我们知道 volatile 函数会影响SQL的执行性能，比如：volatile 类型函数无法建函数索引、volatile 函数针对每条记录都要执行一次。本篇的例子主要讲述 volatile 类型的函数还会影响子查询的提升。

1、构建例子

create table t1(id1 integer,name1 varchar(9),addr1 text);
create table t2(id2 integer,name2 varchar(9),addr2 text);
insert into t1 select generate_series(1,1000000),generate_series(1,1000000),'abc';
insert into t2 select generate_series(1,1000000),generate_series(1,1000000),'abc';
create index ind_t1 on t1(id1);
create index ind_t2 on t2(id2);

2、volatile 函数与执行计划

对于 t2 表的访问无法使用索引。

test=# \df+ replace                                                                                                              
                                                                                      List of functions
 Schema |  Name   | Result data type  | Argument data types | Type | Volatility | Parallel | Owner  | Security | Access privileges | Language |           Source code           | Description 
--------+---------+-------------------+---------------------+------+------------+----------+--------+----------+-------------------+----------+---------------------------------+-------------
 sys    | replace | character varying | text, text, text    | func | volatile   | safe     | system | invoker  |                   | c        | ora_replace_text                | 


test=# explain select id1,name1 from t1 a ,(select id2,replace(id2,'b','B') name2 from t2 ) b where a.id1=b.id2 and name1='123'; 
                             QUERY PLAN                              
---------------------------------------------------------------------
 Hash Join  (cost=17935.01..52120.02 rows=1 width=10)
   Hash Cond: (t2.id2 = a.id1)
   ->  Seq Scan on t2  (cost=0.00..20435.00 rows=1000000 width=36)
   ->  Hash  (cost=17935.00..17935.00 rows=1 width=10)
         ->  Seq Scan on t1 a  (cost=0.00..17935.00 rows=1 width=10)
               Filter: ((name1)::text = '123'::text)
(6 rows)

不使用子查询情况下的，可以使用索引：

test=# explain select  id1,name1,id2,replace(id2,'b','B') name2 from t1 a,t2 b where a.id1=b.id2 and name1='123'; 
                                  QUERY PLAN                                  
------------------------------------------------------------------------------
 Nested Loop  (cost=0.42..17943.46 rows=1 width=46)
   ->  Seq Scan on t1 a  (cost=0.00..17935.00 rows=1 width=10)
         Filter: ((name1)::text = '123'::text)
   ->  Index Only Scan using ind_t2 on t2 b  (cost=0.42..8.44 rows=1 width=4)
         Index Cond: (id2 = a.id1)
(5 rows)

3、immutable 函数与执行计划

改成immutable 函数后，子查询可以提升，从而能够使用索引。

test=# \df+ replace
                                                                                      List of functions
 Schema |  Name   | Result data type  | Argument data types | Type | Volatility | Parallel | Owner  | Security | Access privileges | Language |           Source code           | Description 
--------+---------+-------------------+---------------------+------+------------+----------+--------+----------+-------------------+----------+---------------------------------+-------------
 sys    | replace | character varying | text, text, text    | func | immutable  | safe     | system | invoker  |                   | c        | ora_replace_text                | 


test=# explain select id1,name1 from t1 a ,(select id2,replace(id2,'b','B') name2 from t2 ) b where a.id1=b.id2 and name1='123'; 
                                 QUERY PLAN                                 
----------------------------------------------------------------------------
 Nested Loop  (cost=0.42..17943.45 rows=1 width=10)
   ->  Seq Scan on t1 a  (cost=0.00..17935.00 rows=1 width=10)
         Filter: ((name1)::text = '123'::text)
   ->  Index Only Scan using ind_t2 on t2  (cost=0.42..8.44 rows=1 width=4)
         Index Cond: (id2 = a.id1)
(5 rows)