] [ VERBOSE ] [ table ] VACUUM [ FULL | FREEZE ] [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ]

描述
VACUUM 回收已删除元组占据的存储空间。 在一般的 PostgreSQL 操作里， 那些已经 DELETE 的元组或者被 UPDATE 过后过时的元组是没有从它们所属的表中物理删除的； 在完成VACUUM 之前它们仍然存在。 因此我们有必须周期地运行 VACUUM， 特别是在常更新的表上。

如果没有参数，VACUUM 处理当前数据库里每个表， 如果有参数，VACUUM 只处理那个表。

VACUUM ANALYZE 先执行一个 VACUUM 然后是给每个选定的表执行一个 ANALYZE。 对于日常维护脚本而言，这是一个很方便的组合。参阅 ANALYZE 获取更多有关其处理的细节。

简单的 VACUUM （没有FULL） 只是简单地回收空间并且令其可以再次使用。这种形式的命令可以和对表的普通读写并行操作， 因为没有请求排他锁。VACUUM FULL 执行更广泛的处理，包括跨块移动元组，以便把表压缩到最少的磁盘块数目里。 这种形式要慢许多并且在处理的时候需要在表上施加一个排它锁。

FREEZE 是一种特殊用途的选项，它导致元组尽可能快地标记为"冻结(frozen)"， 而不是等到它们已经相当老的时候才标记。如果在同一个数据库上没有其它运行着的事务的时候完成这个命令， 那么系统就保证在数据库里的所有元组都是"冻结(frozen)"的， 因此不会有事务 ID 重叠的问题，而和数据库未清理的时间没有关系。 我们不建议把 FREEZE 用做日常用途。我们用它的唯一目的是准备和用户定义的模板数据库联接的时候， 或者是其它完全是只读的， 不会等到日常维护性 VACUUM 操作的数据库。 参阅 Chapter 22 获取细节。

参数


FULL
选择"完全"清理，这样可以恢复更多的空间， 但是花的时间更多并且在表上施加了排它锁。

FREEZE
选择激进的元组"冻结"。

VERBOSE
为每个表打印一份详细的清理工作报告。

ANALYZE
更新用于优化器的统计信息，以决定执行查询的最有效方法。

table
要清理的表的名称（可以有模式修饰）。缺省时是当前数据库中的所有表。

column
要分析的具体的列/字段名称。缺省是所有列/字段。

输出
如果声明了 VERBOSE，VACUUM 发出过程信息， 以表明当前正在处理那个表。各种有关这些表的统计也会打印出来。

注意
我们建议在经常VACUUMM（清理）（至少每晚一次）生产数据库， 以保证不断地删除失效的行。尤其是在增删了大量记录之后， 对受影响的表执行 VACUUM ANALYZE 命令是一个很好的习惯。这样做将更新系统目录为最近的更改，并且允许 PostgreSQL 查询优化器在规划用户查询时有更好的选择。

我们不建议日常使用 FULL 选项，但是可以在特殊情况下使用。 一个例子就是在你删除了一个表的大部分行之后，希望从物理上缩小该表以减少磁盘空间占用。VACUUM FULL 通常要比单纯的 VACUUM 收缩更多表的尺寸。

例子
下面是一个在 regression （蜕变）数据库里某个表上执行 VACUUM的一个例子：

regression=# VACUUM VERBOSE ANALYZE onek; INFO: vacuuming "public.onek" INFO: index "onek_unique1" now contains 1000 tuples in 14 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.08u sec elapsed 0.18 sec. INFO: index "onek_unique2" now contains 1000 tuples in 16 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.00s/0.07u sec elapsed 0.23 sec. INFO: index "onek_hundred" now contains 1000 tuples in 13 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.08u sec elapsed 0.17 sec. INFO: index "onek_stringu1" now contains 1000 tuples in 48 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.09u sec elapsed 0.59 sec. INFO: "onek": removed 3000 tuples in 108 pages DETAIL: CPU 0.01s/0.06u sec elapsed 0.07 sec. INFO: "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages DETAIL: 0 dead tuples cannot be removed yet. There were 0 unused item pointers. 0 pages are entirely empty. CPU 0.07s/0.39u sec elapsed 1.56 sec. INFO: analyzing "public.onek" INFO: "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows VACUUM




兼容性
SQL92里没有 VACUUM 语句。




　　这是手动的
　　PostgreSQL 推薦手動 VACUUM 的時機與目的
　　http://postgresql-chinese.blogspot.com/2007/03/postgresql-vacuum.html
　　既然它都推荐自动，那就只有看自动了
　　默认自动整理是关闭的
　　编辑postgresql.conf
　　#------------------------------------------------------------------------------
# AUTOVACUUM PARAMETERS
#------------------------------------------------------------------------------
　　#autovacuum = on   # Enable autovacuum subprocess?  'on'
     # requires track_counts to also be on.
#log_autovacuum_min_duration = -1 # -1 disables, 0 logs all actions and
     # their durations, > 0 logs only
     # actions running at least this number
     # of milliseconds.
#autovacuum_max_workers = 3  # max number of autovacuum subprocesses
#autovacuum_naptime = 1min  # time between autovacuum runs
#autovacuum_vacuum_threshold = 50 # min number of row updates before
     # vacuum
#autovacuum_analyze_threshold = 50 # min number of row updates before
     # analyze
#autovacuum_vacuum_scale_factor = 0.2 # fraction of table size before vacuum
#autovacuum_analyze_scale_factor = 0.1 # fraction of table size before analyze
#autovacuum_freeze_max_age = 200000000 # maximum XID age before forced vacuum
     # (change requires restart)
#autovacuum_vacuum_cost_delay = 20ms # default vacuum cost delay for
     # autovacuum, in milliseconds;
     # -1 means use vacuum_cost_delay
#autovacuum_vacuum_cost_limit = -1 # default vacuum cost limit for
     # autovacuum, -1 means use
     # vacuum_cost_limi
　　但是，在这个的上面，还有一个设置，改善vacuum的时机吧，从最后一个URL的测试结果来看，合适的设置应该会好很多
　　没太看懂
　　# - Cost-Based Vacuum Delay -
　　#vacuum_cost_delay = 0ms  # 0-100 milliseconds
#vacuum_cost_page_hit = 1  # 0-10000 credits
#vacuum_cost_page_miss = 10  # 0-10000 credits
#vacuum_cost_page_dirty = 20  # 0-10000 credits
#vacuum_cost_limit = 200  # 1-10000 credits
　　http://www.postgresql.org/docs/current/static/runtime-config-resource.html

写道
During the execution of VACUUM and ANALYZE commands, the system maintains an internal counter that keeps track of the estimated cost of the various I/O operations that are performed. When the accumulated cost reaches a limit (specified by vacuum_cost_limit), the process performing the operation will sleep for a while (specified byvacuum_cost_delay). Then it will reset the counter and continue execution.

The intent of this feature is to allow administrators to reduce the I/O impact of these commands on concurrent database activity. There are many situations in which it is not very important that maintenance commands like VACUUM and ANALYZE finish quickly; however, it is usually very important that these commands do not significantly interfere with the ability of the system to perform other database operations. Cost-based vacuum delay provides a way for administrators to achieve this.

This feature is disabled by default for manually issued VACUUM commands. To enable it, set the vacuum_cost_delay variable to a nonzero value.



vacuum_cost_delay (integer)
The length of time, in milliseconds, that the process will sleep when the cost limit has been exceeded. The default value is zero, which disables the cost-based vacuum delay feature. Positive values enable cost-based vacuuming. Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; settingvacuum_cost_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10.

When using cost-based vacuuming, appropriate values for vacuum_cost_delay are usually quite small, perhaps 10 or 20 milliseconds. Adjusting vacuum's resource consumption is best done by changing the other vacuum cost parameters.

vacuum_cost_page_hit (integer)
The estimated cost for vacuuming a buffer found in the shared buffer cache. It represents the cost to lock the buffer pool, lookup the shared hash table and scan the content of the page. The default value is one.

vacuum_cost_page_miss (integer)
The estimated cost for vacuuming a buffer that has to be read from disk. This represents the effort to lock the buffer pool, lookup the shared hash table, read the desired block in from the disk and scan its content. The default value is 10.

vacuum_cost_page_dirty (integer)
The estimated cost charged when vacuum modifies a block that was previously clean. It represents the extra I/O required to flush the dirty block out to disk again. The default value is 20.

vacuum_cost_limit (integer)
The accumulated cost that will cause the vacuuming process to sleep. The default value is 200.

Note: There are certain operations that hold critical locks and should therefore complete as quickly as possible. Cost-based vacuum delays do not occur during such operations. Therefore it is possible that the cost accumulates far higher than the specified limit. To avoid uselessly long delays in such cases, the actual delay is calculated as vacuum_cost_delay * accumulated_balance / vacuum_cost_limit with a maximum of vacuum_cost_delay * 4.


[size=1em] 
[size=1em] 
[size=1em]以下貌似是测试结果
[size=1em]http://developer.postgresql.org/~wieck/vacuum_cost/