postgresql如何从表中高效的随机获取一条记录

select C_BH from db_scld.t_scld_cprscjl  order by `random()` LIMIT 1;
select  c_jdrybm from db_scld.t_jdry 
      where c_bmbm = v_scdd and c_sfyx =\'1\' and c_ryzszt not in (\'05\',\'12\',\'11\',\'07\',\'09\',\'13\')  order by `random()` limit 1 
      
 db_jdsjpt=# explain analyze select C_BH from db_scld.t_scld_cprscjl  order by random() LIMIT 1;
                                                             QUERY PLAN                                                         
---------------------------------------------------------------------------------------------------
 Limit  (cost=61029.94..61029.94 rows=1 width=41) (actual time=587.193..587.193 rows=1 loops=1)
   ->  Sort  (cost=61029.94..63172.22 rows=856911 width=41) (actual time=587.185..587.185 rows=1 loops=1)
         Sort Key: (random())
         Sort Method: top-N heapsort  Memory: 25kB
         ->  Seq Scan on t_scld_cprscjl  (cost=0.00..56745.39 rows=856911 width=41) (actual time=0.019..380.139 rows=854682 loop
s=1)
 Planning time: 1.179 ms
 Execution time: 587.242 ms
(7 rows)
--表总数量
 db_jdsjpt=# select count(*) from db_scld.t_scld_cprscjl;
 count  
--------
 854682
(1 row)

随机获取一条记录random()

random()耗时：Time: 389.818 ms

--随机获取一条耗时
db_jdsjpt=# select C_BH from db_scld.t_scld_cprscjl  order by random() LIMIT 1;
               c_bh               
----------------------------------
 6d861b011c854040bf5b731f49d40b48
(1 row)

Time: 389.818 ms

改写1

offset耗时：Time: 60.022 ms

--offset可以走索引，少了排序操作
db_jdsjpt=# select C_BH from db_scld.t_scld_cprscjl  offset floor(random()*854682) LIMIT 1;
               c_bh               
----------------------------------
 f90301bd8ac2485196ffae32ee70345c
(1 row)

Time: 60.022 ms

db_jdsjpt=# explain analyze select C_BH from db_scld.t_scld_cprscjl  offset floor(random()*854682) LIMIT 1;
                                                                         QUERY PLAN                
                                
---------------------------------------------------------------------------------------------------
 Limit  (cost=3747.64..3747.68 rows=1 width=33) (actual time=30.758..30.759 rows=1 loops=1)
   ->  Index Only Scan using i_corscjl_cprscbh_ on t_scld_cprscjl  (cost=0.42..37472.65 rows=854682 width=33) (actual time=0.
047..25.808 rows=81993 loops=1)
         Heap Fetches: 0
 Planning time: 0.228 ms
 Execution time: 30.802 ms
(5 rows)

Time: 31.779 ms

改写2

pg从9.5开始提供抽样函数

使用tablesample抽样的过程中比例不能太低,否则可能获取不到结果，且不能带有过滤条件

system耗时： Time: 0.639 ms

system：随机性较差，效率高

--改写后耗时
db_jdsjpt=# select c_bh from db_scld.t_scld_cprscjl  tablesample system(0.1) limit 1;
               c_bh               
----------------------------------
 e2fce25399db42f0bf49faf8e7214d5f
(1 row)

Time: 0.639 ms

--system随机抽样以块为单位所以更快
db_jdsjpt=# explain analyze  select c_bh from db_scld.t_scld_cprscjl  tablesample system(0.1) limit 1;
                                                      QUERY PLAN                                                      
---------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..0.23 rows=1 width=33) (actual time=0.105..0.105 rows=1 loops=1)
   ->  Sample Scan on t_scld_cprscjl  (cost=0.00..192.55 rows=855 width=33) (actual time=0.102..0.102 rows=1 loops=1)
         Sampling: system (\'0.1\'::real)
 Planning time: 0.190 ms
 Execution time: 0.134 ms
(5 rows)

Time: 1.182 ms

改写3

bernoulli:随机性更好，但效率比system要低

bernoullih耗时：Time: 0.822 ms

db_jdsjpt=# select c_bh from db_scld.t_scld_cprscjl  tablesample bernoulli(0.1) limit 1;
               c_bh               
----------------------------------
 7ec30761ffd04bd9ad77797a33645a84
(1 row)

Time: 0.822 ms

--bernoulli以行为单位进行抽样，比system效率低点
db_jdsjpt=# explain analyze select c_bh from db_scld.t_scld_cprscjl  tablesample bernoulli(0.1) limit 1;
                                                       QUERY PLAN                                                       
---------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..53.85 rows=1 width=33) (actual time=1.410..1.411 rows=1 loops=1)
   ->  Sample Scan on t_scld_cprscjl  (cost=0.00..46042.55 rows=855 width=33) (actual time=1.408..1.408 rows=1 loops=1)
         Sampling: bernoulli (\'0.1\'::real)
 Planning time: 0.446 ms
 Execution time: 1.436 ms
(5 rows)

Time: 25.770 ms

同理另外一条sql也可用同样的方式，且在c_bmbm字段上面加上索引

当有条件的时候可以使用offset获取，offset的值也可以通过for循环传入

db_jdsjpt=# select count(*) from db_scld.t_jdry;
 count  
--------
 214819
(1 row)

db_jdsjpt=# select  c_jdrybm from db_scld.t_jdry  where c_bmbm = \'4402222804\' and c_sfyx =\'1\' and c_ryzszt not in (\'05\',\'12\',\'11\',\'07\',\'09\',\'13\') offset floor(random()*214819) limit 1; 
 c_jdrybm 
----------
(0 rows)

Time: 1.924 ms

对比

使用offset的时候和n的大小有关系，当n越大，扫描的索引块越多，就越大，但是相对于order by random()的方式仍然要快。

注意

system(0.1)等于百分之零点一，也就是抽样千分之一 854682*0.001=854，大概每次抽取854条记录

--system
db_jdsjpt=#  select count(*) from db_scld.t_scld_cprscjl  tablesample system(0.1) ;
 count 
-------
   592
(1 row)

Time: 1.499 ms

--bernoulli
db_jdsjpt=#  select count(*) from db_scld.t_scld_cprscjl  tablesample bernoulli(0.1) ;
 count 
-------
   840
(1 row)

Time: 86.037 ms
这里可以看出bernoulli效率比system要低

结语

1.随机获取表中的一条数据，当表中数据较小时使用random感觉不明显，当数据量大时random由于每次都需要排序操作，导致随机获取一条的成本较高

4.随机获取一条记录可以使用limit 1 offset n-1的方式，或者使用随机抽样的方式

5.无论是使用limit 1 offset n还是使用tablesample随机抽样方式都需要知道表中的数据量，不能超过表数据量