總結：
1、主鍵和復合主鍵，查詢性能相同（索引高度相同，恰當的運用索引）。
2、主鍵和復合主鍵，(update,insert)性能不同(因為復合主鍵會用更多的塊來創建索引，所以update,insert性能低)
實驗思路：
1、 建立實驗表，及主鍵，聯合2個主鍵，聯合3個主鍵
2、 查看索引的結構
3、查看條件相同的，執行計划（來確定主鍵和復合主鍵的效率）
一、 建立實驗表；test1為單主鍵為1個column，test2為聯合主鍵2個columns，test3為聯合主鍵3個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、 查看索引的結構
1、先查看一下建立的表對應的索引
SQL> select index_name,table_name from user_indexes;
INDEX_NAME TABLE_NAME
------------------------------ ------------------------------
SYS_C005198 TEST1
SYS_C005199 TEST2
SYS_C005200 TEST3
2、寫個儲存過程來給實驗表插入數據
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
總結：根據B-TREE索引的結構特點。說明主鍵和聯合主鍵，同樣的數據聯合主鍵需要更多的資源來維護索引。（聯合主鍵索引因為用了更多的塊，所以update,insert會比主鍵索引慢一些。至於查詢下面研究）
三、 查看相同情況下，主鍵的效率。
1、 語句都讓其走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：通過執行SQL走INDEX UNIQUE SCAN索引的情況，分析執行計划得到的結果是主鍵和聯合主鍵性能相同：
（我們關注的：
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。）
總結：主鍵和聯合主鍵，應用B-tree索引的情況下，如果我們的索引高度相同,且正確的應用索引。這樣的情況下我們查詢性能是相同的。
歡迎大家給與糾正錯誤，共同提升！