PostgreSQL CLUSTER意在将表按照索引的顺序排布.
可以通过ctid来观察这个排布, 或者通过pg_stats.correlation来观察这个排布.
下面来举个例子 :
创建测试表 :
digoal=> create table test (id int, val numeric);
CREATE TABLE
插入测试数据 :
digoal=> insert into test select generate_series(1,100000),random();
INSERT 0 100000
表分析 :
digoal=> vacuum analyze test;
VACUUM
查询表的物理分布和对应列的值离散情况 :
可以看出id列的顺序和表的物理分布一致 :
digoal=> select correlation from pg_stats where schemaname='digoal' and tablename='test' and attname='id';
correlation
-------------
1
(1 row)
digoal=> select correlation from pg_stats where schemaname='digoal' and tablename='test' and attname='val';
0.00625629
查询val的最小值的物理存储位置 :
digoal=> select ctid,id,val from test where val=(select min(val) from test);
ctid | id | val
-----------+-------+------------------------
(380,154) | 70420 | 0.00000077439472079277
查询id的最小值的物理存储位置 :
因为测试数据是generate_series(1,100000)生成的, 所以ID列的值与物理分布相同 :
digoal=> select ctid,id,val from test where id=(select min(id) from test);
ctid | id | val
-------+----+-------------------
(0,1) | 1 | 0.645392156671733
创建索引 :
digoal=> create index idx_test_1 on test(id);
CREATE INDEX
digoal=> create index idx_test_2 on test(val);
使用cluster重新对表进行物理分布 :
digoal=> cluster test using idx_test_2;
CLUSTER
再次查看val的最小值的物理存储位置 :
说明表的物理分布已经和idx_test_2的排序相同了.
ctid | id | val
-------+-------+------------------------
(0,1) | 70420 | 0.00000077439472079277
再次查看id的最小值的物理存储位置 :
ctid | id | val
----------+----+-------------------
(349,37) | 1 | 0.645392156671733
分析表 :
可以看出现在表的物理分布和val列的值顺序一致.
-0.00118176
现在使用idx_test_1这个索引来重分布test表 :
结果不再解释.
digoal=> cluster verbose test using idx_test_1;
INFO: clustering "digoal.test" using index scan on "idx_test_1"
INFO: "test": found 0 removable, 100000 nonremovable row versions in 542 pages
DETAIL: 0 dead row versions cannot be removed yet.
CPU 0.00s/0.15u sec elapsed 0.19 sec.
0.00780408
cluster的好处 :
1. 因为PostgreSQL 统计了表的物理存储顺序和每一列值的顺态值, 在执行计划选择时, 可以用到这个顺态值用作计算走索引的成本.
这个值越接近0, 说明表的物理分布上这个列的值比较离散, 走索引的成本越高;
反之这个值越接近1或者-1, 说明表的物理分布上这个列的值比较有序, 走索引的成本越低;
2. cluster 后, 表的物理分布就和索引一致了, 观察上面ctid的变化就可以得知. cluster完后查看pg_stats.correlation会等于1.
3. 注意cluster是一次性的, 在这个表做了dml 后, 物理分布又会被打乱.
4. 结合块设备的read ahead, cluster后, 如果执行计划走这个cluster了的索引取数据(如几百条到几万条[取数在全表来说是比较少的时候]), 可以减少大量的物理磁盘读请求.
cluster加哪些锁?
SESSION 1 :
digoal=> begin;
BEGIN
digoal=> select pg_backend_pid();
pg_backend_pid
----------------
10222
SESSION 2 :
digoal=> select pid,locktype,database,relation,granted,mode,b.relname from pg_locks a,pg_class b where a.relation=b.oid and pid=10222;
pid | locktype | database | relation | granted | mode | relname
-------+----------+----------+----------+---------+---------------------+------------
10222 | relation | 16386 | 89731 | t | AccessShareLock | idx_test_2
10222 | relation | 16386 | 89731 | t | AccessExclusiveLock | idx_test_2
10222 | relation | 16386 | 89729 | t | AccessShareLock | idx_test_1
10222 | relation | 16386 | 89729 | t | AccessExclusiveLock | idx_test_1
10222 | relation | 16386 | 89726 | t | ShareLock | test
10222 | relation | 16386 | 89726 | t | AccessExclusiveLock | test
(6 rows)
对照src/include/storage/lock.h的定义, 可以看出这里总共涉及了以下锁 :
test表的ShareLock 是CREATE INDEX (WITHOUT CONCURRENTLY)申请的.
test表的AccessExclusiveLock 是重建表申请的,
索引的AccessShareLock和AccessExclusiveLock 是重建索引申请的.
也就是说cluster不但要重建表还要重建索引.
AccessExclusiveLock和所有锁冲突, 因此select也被阻断 :
digoal=> cluster test using idx_test_1;
digoal=> select * from test limit 1;
waiting...
SESSION 3 :
digoal=> select pid,locktype,database,relation,granted,mode,b.relname from pg_locks a,pg_class b where a.relation=b.oid;
pid | locktype | database | relation | granted | mode | relname
-------+----------+----------+----------+---------+---------------------+----------------------------
12044 | relation | 16386 | 2663 | t | AccessShareLock | pg_class_relname_nsp_index
12044 | relation | 16386 | 2662 | t | AccessShareLock | pg_class_oid_index
12044 | relation | 16386 | 1259 | t | AccessShareLock | pg_class
12044 | relation | 16386 | 11069 | t | AccessShareLock | pg_locks
10759 | relation | 16386 | 89762 | f | AccessShareLock | test
5685 | relation | 16386 | 89768 | t | AccessShareLock | idx_test_1
5685 | relation | 16386 | 89768 | t | AccessExclusiveLock | idx_test_1
5685 | relation | 16386 | 89765 | t | AccessExclusiveLock | pg_toast_89762
5685 | relation | 16386 | 89762 | t | ShareLock | test
5685 | relation | 16386 | 89762 | t | AccessExclusiveLock | test
5685 | relation | 16386 | 89769 | t | AccessShareLock | idx_test_2
5685 | relation | 16386 | 89769 | t | AccessExclusiveLock | idx_test_2
(12 rows)
pid=10759的会话正在等待test表的AccessShareLock.
因此filenode也是发生变化的, 如下 :
digoal=> \d+ test
Table "digoal.test"
Column | Type | Modifiers | Storage | Stats target | Description
----------+-----------------------------+-----------+---------+--------------+-------------
id | integer | | plain | |
crt_time | timestamp without time zone | | plain | |
Indexes:
"idx_test_1" btree (id)
"idx_test_2" btree (crt_time DESC) CLUSTER
Has OIDs: no
digoal=> select pg_relation_filepath('test'::regclass);
pg_relation_filepath
----------------------------------------------
pg_tblspc/16385/PG_9.2_201204301/16386/89752
digoal=> select pg_relation_filepath('idx_test_1'::regclass);
pg_tblspc/16385/PG_9.2_201204301/16386/89755
digoal=> select pg_relation_filepath('idx_test_2'::regclass);
pg_tblspc/16385/PG_9.2_201204301/16386/89756
执行cluster后, 表, 索引的文件名都发生了变化 :
pg_tblspc/16385/PG_9.2_201204301/16386/89757
pg_tblspc/16385/PG_9.2_201204301/16386/89760
pg_tblspc/16385/PG_9.2_201204301/16386/89761
【参考】
1. src/backend/commands/cluster.c
2.
pg_stats.correlation
Statistical correlation between physical row ordering and logical ordering of the column values.
This ranges from -1 to +1.
When the value is near -1 or +1, an index scan on the column will be estimated to be cheaper than when it is near zero, due to reduction of random access to the disk.
(This column is null if the column data type does not have a < operator.)
digoal=> \d pg_stats View "pg_catalog.pg_stats"
Column | Type | Modifiers
------------------------+----------+-----------
schemaname | name |
tablename | name |
attname | name |
inherited | boolean |
null_frac | real |
avg_width | integer |
n_distinct | real |
most_common_vals | anyarray |
most_common_freqs | real[] |
histogram_bounds | anyarray |
correlation | real |
most_common_elems | anyarray |
most_common_elem_freqs | real[] |
elem_count_histogram | real[] |
3. src/include/storage/lock.h
#define NoLock 0
#define AccessShareLock 1 /* SELECT */
#define RowShareLock 2 /* SELECT FOR UPDATE/FOR SHARE */
#define RowExclusiveLock 3 /* INSERT, UPDATE, DELETE */
#define ShareUpdateExclusiveLock 4 /* VACUUM (non-FULL),ANALYZE, CREATE
* INDEX CONCURRENTLY */
#define ShareLock 5 /* CREATE INDEX (WITHOUT CONCURRENTLY) */
#define ShareRowExclusiveLock 6 /* like EXCLUSIVE MODE, but allows ROW
* SHARE */
#define ExclusiveLock 7 /* blocks ROW SHARE/SELECT...FOR
* UPDATE */
#define AccessExclusiveLock 8 /* ALTER TABLE, DROP TABLE, VACUUM
* FULL, and unqualified LOCK TABLE */
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