一、开发背景
DB: Postgresql 9.3.0
cas=# select oid,rolname from pg_authid where oid in (10,327299); oid | rolname -----+---------- 10 | postgres 327299| usr_cas (1 row) cas=# select lomowner,count(1) from pg_largeobject_Metadata group by 1; lomowner | count ----------+-------- 10 | 292408 327299 | 382123 (2 row)二、清理
需要清理两部分，postgres用户的大对象与usr_cas用户的大对象，前者是用postgres连接时创建的，需要全部删除，后者存在部分用户数据已删但大对象没删的数据，也需要删除。
1.lo_unlink删除
删除通常使用自带的lo_unlink()函数，于是使用了以下命令，但爆出问题 out of shared memory
cas=# select lo_unlink(oid) from pg_largeobject_Metadata where lomowner = 10; WARNING: out of shared memory ERROR: out of shared memory HINT: You might need to increase max_locks_per_transaction. cas=# show max_locks_per_transaction ; max_locks_per_transaction --------------------------- 64 (1 row)这个提示比较明显，一个sql把所有的大对象在一个事务里完成，但分配的内存不够，所以失败了，要增加max_locks_per_transaction参数值，这个值默认是64。其实也可以换个角度删除，不把所有的大对象在一个事务里删除，而是分批次执行，因为要删除的数据量其实也不算多，就考虑了后者。
--多执行以下命令几次就可以了，每次删2W，执行10几次就够了，也可以放脚本里写，一次执行 cas=# select lo_unlink(oid) from pg_largeobject_Metadata where lomowner = 10 limit 20000;2.vacuumlo删除
清理完postgres的用户数据以后，接着要清理usr_cas用户的大对象数据，要写脚本逐个比对比较麻烦，而且效率也不一定好。这可以使用自带的vacuumlo的小工具。这个工具是通过大对象的OID与用户表中的oid进行关联比对，然后逐一删除，所以在设计大对象用户表时，虽然也可以使用int类型存储oid值，但是对后期的维护不方便，推荐使用oid类型。 如果这个工具没有安装，可以在contrib/vacuumlo下面make && make install安装一下即可
简介如下：
[postgres@kenyon-primary ~]$ vacuumlo --help vacuumlo removes unreferenced large objects from databases. Usage: vacuumlo [OPTION]... dbnAME... Options: -l LIMIT commit after removing each LIMIT large objects -n don't remove large objects,just show what would be done -v write a lot of progress messages -V,--version output version information,then exit -?,--help show this help,then exit Connection options: -h HOSTNAME database server host or socket directory -p PORT database server port -U USERNAME user name to connect as -w never prompt for password -W force password prompt Report bugs to .使用:
--显示要清理的数据，不清理，只显示 [postgres@kenyon-primary ~]$ vacuumlo -n cas -v Connected to database "cas" Test run: no large objects will be removed! Checking expiration_policy in public.serviceticket Checking service in public.serviceticket Checking expiration_policy in public.ticketgrantingticket Checking authentication in public.ticketgrantingticket Checking services_granted_access_to in public.ticketgrantingticket Would remove 382143 large objects from database "cas". --清理，可以加个“l”参数，每隔这个参数提交一次 [postgres@kenyon-primary ~]$ vacuumlo cas -v -l 1000 Connected to database "cas" Test run: no large objects will be removed! Checking expiration_policy in public.serviceticket Checking service in public.serviceticket Checking expiration_policy in public.ticketgrantingticket Checking authentication in public.ticketgrantingticket Checking services_granted_access_to in public.ticketgrantingticket Would remove 382143 large objects from database "cas".清理完毕再看一下
cas=# select pg_size_pretty(pg_database_size('cas'));
pg_size_pretty
----------------
1.3 GB
(1 row)
--空间还没有收缩，使用vacuum full analyze
cas=# vacuum full analyze verbose pg_largeobject;
INFO: vacuuming "pg_catalog.pg_largeobject"
INFO: scanned index "pg_largeobject_loid_pn_index" to remove 88928 row versions
DETAIL: cpu 0.01s/0.24u sec elapsed 0.26 sec.
INFO: "pg_largeobject": removed 88928 row versions in 6833 pages
DETAIL: cpu 0.00s/0.02u sec elapsed 0.02 sec.
INFO: index "pg_largeobject_loid_pn_index" Now contains 948117 row versions in 4120 pages
DETAIL: 88928 index row versions were removed.
1516 index pages have been deleted,1269 are currently reusable.
cpu 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_largeobject": found 88928 removable,52 nonremovable row versions in 6891 out of 109226 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 2329 unused item pointers.
0 pages are entirely empty.
cpu 0.03s/0.32u sec elapsed 0.35 sec.
INFO: analyzing "pg_catalog.pg_largeobject"
INFO: "pg_largeobject": scanned 30000 of 109226 pages,containing 260529 live rows and 0 dead rows; 30000 rows in sample,947568 estimated total rows
VACUUM
cas=# select pg_size_pretty(pg_relation_size('pg_largeobject'));
pg_size_pretty
----------------
8192 KB
(1 row) 整个世界清静了。 写成脚本的方式，定期执行
[postgres@kenyon-primary ~]$ more cas_rm_lo.sh #!/bin/bash ###################################################### ## ## purpose:Rm the cas's large object and free space ## ## author :Kenyon ## ## created:2014-01-22 ## ##################################################### source /home/postgres/.bash_profile vacuumlo cas -l 1000 -v psql -d cas -c "vacuum full analyze verbose pg_largeobject;" psql -d cas -c "vacuum full analyze verbose pg_largeobject_Metadata;"三、总结 在使用开源的一些工具时，如果有使用一些大对象，需要注意一下程序清理用户数据时是否会同步删除大对象数据。
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