Postgres的索引01

一.PG 9.3有以下索引类型

1.b-tree

• 1.1支持前导模糊查询，如xxx%或者^'xxx'
• 1.2忽略大小写字符前导模糊查询，如ILIKE 'XXX%'或者~*'^xxx'
• 1.3支持常见的条件运算符< = <= = >= >

2.hash

• 仅支持=条件运算符

3.gin

• 支持多列值索引，例如数据类型，全文检索类型
• <@ 被包含 array[1,2,3] <@ array[2,3,4]
• @> 包含 array[1,2,3] @> array[2]
• = 相等 array[1,2,3] = array[1,2,3]
• && 相交 array[1,2,3]&& array[2]

4.gist

• 不是单类索引，算是一种索引框架，支持许多不同的索引策略，可以自定义条件运算符
• 支持近邻排序，如取某一个点的10个近邻

select * from places order by localtion <-> point '(101,456)' limit 10;

• << -- 严格在左侧, 例如circle '((0,0),1)' << circle '((5,0),1)'
• &< -- 表示左边的平面体不会扩展到超过右边的平面体的右边. 例如box '((0,0),(1,1))' &< box '((0,0),(2,2))'
• &> -- 表示左边的平面体不会扩展到超过右边的平面体的左边. 例如box '((0,0),(3,3))' &> box '((0,0),(2,2))'
• >> -- 严格在右
• <<| -- 严格在下
• &<| -- 不会扩展到超出上面
• |&> -- 不会扩展到超出下面
• |>> -- 严格在上
• @> -- 包含
• <@ -- 被包含
• ~= -- 相同
• && -- 相交
  

  http://www.postgresql.org/docs/9.3/static/functions-geometry.html

5.sp-gist

• 与gist类似，也是一张索引框架，支持基于磁盘存储的非平衡数据结构，如四叉树、k-d树、radix树
• 支持操作符 << >> ~= <@
• <^ 在下面，circle'((0,0),1)' <^ circle'((0,5),1) 左边的圆在右边的圆的下边
• >^ 在上面，circle'((0,5),1)' 》^ circle'((0,0),1) 左边的圆在右边的圆的上边

二.使用索引的好处

1.利用索引进行排序减少CPU开销

• 1.1 查询条件就是索引列

postgres=# \c db1
You are now connected to database "db1" as user "yzw".
db1=# create table test(id int,info text,crt_time timestamp);
CREATE TABLE
db1=# insert into test select generate_series(1,10000), md5(random()::text),clock_timestamp();
INSERT 0 10000
db1=# create index idx_test_1 on test(id);
CREATE INDEX
db1=# explain analyze select * from test where id<100 order by id;
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=396.80..405.13 rows=3333 width=44) (actual time=0.106..0.111 rows=99 loops=1)
   Sort Key: id
   Sort Method: quicksort  Memory: 32kB
   ->  Bitmap Heap Scan on test  (cost=66.12..201.78 rows=3333 width=44) (actual time=0.050..0.059 rows=99 loops=1)
         Recheck Cond: (id < 100)
         Heap Blocks: exact=1
         ->  Bitmap Index Scan on idx_test_1  (cost=0.00..65.28 rows=3333 width=0) (actual time=0.036..0.036 rows=99 loops=1)
               Index Cond: (id < 100)
 Planning time: 0.520 ms
 Execution time: 0.178 ms
(10 rows)

• 1.2 查询条件不是索引列

db1=# explain analyze select * from test where info='c969799412fed1c8f91eff5e65353a85' order by id;
                                              QUERY PLAN
-------------------------------------------------------------------------------------------------------
 Sort  (cost=219.01..219.01 rows=1 width=45) (actual time=1.112..1.112 rows=1 loops=1)
   Sort Key: id
   Sort Method: quicksort  Memory: 25kB
   ->  Seq Scan on test  (cost=0.00..219.00 rows=1 width=45) (actual time=0.011..1.104 rows=1 loops=1)
         Filter: (info = 'c969799412fed1c8f91eff5e65353a85'::text)
         Rows Removed by Filter: 9999
 Planning time: 0.081 ms
 Execution time: 1.129 ms
(8 rows)

> 为何都有排序的节点Sort Key？

# 关闭enable_seqscan全表扫描后，查询索引列没有了排序节点
db1=# set enable_seqscan=off;
SET
db1=# explain analyze select * from test where id<100 order by id;
                                                      QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_test_1 on test  (cost=0.29..10.04 rows=100 width=45) (actual time=0.005..0.016 rows=99 loops=1)
   Index Cond: (id < 100)
 Planning time: 0.119 ms
 Execution time: 0.034 ms
(4 rows)

enable_seqscan 9.4默认是on，9.3是off？

2.加速带条件的查询，删除，更新

• 2.1 正常开启全表扫描和索引扫描情况下，有索引的列查找走索引

db1=# set enable_seqscan=on;
SET
db1=# explain analyze select * from test where id=1;
                                                    QUERY PLAN
------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_test_1 on test  (cost=0.29..8.30 rows=1 width=45) (actual time=0.014..0.015 rows=1 loops=1)
   Index Cond: (id = 1)
 Planning time: 0.067 ms
 Execution time: 0.032 ms
(4 rows)

• 2.2在没有索引条件下的查询效率，即使有索引列也会走全表扫描

db1=# show enable_indexscan;
 enable_indexscan
------------------
 on
(1 row)

db1=# show enable_bitmapscan;
 enable_bitmapscan
-------------------
 on
(1 row)

db1=# set enable_indexscan=off,enable_bitmapscan=off;
db1=# set enable_indexscan=off;set enable_bitmapscan=off;
SET
SET
db1=# show enable_indexscan;show enable_bitmapscan;
 enable_indexscan
------------------
 off
(1 row)

 enable_bitmapscan
-------------------
 off
(1 row)
# 关闭索引后，变成全表扫描了
db1=# explain analyze select * from test where id=1;
                                           QUERY PLAN
-------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..219.00 rows=1 width=45) (actual time=0.012..0.943 rows=1 loops=1)
   Filter: (id = 1)
   Rows Removed by Filter: 9999
 Planning time: 0.138 ms
 Execution time: 0.971 ms
(5 rows)

• 2.3 加速join操作

db1=# set enable_indexscan=on;set enable_bitmapscan=on;
SET
SET
db1=# insert into test1 select generate_series(1,10000), md5(random()::text),clock_timestamp();
INSERT 0 10000

test1表没有建索引，走全表扫描，test表走id索引，并且出现嵌套循环

db1=# explain analyze select t1.*,t2.* from test t1 join test1 t2 on(t1.id=t2.id and t2.id=1);
                                                        QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.29..227.31 rows=1 width=90) (actual time=0.032..0.896 rows=1 loops=1)
   ->  Index Scan using idx_test_1 on test t1  (cost=0.29..8.30 rows=1 width=45) (actual time=0.019..0.020 rows=1 loops=1)
         Index Cond: (id = 1)
   ->  Seq Scan on test1 t2  (cost=0.00..219.00 rows=1 width=45) (actual time=0.010..0.873 rows=1 loops=1)
         Filter: (id = 1)
         Rows Removed by Filter: 9999
 Planning time: 0.124 ms
 Execution time: 0.927 ms
(8 rows)

给test1表增加索引后，也走索引，test1表的索引数据在内存，因此速度更快

db1=# create index idx_test1_id on test1(id);
CREATE INDEX
db1=# explain analyze select t1.*,t2.* from test t1 join test1 t2 on(t1.id=t2.id and t2.id=1);
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.57..16.62 rows=1 width=90) (actual time=0.033..0.034 rows=1 loops=1)
   ->  Index Scan using idx_test_1 on test t1  (cost=0.29..8.30 rows=1 width=45) (actual time=0.011..0.012 rows=1 loops=1)
         Index Cond: (id = 1)
   ->  Index Scan using idx_test1_id on test1 t2  (cost=0.29..8.30 rows=1 width=45) (actual time=0.020..0.020 rows=1 loops=1)
         Index Cond: (id = 1)
 Planning time: 0.240 ms
 Execution time: 0.059 ms
(7 rows)

merge join，两个join的表按照join列做好排序后，再进行join，也能用上索引，通常来说，能够使用merge join的地方，使用hash join更快

db1=# show enable_hashjoin;
 enable_hashjoin
-----------------
 on
(1 row)

db1=# show enable_mergejoin;
 enable_mergejoin
------------------
 on
(1 row)
# 关闭hashjoin
set enable_hashjoin=off;
db1=# explain analyze select t1.*,t2.* from test t1 join test1 t2 on t1.id=t2.id;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=0.57..884.57 rows=10000 width=90) (actual time=0.020..10.837 rows=10000 loops=1)
   Merge Cond: (t1.id = t2.id)
   ->  Index Scan using idx_test_1 on test t1  (cost=0.29..367.29 rows=10000 width=45) (actual time=0.006..2.453 rows=10000 loops=1)
   ->  Index Scan using idx_test1_id on test1 t2  (cost=0.29..367.29 rows=10000 width=45) (actual time=0.006..3.625 rows=10000 loops=1)
 Planning time: 0.309 ms
 Execution time: 11.304 ms
(6 rows)
# 如果没有索引，效率最差，先全表扫描，然后排序，再join
db1=# explain analyze select t1.*,t2.* from test t1 join test1 t2 on t1.id=t2.id;
                                                       QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=1716.77..1916.77 rows=10000 width=90) (actual time=3.090..7.286 rows=10000 loops=1)
   Merge Cond: (t1.id = t2.id)
   ->  Sort  (cost=858.39..883.39 rows=10000 width=45) (actual time=1.571..2.007 rows=10000 loops=1)
         Sort Key: t1.id
         Sort Method: quicksort  Memory: 1166kB
         ->  Seq Scan on test t1  (cost=0.00..194.00 rows=10000 width=45) (actual time=0.005..0.789 rows=10000 loops=1)
   ->  Sort  (cost=858.39..883.39 rows=10000 width=45) (actual time=1.514..2.039 rows=10000 loops=1)
         Sort Key: t2.id
         Sort Method: quicksort  Memory: 1166kB
         ->  Seq Scan on test1 t2  (cost=0.00..194.00 rows=10000 width=45) (actual time=0.003..0.748 rows=10000 loops=1)
 Planning time: 0.171 ms
 Execution time: 7.614 ms
(12 rows)
# 自动使用hash join
db1=# set enable_hashjoin=on;set enable_indexscan=on;set enable_bitmapscan=on;
SET
db1=# explain analyze select t1.*,t2.* from test t1 join test1 t2 on t1.id=t2.id;
                                                       QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=319.00..763.00 rows=10000 width=90) (actual time=2.208..7.150 rows=10000 loops=1)
   Hash Cond: (t1.id = t2.id)
   ->  Seq Scan on test t1  (cost=0.00..194.00 rows=10000 width=45) (actual time=0.005..0.966 rows=10000 loops=1)
   ->  Hash  (cost=194.00..194.00 rows=10000 width=45) (actual time=2.160..2.160 rows=10000 loops=1)
         Buckets: 1024  Batches: 1  Memory Usage: 782kB
         ->  Seq Scan on test1 t2  (cost=0.00..194.00 rows=10000 width=45) (actual time=0.003..0.959 rows=10000 loops=1)
 Planning time: 0.211 ms
 Execution time: 7.502 ms
(8 rows)

3.加速外键约束更新和删除操作

create table p(id int primary key, info text, crt_time timestamp);
create table f(id int primary key, p_id int references p(id) on delete cascade on update cascade, info text, crt_time timestamp);
insert into p select generate_series(1,10000), md5(random()::text), clock_timestamp();
insert into f select generate_series(1,10000), generate_series(1,10000), md5(random()::text), clock_timestamp();

f表的p_id列未加索引情况下

db1=# explain (analyze,verbose,costs,buffers,timing) update p set id=1 where id=0;
                                                       QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
 Update on public.p  (cost=0.29..8.30 rows=1 width=47) (actual time=0.053..0.053 rows=0 loops=1)
   Buffers: shared hit=7
   ->  Index Scan using p_pkey on public.p  (cost=0.29..8.30 rows=1 width=47) (actual time=0.019..0.019 rows=1 loops=1)
         Output: 1, info, crt_time, ctid
         Index Cond: (p.id = 0)
         Buffers: shared hit=3
 Planning time: 0.068 ms
 Trigger RI_ConstraintTrigger_a_16424 for constraint f_p_id_fkey on p: time=1.225 calls=1 # p表上耗时长
 Trigger RI_ConstraintTrigger_c_16426 for constraint f_p_id_fkey on f: time=0.068 calls=1
 Execution time: 1.377 ms
(10 rows)

增加p表索引后

create index idx_f_1 on f(p_id);
db1=#  explain (analyze,verbose,costs,buffers,timing) update p set id=0 where id=1;
                                                       QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
 Update on public.p  (cost=0.29..8.30 rows=1 width=47) (actual time=0.055..0.055 rows=0 loops=1)
   Buffers: shared hit=7
   ->  Index Scan using p_pkey on public.p  (cost=0.29..8.30 rows=1 width=47) (actual time=0.022..0.023 rows=1 loops=1)
         Output: 0, info, crt_time, ctid
         Index Cond: (p.id = 1)
         Buffers: shared hit=3
 Planning time: 0.079 ms
 Trigger RI_ConstraintTrigger_a_16424 for constraint f_p_id_fkey on p: time=0.132 calls=1 # p表耗时短
 Trigger RI_ConstraintTrigger_c_16426 for constraint f_p_id_fkey on f: time=0.085 calls=1
 Execution time: 0.307 ms
(10 rows)

4.索引在排他约束中的使用

• 要求左右操作符互换对结果没有影响，例如x=y,y=x结果都是true或者unknown

db1=# CREATE TABLE test2(id int,geo point,EXCLUDE USING btree (id WITH pg_catalog.=));
CREATE TABLE
db1=# insert into test2 (id) values (1);
INSERT 0 1
db1=# insert into test2 (id) values (1);
ERROR:  conflicting key value violates exclusion constraint "test2_id_excl"
DETAIL:  Key (id)=(1) conflicts with existing key (id)=(1).

> 模拟unique

5.加速唯一值约束、排他约束

• 主键
• 唯一键

CREATE TABLE test3(id int,geo point,EXCLUDE USING spGIST (geo WITH pg_catalog.~=));
select * from pg_indexes where tablename='test3';
db1=# select * from pg_indexes where tablename='test3';
 schemaname | tablename |   indexname    | tablespace |                        indexdef
------------+-----------+----------------+------------+---------------------------------------------------------
 public     | test3     | test3_geo_excl |            | CREATE INDEX test3_geo_excl ON test3 USING spgist (geo)
(1 row)

三.索引的弊端

• 随着表的记录块的变迁需要更新，因此会对这类操作带来一定的性能影响
• 块不变更的情况下触发hot特性，可以不需要更新索引
• 写多读少的场景，索引弊端可能大于其好处

四.注意事项

• 1.正常创建索引时，会阻断除查询意外的其他操作
• 2.使用并行CONCURRENTLY选项后，可以允许同时对标的DML操作，但是对于频繁DML的表，这种创建索引的时间非常长
• 3.某些索引不记录WAL，所以如果有利于WAL进行数据恢复的情况，如crash recovery，流复制，warm standby等，这类索引在使用前需要重建（HASH索引）