1、主键和复合主键，查询性能相同（索引高度相同，恰当的运用索引）。

2、主键和复合主键，(update,insert)性能不同(因为复合主键会用更多的块来创建索引，所以update,insert性能低)

实验思路：

1、 建立实验表，及主键，联合2个主键，联合3个主键

2、 查看索引的结构

3、查看条件相同的，执行计划（来确定主键和复合主键的效率）

一、 建立实验表；test1为单主键为1个column，test2为联合主键2个columns，test3为联合主键3个columns

SQL> create table test1(a number,b number,c number,primary key(a));

Table created.

SQL> create table test2(a number,b number,c number,primary key(a,b));

Table created.

SQL> create table test3(a number,b number,c number,primary key(a,b,c));

Table created.

二、 查看索引的结构

1、先查看一下建立的表对应的索引

SQL> select index_name,table_name from user_indexes;

INDEX_NAME TABLE_NAME

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

SYS_C005198 TEST1

SYS_C005199 TEST2

SYS_C005200 TEST3

2、写个储存过程来给实验表插入数据

begin

for i in 1..10000 loop

insert into test1 values(i,i+1,i+2);

commit;

end loop;

end;

Test1

SQL>analyze index SYS_C005198 validate structure;

Index analyzed.

SQL> select HEIGHT,BLOCKS,BR_BLKS,LF_BLKS,LF_ROWS,DEL_LF_ROWS from index_stats ;

HEIGHT BLOCKS BR_BLKS LF_BLKS LF_ROWS DEL_LF_ROWS

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

2 24 1 18 10000 0

Test2

SQL> analyze index SYS_C005199 validate structure;

Index analyzed.

SQL> select HEIGHT,BLOCKS,BR_BLKS,LF_BLKS,LF_ROWS,DEL_LF_ROWS from index_stats ;

HEIGHT BLOCKS BR_BLKS LF_BLKS LF_ROWS DEL_LF_ROWS

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

2 32 1 23 10000 0

Test3

SQL>analyze index SYS_C005200 validate structure;

Index analyzed.

SQL> select HEIGHT,BLOCKS,BR_BLKS,LF_BLKS,LF_ROWS,DEL_LF_ROWS from index_stats ;

HEIGHT BLOCKS BR_BLKS LF_BLKS LF_ROWS DEL_LF_ROWS

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

2 40 1 28 10000 0

总结：根据B-TREE索引的结构特点。说明主键和联合主键，同样的数据联合主键需要更多的资源来维护索引。（联合主键索引因为用了更多的块，所以update,insert会比主键索引慢一些。至于查询下面研究）

三、 查看相同情况下，主键的效率。

1、 语句都让其走INDEX UNIQUE SCAN，看看效率：

Test1

SQL> select a from test1 where a=5555;

A

----------

5555

Execution Plan

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

Plan hash value: 2716871853

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

-

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time

|

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

-

| 0 | SELECT STATEMENT | | 1 | 13 | 1 (0)| 00:00:01

|

|* 1 | INDEX UNIQUE SCAN| SYS_C005198 | 1 | 13 | 1 (0)| 00:00:01

|

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

-

Predicate Information (identified by operation id):

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

1 - access("A"=5555)

Statistics

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

1 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

405 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

Test2

SQL> select a,b from test2 where a=5555 and b=5556;

A B

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

5555 5556

Execution Plan

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

Plan hash value: 3210951477

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

-

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time

|

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

-

| 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:01

|

|* 1 | INDEX UNIQUE SCAN| SYS_C005199 | 1 | 26 | 1 (0)| 00:00:01

|

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

-

Predicate Information (identified by operation id):

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

1 - access("A"=5555 AND "B"=5556)

Statistics

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

1 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

460 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

Test3

SQL> select a,b,c from test3 where a=5555 and b=5556 and c=5557;

A B C

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

5555 5556 5557

Execution Plan

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

Plan hash value: 1852305570

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

-

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time

|

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

-

| 0 | SELECT STATEMENT | | 1 | 39 | 1 (0)| 00:00:01

|

|* 1 | INDEX UNIQUE SCAN| SYS_C005200 | 1 | 39 | 1 (0)| 00:00:01

|

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

-

Predicate Information (identified by operation id):

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

1 - access("A"=5555 AND "B"=5556 AND "C"=5557)

Statistics

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

1 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

515 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

分析：通过执行SQL走INDEX UNIQUE SCAN索引的情况，分析执行计划得到的结果是主键和联合主键性能相同：

（我们关注的：

1 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

消耗一致和COST消耗一致。）

总结：主键和联合主键，应用B-tree索引的情况下，如果我们的索引高度相同,且正确的应用索引。这样的情况下我们查询性能是相同的。

欢迎大家给与纠正错误，共同提升！