nulls first & nulls last 对索引影响

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nulls first & nulls last 对索引影响

rudy_gao 2016-05-04 14:18:10 浏览511
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-- 当我们需要排序字段时,比如order by name,如果name字段定义时没有not null时,就有可能涉及到null值的排序
-- 如果不注意,可能会造成隐藏的bug,pg默认null是无穷大,在升序时排在最后面,当然在排序时也可以指定 nulls first 或 nulls last
-- 具体使用方法在此不在复述,本文主要是讲的是在创建索引时指定 nulls first 或 nulls last 对查询的影响


-- 基础创建索引语法如下
CREATE INDEX [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
    
-- 在升序或降序时的默认值
-- NULLS FIRST
Specifies that nulls sort before non-nulls. This is the default when DESC is specified.
-- NULLS LAST
Specifies that nulls sort after non-nulls. This is the default when DESC is not specified.



--创建测试表
create table t as select n id ,'rudy'||n as name ,n||'password' as password,now() + (n||' second')::interval as create_date from generate_seies(1,1000000) n;

postgres=# update t set name = null where mod(id,2)=0;
UPDATE 500000
-- 构造随机的null值,本次测试取对半
postgres=# create table t1 as select * from t; 
SELECT 1000000
postgres=# create index on t(name);
CREATE INDEX
postgres=# create index on t1(name nulls first);
CREATE INDEX

-- 在不指定 null first 时,由于pg默认null无限大,故当排序为升序时,t表能够使用索引,t1表却是使用全表扫描
postgres=# explain verbose select * from t where id>1000 order by name limit 10;
                                        QUERY PLAN                                         
-------------------------------------------------------------------------------------------
 Limit  (cost=0.42..1.26 rows=10 width=33)
   Output: id, name, password, create_date
   ->  Index Scan using idx_t_name on public.t  (cost=0.42..83387.28 rows=999027 width=33)
         Output: id, name, password, create_date
         Filter: (t.id > 1000)
(5 rows)


postgres=# explain verbose select * from t1 where id>1000 order by name limit 10;
                                  QUERY PLAN                                   
-------------------------------------------------------------------------------
 Limit  (cost=40949.92..40949.94 rows=10 width=33)
   Output: id, name, password, create_date
   ->  Sort  (cost=40949.92..43447.52 rows=999041 width=33)
         Output: id, name, password, create_date
         Sort Key: t1.name
         ->  Seq Scan on public.t1  (cost=0.00..19361.00 rows=999041 width=33)
               Output: id, name, password, create_date
               Filter: (t1.id > 1000)
(8 rows)


-- 在指定 nulls first 时,由于t1表创建索引时null值放在最前面,而且排序字段为升序,所以t1表使用索引,t表使用全表扫描
postgres=# explain verbose select * from t where id>1000 order by name nulls first limit 10; 
                                  QUERY PLAN                                  
------------------------------------------------------------------------------
 Limit  (cost=46095.61..46095.64 rows=10 width=33)
   Output: id, name, password, create_date
   ->  Sort  (cost=46095.61..48593.18 rows=999027 width=33)
         Output: id, name, password, create_date
         Sort Key: t.name NULLS FIRST
         ->  Seq Scan on public.t  (cost=0.00..24507.00 rows=999027 width=33)
               Output: id, name, password, create_date
               Filter: (t.id > 1000)
(8 rows)


postgres=# explain verbose select * from t1 where id>1000 order by name nulls first limit 10;
                                         QUERY PLAN                                          
---------------------------------------------------------------------------------------------
 Limit  (cost=0.42..0.98 rows=10 width=33)
   Output: id, name, password, create_date
   ->  Index Scan using t1_name_idx on public.t1  (cost=0.42..55921.45 rows=999041 width=33)
         Output: id, name, password, create_date
         Filter: (t1.id > 1000)
(5 rows)



-- 综上,为什么会出现这样的结果,因为我们默认创建的btree索引,其叶子结点的数据是有序排列的,当创建索引不指定nulls first时,pgl默认把null值放在叶子节点的最后
-- 如果排序时只是order by name,未指定nulls first,pg只需要根据索引顺序的返回需要的数据则可,否则,如果order by name nulls last,pg如果使用索引,
-- 其可能先在叶子未尾节点返回null值的数据,再在叶子起始节点开始返回数据(假设需要返回10条数据,null值为5条,非null值有5条),其明显此时使用索引不是高效的

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