PostgreSQL 9.5 new feature - BRIN (block range index) index
2015-10-14 21:29
435 查看
![](http://postgres.cn/images/pg2015_conf.png)
Postgres2015全国用户大会将于11月20至21日在北京丽亭华苑酒店召开。本次大会嘉宾阵容强大,国内顶级PostgreSQL数据库专家将悉数到场,并特邀欧洲、俄罗斯、日本、美国等国家和地区的数据库方面专家助阵:
Postgres-XC项目的发起人铃木市一(SUZUKI Koichi)
Postgres-XL的项目发起人Mason Sharp
pgpool的作者石井达夫(Tatsuo Ishii)
PG-Strom的作者海外浩平(Kaigai Kohei)
Greenplum研发总监姚延栋
周正中(德哥), PostgreSQL中国用户会创始人之一
汪洋,平安科技数据库技术部经理
……
2015年度PG大象会报名地址:http://postgres2015.eventdove.com/PostgreSQL中国社区: http://postgres.cn/PostgreSQL专业1群: 3336901(已满)PostgreSQL专业2群: 100910388PostgreSQL专业3群: 150657323 |
![]() |
比如一张表有1000个数据块,我们建议一个BRIN在ID(假设这个表有ID字段)上的索引。
BRIN默认是每128个连续数据块区间存储一个字段取值的区间,所以这个索引的信息量是将1000个数据块划分为几个连续的128个块的区间,然后存储每个区间ID值的取值范围。
很显然,BRIN索引时lossy索引(即有损索引),那么我们并不能直接从索引中精确匹配要查询的记录,但是通过索引我们可以将查询范围缩小到最小128个连续的数据块(假设我们要找的值落在这个区间)。
以上是BRIN大概的原理,那么BRIN可以用在什么场景呢?
一个非常好的场景是流式日志数据,比如用户行为,大批量的数据按时间顺序不停的插入数据表。
我们如果要按照时间来访问这样的数据,以往我们需要创建BTREE索引,可以范围查询或者精确匹配。但是BTREE索引需要存储的信息量较大,如果数据量很大,索引也很庞大。
BRIN的话,索引可以变得很小,而且因为数据是按照时间顺序插入的,所以BRIN的信息量也很大,因为每个连续的数据块区间存储的时间范围和其他连续的数据块区间独立性很好,即不会出现大量数据交叉,如果有大量较差,那么使用BRIN检索还不如全表扫描。
BRIN可认为是全表扫描的切片,如果数据值分布和物理值分布的相关性很好,那么BRIN无疑是非常好的选择。
这里说到的相关性,大家可以参考统计学的知识,或者参考我之前写过的一篇文章。
http://blog.163.com/digoal@126/blog/static/163877040201512810112541/
接下来我们测试一下BRIN对于相关性好和相关性差的数据,以及他们的性能。
postgres=# create table t1(id int,info text);CREATE TABLEpostgres=# create table t2(id int,info text);CREATE TABLEpostgres=# insert into t1 select generate_series(1,10000000),md5(random()::text);INSERT 0 10000000
以下数据ID和物理存储相关性非常差。
postgres=# insert into t2 select id,md5(random()::text) from generate_series(1,10000000) as t(id) order by random();INSERT 0 10000000postgres=# analyze t1;ANALYZEpostgres=# analyze t2;ANALYZE
查询他们的相关性。显然T2表的物理存储和实际值顺序相关性很差。
postgres=# select correlation from pg_stats where tablename='t1' and attname='id'; correlation ------------- 1(1 row)postgres=# select correlation from pg_stats where tablename='t2' and attname='id'; correlation ------------- 0.00805771(1 row)
创建索引,创建索引的速度明显比BTREE索引快,因为BRIN只需要存储值区间,瘦得很。
postgres=# create index idx_t1_id on t1 using brin (id);CREATE INDEXpostgres=# create index idx_t2_id on t2 using brin (id);CREATE INDEX
我们看看索引的大小和表的大小,从BRIN的原理我们可以想象索引肯定很小,表650MB,索引才192K。
postgres=# \di+ List of relations Schema | Name | Type | Owner | Table | Size | Description --------+-----------+-------+----------+-------+--------+------------- public | idx_t1_id | index | postgres | t1 | 192 kB | public | idx_t2_id | index | postgres | t2 | 192 kB | (2 rows)postgres=# \dt+ t1 List of relations Schema | Name | Type | Owner | Size | Description --------+------+-------+----------+--------+------------- public | t1 | table | postgres | 650 MB | (1 row)postgres=# \dt+ t2 List of relations Schema | Name | Type | Owner | Size | Description --------+------+-------+----------+--------+------------- public | t2 | table | postgres | 650 MB | (1 row)
来看看实际的查询差别就知道,BRIN有多么适合流式数据了。
postgres=# explain analyze select * from t1 where id>=1000 and id<=5000; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on t1 (cost=50.98..9767.60 rows=3803 width=37) (actual time=0.351..13.732 rows=4001 loops=1) Recheck Cond: ((id >= 1000) AND (id <= 5000)) Rows Removed by Index Recheck: 57567 Heap Blocks: lossy=128 -> Bitmap Index Scan on idx_t1_id (cost=0.00..50.03 rows=3803 width=0) (actual time=0.104..0.104 rows=1280 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.111 ms Execution time: 14.019 ms(8 rows)
对于相关性差的,还不如全表扫描。
postgres=# explain analyze select * from t2 where id>=1000 and id<=5000; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on t2 (cost=49.78..9549.73 rows=3686 width=37) (actual time=2.806..2268.044 rows=4001 loops=1) Recheck Cond: ((id >= 1000) AND (id <= 5000)) Rows Removed by Index Recheck: 9995999 Heap Blocks: lossy=20791 -> Bitmap Index Scan on idx_t2_id (cost=0.00..48.86 rows=3686 width=0) (actual time=2.019..2.019 rows=208640 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.195 ms Execution time: 2268.590 ms(8 rows)
t2全表扫描
postgres=# set enable_bitmapscan=off;SETpostgres=# explain analyze select * from t2 where id>=1000 and id<=5000; QUERY PLAN ----------------------------------------------------------------------------------------------------------- Seq Scan on t2 (cost=0.00..170791.00 rows=3686 width=37) (actual time=0.593..1881.929 rows=4001 loops=1) Filter: ((id >= 1000) AND (id <= 5000)) Rows Removed by Filter: 9995999 Planning time: 0.109 ms Execution time: 1882.397 ms(5 rows)
接下来BRIN和BTREE索引对比一下。
postgres=# create index idx_t1_id_bt on t1 using btree (id);CREATE INDEXpostgres=# create index idx_t2_id_bt on t2 using btree (id);CREATE INDEXpostgres=# set enable_bitmapscan=on;SETpostgres=# drop index idx_t1_id;DROP INDEXpostgres=# drop index idx_t2_id;DROP INDEXpostgres=# explain analyze select * from t1 where id>=1000 and id<=5000; QUERY PLAN -------------------------------------------------------------------------------------------------------------------------- Index Scan using idx_t1_id_bt on t1 (cost=0.43..102.04 rows=3880 width=37) (actual time=0.023..1.048 rows=4001 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.412 ms Execution time: 1.318 ms(4 rows)
postgres=# explain analyze select * from t2 where id>=1000 and id<=5000; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on t2 (cost=53.05..10056.68 rows=3962 width=37) (actual time=1.932..8.304 rows=4001 loops=1) Recheck Cond: ((id >= 1000) AND (id <= 5000)) Heap Blocks: exact=3642 -> Bitmap Index Scan on idx_t2_id_bt (cost=0.00..52.05 rows=3962 width=0) (actual time=1.143..1.143 rows=4001 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.379 ms Execution time: 8.621 ms(7 rows)
我们看到btree索引查询性能是提高了,但是索引大小你看看有多大?
postgres=# \di+ List of relations Schema | Name | Type | Owner | Table | Size | Description --------+--------------+-------+----------+-------+--------+------------- public | idx_t1_id_bt | index | postgres | t1 | 213 MB | public | idx_t2_id_bt | index | postgres | t2 | 213 MB | (2 rows)
接下调整brin索引的精度提高查询效率,我们了解到默认的brin是存储128个连续的数据块区间的,这个值越小,精度越高。
postgres=# create index idx_t1_id on t1 using brin (id) with (pages_per_range=1);CREATE INDEXpostgres=# create index idx_t2_id on t2 using brin (id) with (pages_per_range=1);CREATE INDEXpostgres=# \di+ List of relations Schema | Name | Type | Owner | Table | Size | Description --------+--------------+-------+----------+-------+--------+------------- public | idx_t1_id | index | postgres | t1 | 672 kB | public | idx_t1_id_bt | index | postgres | t1 | 213 MB | public | idx_t2_id | index | postgres | t2 | 672 kB | public | idx_t2_id_bt | index | postgres | t2 | 213 MB | (4 rows)postgres=# drop index idx_t1_id_bt;DROP INDEXpostgres=# drop index idx_t2_id_bt;DROP INDEXpostgres=# explain analyze select * from t1 where id>=1000 and id<=5000; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------ Bitmap Heap Scan on t1 (cost=110.98..9827.60 rows=3803 width=37) (actual time=9.487..10.571 rows=4001 loops=1) Recheck Cond: ((id >= 1000) AND (id <= 5000)) Rows Removed by Index Recheck: 328 Heap Blocks: lossy=9 -> Bitmap Index Scan on idx_t1_id (cost=0.00..110.03 rows=3803 width=0) (actual time=9.449..9.449 rows=90 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.141 ms Execution time: 10.853 ms(8 rows)postgres=# explain analyze select * from t2 where id>=1000 and id<=5000; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on t2 (cost=109.78..9609.73 rows=3686 width=37) (actual time=10.407..481.673 rows=4001 loops=1) Recheck Cond: ((id >= 1000) AND (id <= 5000)) Rows Removed by Index Recheck: 2125867 # 看看精度不高的后果,取4001条数据却额外扫描了2125867条无用数据 Heap Blocks: lossy=4428 -> Bitmap Index Scan on idx_t2_id (cost=0.00..108.86 rows=3686 width=0) (actual time=10.364..10.364 rows=44280 loops=1) Index Cond: ((id >= 1000) AND (id <= 5000)) Planning time: 0.106 ms Execution time: 482.077 ms(8 rows)
精度提高后,扫描效率有一定的提升。(对于相关度不高的就不要用BRIN了,精度提高到1都于事无补的,无用功太多)当然相比btree还有差距,不过对于大数据场景,我们还要考虑数据的插入性能,对于btree插入性能好还是brin的插入性能好呢?
我这里简单的测试了一下,并未涉及并发处理,已经可以明显的了解到btree索引对数据插入带来的开销更大。
postgres=# \d t1 Table "public.t1" Column | Type | Modifiers --------+---------+----------- id | integer | info | text | Indexes: "idx_t1_id" brin (id) WITH (pages_per_range=1)postgres=# \timingTiming is on.postgres=# insert into t1 select generate_series(1,1000000);INSERT 0 1000000Time: 2152.527 mspostgres=# drop index idx_t1_id;DROP INDEXTime: 9.527 mspostgres=# create index idx_t1_id_bt on t1 using btree (id);CREATE INDEXTime: 29659.752 mspostgres=# insert into t1 select generate_series(1,1000000);INSERT 0 1000000Time: 5407.971 ms
最后,我们同样可以使用pageinspect来观测brin索引的内容。
postgres=# create extension pageinspect;CREATE EXTENSIONpostgres=# select * from brin_page_items(get_raw_page('idx_t1_id',10),'idx_t1_id'); itemoffset | blknum | attnum | allnulls | hasnulls | placeholder | value ------------+--------+--------+----------+----------+-------------+---------------------- 1 | 2176 | 1 | f | f | f | {1046657 .. 1047137} 2 | 2177 | 1 | f | f | f | {1047138 .. 1047618} 3 | 2178 | 1 | f | f | f | {1047619 .. 1048099} 4 | 2179 | 1 | f | f | f | {1048100 .. 1048580} 5 | 2180 | 1 | f | f | f | {1048581 .. 1049061} 6 | 2181 | 1 | f | f | f | {1049062 .. 1049542} 7 | 2182 | 1 | f | f | f | {1049543 .. 1050023}。。。
例如我们看到2176 这个数据块的ID取值区间是{1046657 .. 1047137},我们使用ctid来验证一下.
postgres=# select min(id),max(id) from t1 where ctid::text ~ E'^\\(2176,'; min | max ---------+--------- 1046657 | 1047137(1 row)# 完全正确
其他还有几个pageinspect的函数:
postgres=# SELECT brin_page_type(get_raw_page('idx_t1_id', id)) from generate_series(0,10) t(id); brin_page_type ---------------- meta revmap revmap revmap revmap revmap regular regular regular regular regular(11 rows)postgres=# SELECT * FROM brin_metapage_info(get_raw_page('idx_t1_id', 0)); magic | version | pagesperrange | lastrevmappage ------------+---------+---------------+---------------- 0xA8109CFA | 1 | 1 | 5(1 row)postgres=# SELECT * FROM brin_revmap_data(get_raw_page('idx_t1_id', 1)) limit 5; pages ----------- (18,1105) (18,1106) (18,1107) (18,1108) (18,1109)(5 rows)
截止目前,PostgreSQL可以支持btree,hash,gin,gist,spgist,brin共6种索引访问方法。用户可以根据实际应用场景选择合适的索引。
[参考]
1. http://blog.163.com/digoal@126/blog/static/163877040201512810112541/
2. http://www.postgresql.org/docs/devel/static/brin.html
3. http://www.postgresql.org/docs/devel/static/sql-createindex.html
BRIN indexes accept a different parameter:
pages_per_rangeDefines the number of table blocks that make up one block range for each entry of a BRIN index (see Section 60.1 for more details). The default is 128.
相关文章推荐
- Android之获取手机上的图片和视频缩略图thumbnails
- 数据库链接字符串查询网站
- DB2实例管理
- DB2实例管理
- 保障MySQL数据安全的14个最佳方法
- mysql问答汇集
- 创建一个空的IBM DB2 ECO数据库的方法
- 选定虚拟主机 性能凸显优势
- Access 2000 数据库 80 万记录通用快速分页类
- 开通一个数据库失败的原因的和解决办法
- 一个简单的asp数据库操作类
- 修改一行代码提升 Postgres 性能 100 倍
- CentOS下DB2数据库安装过程详解
- PostgreSQL新手入门教程
- EasyASP v1.5发布(包含数据库操作类,原clsDbCtrl.asp)第1/2页
- PostgreSQL教程(十):性能提升技巧
- PostgreSQL教程(二):模式Schema详解
- PostgreSQL教程(十三):数据库管理详解