考虑一个主键作为列“id”的表
必须查询 id 范围为 [ 1, 2, 3, 4 ...]
有 2 个选项 -
1)
Select * from Table where id = 1;
Select * from Table where id = 2;
Select * from Table where id = 3;
Select * from Table where id = 4;
2)
Select * from Table where id in ( 1, 2, 3, 4 );
两者中哪一个在 Oracle 作为数据库和使用 Spring JDBC 模板进行持久性方面的性能更好。假设数据集 1、2、3、4 将在 Java 数据结构的限制范围内，则从应用程序的角度抛开内存限制。
它使用数据库连接池。
我宁愿使用第二个查询，因为它只查询数据库一次。除此之外，您也只能在数据库上连接一次。
请记住，数据库连接成本很高（消耗大量资源）。
为了备份 JW 和 a_horse_with_no_name 的语句，这里是一个测试，都使用相同的连接，因此这里忽略连接（池）问题。
首先使用主键创建表并为优化器收集统计信息：
SQL> create table mytable
2 as
3 select level id
4 , lpad('*',1000,'*') filler
5 from dual
6 connect by level <= 10000
7 /
Table created.
SQL> alter table mytable add constraint my_pk primary key (id)
2 /
Table altered.
SQL> exec dbms_stats.gather_table_stats(user,'mytable')
PL/SQL procedure successfully completed.
接下来，将所有语句放在共享池中，作为热身，切断大部分查询的输出：
SQL> select * from mytable where id = 1
2 /
ID
----------
FILLER
---------------------------------------------------------------------------------------------------------------------------------------
1
***************************************************************************************************************************************
***************************************************************************************************************************************
***************************************************************************************************************************************
***************************************************************************************************************************************
***************************************************************************************************************************************
***************************************************************************************************************************************
***************************************************************************************************************************************
*******************************************************
1 row selected.
SQL> select * from mytable where id = 2
2 /
1 row selected.
SQL> select * from mytable where id = 3
2 /
1 row selected.
SQL> select * from mytable where id = 4
2 /
1 row selected.
SQL> select * from mytable where id in (1,2,3,4)
2 /
4 rows selected.
接下来，检查自动跟踪输出，以查看数据库必须执行的工作：
SQL> set autotrace traceonly
SQL> select * from mytable where id = 1
2 /
1 row selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3280897506
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1005 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 1 | 1005 | 2 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | MY_PK | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=1)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
1387 bytes sent via SQL*Net to client
448 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select * from mytable where id = 2
2 /
1 row selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3280897506
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1005 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 1 | 1005 | 2 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | MY_PK | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
1387 bytes sent via SQL*Net to client
448 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select * from mytable where id = 3
2 /
1 row selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3280897506
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1005 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 1 | 1005 | 2 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | MY_PK | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=3)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
1387 bytes sent via SQL*Net to client
448 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select * from mytable where id = 4
2 /
1 row selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3280897506
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1005 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 1 | 1005 | 2 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | MY_PK | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=4)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
1387 bytes sent via SQL*Net to client
448 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select * from mytable where id in (1,2,3,4)
2 /
4 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1637292604
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 4020 | 4 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 4 | 4020 | 4 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | MY_PK | 4 | | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("ID"=1 OR "ID"=2 OR "ID"=3 OR "ID"=4)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
8 consistent gets
0 physical reads
0 redo size
2435 bytes sent via SQL*Net to client
448 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
SQL> set autotrace off
四次 3 一致得到对 8 一致得到。最后，让我们通过执行所有语句 10,000 次来为这两种变体计时：
SQL> set timing on
SQL> declare
2 v_id number;
3 v_filler varchar2(1000);
4 begin
5 for i in 1 .. 10000
6 loop
7 select *
8 into v_id, v_filler
9 from mytable
10 where id = 1
11 ;
12 select *
13 into v_id, v_filler
14 from mytable
15 where id = 2
16 ;
17 select *
18 into v_id, v_filler
19 from mytable
20 where id = 3
21 ;
22 select *
23 into v_id, v_filler
24 from mytable
25 where id = 4
26 ;
27 end loop;
28 end;
29 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:01.03
SQL> /
PL/SQL procedure successfully completed.
Elapsed: 00:00:01.00
SQL> /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.99
SQL> declare
2 v_id number;
3 v_filler varchar2(1000);
4 begin
5 for i in 1 .. 10000
6 loop
7 for r in (select * from mytable where id in (1,2,3,4))
8 loop
9 v_id := r.id;
10 v_filler := r.filler;
11 end loop;
12 end loop;
13 end;
14 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.41
SQL> /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.39
SQL> /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.39
SQL> set timing off
在这里，您可以看到向数据库发送四倍多的语句的开销最多。根据索引深度和聚类因子，这里和那里可能存在一些变化，但一个查询与四个查询仍然是最大的区别。
所以，希望这个线程的读者不会再说“已知 IN 子句很昂贵”:-)
问候，
罗布。
我认为这个问题是过早优化的一个很好的例子，为什么应该避免它。
您正在尝试从表中获取多个 ID 的行。要使用的明显查询应该是以最简单的方式满足要求的查询。除非有很好的理由，否则我总是会考虑使用：
Select * from Table where id in ( 1, 2, 3, 4 );
这使后来的开发人员清楚地知道发生了什么以及为什么。它还将优化任务留给数据库来处理，这就是它的用途。
如果在编写此代码后您发现存在性能问题，那么是时候考虑优化了。
对于任何类型的代码问题，最简单的解决方案通常是最好的。
我确实意识到我在这里有点离题并且没有很多知识 JDBC 但确实有很多 Oracle 知识
我将假设您正在使用 PreparedStatment
select * from TABLE where ID=?;
在 Oracle 中，使用 PreparedStatement 将允许 Oracle 缓存 SQL 的执行计划生成。因此传递 1 或 2 或 1000 的 ID 将使用相同的缓存 Oracle 计划。
甲骨文将对待
select * from TABLE where ID in (1, 2, 3);
和
select * from TABLE where ID in (4, 8, 15, 16, 23, 42);
作为两个不同的查询并为两者生成不同的执行计划。因此，从 Oracle 的角度来看，类型 1 更可取。
实际上，如果您在 ID 上放置索引，您将不会注意到两种样式之间的实质性差异。
与性能调优一样，“不要猜测，做测试”。使用 YourKit 甚至 System.currentTimeMillis() 来查看真实世界的数字。也不要为了微不足道的性能提升而牺牲代码清晰度。
您可能有一个选项3...
我的问题是您是如何获得这些 ID 值的？
我发现必须使用一组唯一标识符查询表的模式，这是一种奇怪的模式。
您更有可能在其他列上有 (a) 谓词，这些列标识您要检索的完全相同的行集。