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os: ubuntu 16.04
db: postgresql 9.6.8
pg_repack: 1.4.4
CLUSTER 是根据一个索引聚簇一个表，优点是使用这个索引高效，确定是可能会影响其它索引的效率。
语法
peiybdb=# \h cluster
Command: CLUSTER
Description: cluster a table according to an index
Syntax:
CLUSTER [VERBOSE] table_name [ USING index_name ]
CLUSTER [VERBOSE]
实验
peiybdb=# \d+ tmp_t0;
Table "public.tmp_t0"
Column | Type | Modifiers | Storage | Stats target | Description
--------+--------+-----------+---------+--------------+-------------
c0 | bigint | | plain | |
c1 | bigint | not null | plain | |
Indexes:
"tmp_t0_pk" PRIMARY KEY, btree (c1)
peiybdb=# cluster verbose public.tmp_t0 using tmp_t0_pk;
INFO: clustering "public.tmp_t0" using index scan on "tmp_t0_pk"
INFO: "tmp_t0": found 0 removable, 2999983 nonremovable row versions in 16217 pages
DETAIL: 0 dead row versions cannot be removed yet.
CPU 0.20s/7.75u sec elapsed 9.06 sec.
CLUSTER
peiybdb=#
执行 CLUSTER 时的锁
peiybdb=# select oid,relname from pg_class where relname in ('tmp_t0','tmp_t0_pk');
oid | relname
---------+-----------
1447752 | tmp_t0_pk
1447603 | tmp_t0
(2 rows)
peiybdb=# select * from pg_locks where pid = '29475';
locktype | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid | mode | granted | fastpath
---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+---------------------+---------+----------
virtualxid | | | | | 3/4278 | | | | | 3/4278 | 29475 | ExclusiveLock | t | t
relation | 1406609 | 1453912 | | | | | | | | 3/4278 | 29475 | AccessExclusiveLock | t | f
relation | 1406609 | 1447752 | | | | | | | | 3/4278 | 29475 | AccessShareLock | t | f
relation | 1406609 | 1447752 | | | | | | | | 3/4278 | 29475 | AccessExclusiveLock | t | f
relation | 1406609 | 1447603 | | | | | | | | 3/4278 | 29475 | ShareLock | t | f
relation | 1406609 | 1447603 | | | | | | | | 3/4278 | 29475 | AccessExclusiveLock | t | f
transactionid | | | | | | 454353304 | | | | 3/4278 | 29475 | ExclusiveLock | t | f
(7 rows)
可以看到在对表做 cluster 时，表的锁为 ShareLock、AccessExclusiveLock。
有 AccessExclusiveLock 时，表是做不了任何操作的，包括select，在生产环境慎用。最好使用 pg_repack 操作。