我对PostgreSQL 中 index scan 与 seq scan 的对比学习

开始

数据量很小的时候，我们可以看到，seq scan 比 index scan 更加有效。那是因为 index scan 至少要发生两次I/O，一次是 读取索引块， 一次是读取数据块。当index 很大的时候，情况可能会更加复杂。 

postgres=# select a.relpages, a.reltuples, a.relfilenode,a.reltype,b.typname from pg_class a, pg_type b where a.relname like 'gaotab%' and a.reltype=b.oid;    

 relpages | reltuples | relfilenode | reltype | typname     

----------+-----------+-------------+---------+---------    

        1 |      100 |       16396 |   16386 | gaotab    

数据量为 100条记录。

预估成本：

postgres=# set session enable_seqscan=false;                    

SET                    

postgres=# explain select name from gaotab where id=50;                    

                             QUERY PLAN                                                  

---------------------------------------------------------------------                    

 Index Scan using idx_id on gaotab  (cost=0.00..8.27 rows=1 width=5)                    

   Index Cond: (id = 50)                    

(2 rows)                    

postgres=# set session enable_seqscan=true;        

SET        

        

postgres=# explain select name from gaotab where id=50;        

                      QUERY PLAN                              

------------------------------------------------------        

 Seq Scan on gaotab  (cost=0.00..2.25 rows=1 width=5)        

   Filter: (id = 50)        

(2 rows)        

实际执行：

postgres=# set session enable_seqscan=false;        

SET        

postgres=# explain analyze select name from gaotab where id=50;        

                                                  QUERY PLAN                            

                                       

--------------------------------------------------------------------------------        

-------------------------------        

 Index Scan using idx_id on gaotab  (cost=0.00..8.27 rows=1 width=5) (actual tim        

e=0.112..0.113 rows=1 loops=1)        

   Index Cond: (id = 50)        

 Total runtime: 0.133 ms        

(3 rows)        

postgres=# set session enable_seqscan=true;        

SET        

postgres=# explain analyze select name from gaotab where id=50;        

                                           QUERY PLAN                                   

                        

--------------------------------------------------------------------------------        

----------------        

 Seq Scan on gaotab  (cost=0.00..2.25 rows=1 width=5) (actual time=0.014..0.018         

rows=1 loops=1)        

   Filter: (id = 50)        

   Rows Removed by Filter: 99        

 Total runtime: 0.034 ms        

(4 rows)        

等到数据量大的时候，就是截然不同了。

数据为1000条记录时，通过查询可以看到，已经跨越了7个page：

postgres=# analyze;    

ANALYZE    

postgres=# select a.relpages, a.reltuples, a.relfilenode,a.reltype,b.typname from pg_class a, pg_type b where a.relname like 'gaotab%' and a.reltype=b.oid;    

 relpages | reltuples | relfilenode | reltype | typname     

----------+-----------+-------------+---------+---------    

        7 |      1000 |       16396 |   16386 | gaotab    

(1 row)    

    

postgres=#     

再次预估成本，此时seq scan 已经开始变得不划算了：

postgres=# set session enable_seqscan=false;

SET

postgres=# explain select name from gaotab where id=50;

                             QUERY PLAN                              

---------------------------------------------------------------------

 Index Scan using idx_id on gaotab  (cost=0.00..8.27 rows=1 width=6)

   Index Cond: (id = 50)

(2 rows)



postgres=# set session enable_seqscan=true;

SET

postgres=# explain select name from gaotab where id=50;

                             QUERY PLAN                              

---------------------------------------------------------------------

 Index Scan using idx_id on gaotab  (cost=0.00..8.27 rows=1 width=6)

   Index Cond: (id = 50)

(2 rows)



postgres=# set session enable_indexscan=false;

SET

postgres=# explain select name from gaotab where id=50;

                             QUERY PLAN                              

---------------------------------------------------------------------

 Bitmap Heap Scan on gaotab  (cost=4.26..8.27 rows=1 width=6)

   Recheck Cond: (id = 50)

   ->  Bitmap Index Scan on idx_id  (cost=0.00..4.26 rows=1 width=0)

         Index Cond: (id = 50)

(4 rows)



postgres=# set session enable_bitmapscan=false;

SET

postgres=# explain select name from gaotab where id=50;

                      QUERY PLAN                       

-------------------------------------------------------

 Seq Scan on gaotab  (cost=0.00..19.50 rows=1 width=6)

   Filter: (id = 50)

(2 rows)



postgres=# 

实际执行

postgres=# set session enable_seqscan=false;

SET

postgres=# explain analyze select name from gaotab where id=50;

                                                            QUERY PLAN                                      

                       

------------------------------------------------------------------------------------------------------------

-----------------------

 Index Scan using idx_id on gaotab  (cost=10000000000.00..10000000008.27 rows=1 width=6) (actual time=0.020.

.0.022 rows=1 loops=1)

   Index Cond: (id = 50)

 Total runtime: 0.051 ms

(3 rows)



postgres=# set session enable_seqscan=true;

SET

postgres=# set session enable_indexscan=false;

SET

postgres=# set session enable_bitmapscan=false;

SET

postgres=# explain analyze select name from gaotab where id=50;

                                           QUERY PLAN                                            

-------------------------------------------------------------------------------------------------

 Seq Scan on gaotab  (cost=0.00..19.50 rows=1 width=6) (actual time=0.015..0.095 rows=1 loops=1)

   Filter: (id = 50)

   Rows Removed by Filter: 999

 Total runtime: 0.109 ms

(4 rows)



postgres=# 

[作者：技术者高健@博客园  mail: [email protected] ]

结束