Oracle 基礎篇 --- 索引
##第四. 索引sql
###4.1 定義bash
索引是與表關聯的可選結構。經過建立索引可提升數據更新和檢索的性能。Oracle 索引提供到數據行的直接訪問路徑。服務器
能夠對錶的一個或多個列建立索引。建立索引後，Oracle 服務器會自動維護和使用索引。表數據的更新（如添加新行、更新行或刪除行）會自動傳播到全部相關的索引，這些對用戶來講是徹底透明的。ide
索引還能夠提升實施主鍵和惟一鍵約束條件時的性能。若是沒有索引，則每次對錶執行DML 操做時都會掃描整個表（全表掃描）。性能
###4.2 類型ui
有多種類型的索引結構，能夠根據須要使用。最經常使用的兩種 類型是：code
####4.2.1 B 樹索引orm
默認的索引類型； 採用平衡樹的形式. B 樹索引的鍵值存儲在平衡樹（B 樹）中，這樣能夠快速執行二進制搜索。排序
B 樹索引的結構索引
索引的頂層爲根，它包含指向索引中下一層次的條目。下一層次爲分支塊，它又指向位於索引中下一層次的塊。最底層是葉節點，它包含指向錶行的索引條目。葉塊是雙向關聯的，這便於按鍵值升序或降序掃描索引。
索引葉條目的格式
- 條目頭：存儲列數和鎖定信息
- 鍵列長度/值對：用於定義鍵中的列大小，後面跟隨列值（此類長度/值對的數目就是索引中的最大列數）。
- ROWID：包含鍵值的行的行ID
索引葉條目的特性
在非分區表的B 樹索引中：
- 當多個行具備相同的鍵值時，若是不壓縮索引，鍵值會出現重複
- 當某行包含的全部鍵列爲NULL 時，該行沒有對應的索引條目。所以，當WHERE 子句指定了NULL 時，將始終執行全表掃描
- 由於全部行屬於同一個段，因此要使用受限的ROWID 指向錶行
對索引執行DML 操做的效果
對錶執行DML 操做時，Oracle 服務器會維護全部索引。下面說明對索引執行DML 命令產生的效果：
- 執行插入操做致使在相應塊中插入索引條目。
- 刪除一行只致使對索引條目進行邏輯刪除。已刪除行所佔用的空間不可供後面新的葉條目使用。
- 更新鍵列致使對索引進行邏輯刪除和插入。PCTFREE設置對索引沒有影響，但建立時除外。即便索引塊的空間少於PCTFREE指定的空間，也能夠向索引塊添加新條目。
###4.3 建立
#建立索引
create index emp3_name_ix on
emp3(emp3_name);
#查看索引的信息
select index_name, index_type, table_name, table_type, uniqueness, status
from user_indexes
where table_name = 'EMP3';
INDEX_NAME INDEX_TYPE TABLE_NAME TABLE_TYPE UNIQUENES STATUS
--------------- --------------- --------------- ----------- --------- --------
EMP3_ID_PK NORMAL EMP3 TABLE UNIQUE VALID
EMP3_NAME_IX NORMAL EMP3 TABLE NONUNIQUE VALID
#查看索引對應的列
SQL> select * from user_ind_columns where table_name = 'EMP3';
INDEX_NAME TABLE_NAME COLUMN_NAME COLUMN_POSITION COLUMN_LENGTH CHAR_LENGTH DESC
--------------- --------------- --------------- --------------- ------------- ----------- ----
EMP3_ID_PK EMP3 EMP3_ID 1 22 0 ASC
EMP3_NAME_IX EMP3 EMP3_NAME 1 10 10 ASC
SQL> select * from emp3 where emp3_name = 'qa1';
EMP3_ID EMP3_NAME DEP_ID
---------- ---------- ----------
2 qa1 2
Execution Plan
----------------------------------------------------------
Plan hash value: 215206995
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 33 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP3 | 1 | 33 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP3_NAME_IX | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMP3_NAME"='qa1')
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
675 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
###4.4 決定使用全表掃描仍是使用索引
在大多數狀況下，全表掃描可能會致使更多的物理磁盤輸入輸出，可是全表掃描有時又可能會由於高度並行化的存在而執行的更快。
索引範圍掃描的整體原則是：
- 對於原始排序的表僅讀取少於表記錄數
40％的查詢應該使用索引範圍掃描。 反之，讀取記錄數目多於表記錄數的40％的查詢應該使用全表掃描。
- 對於未排序的表僅讀取少於表記錄數
7％的查詢應該使用索引範圍掃描。 反之，讀取記錄數目多於表記錄數的7％的查詢應該使用全表掃描。
###決定使用全表掃描仍是使用索引
SQL> select index_name, index_type, table_name, uniqueness, status from user_indexes where table_name = 'EMP3';
INDEX_NAME INDEX_TYPE TABLE_NAME UNIQUENESS STATUS
--------------- ---------- --------------- ---------- --------
EMP3_ID_PK NORMAL EMP3 UNIQUE VALID
EMP3_NAME_IX NORMAL EMP3 NONUNIQUE VALID
SQL> select count(*) from emp3;
COUNT(*)
----------
19
#雖然有索引，可是此時是全表掃描
SQL> select * from emp3 where emp3_name = 'qa8';
EMP3_ID EMP3_NAME DEP_ID
---------- ---------- ----------
16 qa8 2
Execution Plan
----------------------------------------------------------
Plan hash value: 2425169977
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 11 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| EMP3 | 1 | 11 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("EMP3_NAME"='qa8')
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
671 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
#往emp3表添加數據
[root@hzvscmdb sql]# more insert_data.sql
#!/bin/bash
i=$1;
while [ $i -le $2 ]
do
sqlplus hr/CCM%lab123@tony1521 <<EOF
insert into emp3 values($i,'$3',$4);
commit;
quit;
EOF
let i=i+1
done
echo "inset into emp3 table"
[root@hzvscmdb sql]# ./insert_data.sql 90 100 dev 1
SQL> select max(emp3_id) from emp3;
MAX(EMP3_ID)
------------
100000
SQL> analyze table emp3 estimate statistics;
Table analyzed.
SQL> select blocks, empty_blocks, num_rows from user_tables where table_name = 'EMP3';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
374 10 101081
#查看一條數據
SQL> select * from emp3 where emp3_name = 'qa33333';
EMP3_ID EMP3_NAME DEP_ID
---------- ---------- ----------
33333 qa33333 2
Execution Plan
----------------------------------------------------------
Plan hash value: 215206995
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 14 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP3 | 1 | 14 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP3_NAME_IX | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMP3_NAME"='qa33333')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
681 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
#不加條件的查詢，索引掃描
SQL> select count(*) from emp3;
COUNT(*)
----------
100000
Execution Plan
----------------------------------------------------------
Plan hash value: 2418373429
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 70 (2)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| EMP3_ID_PK | 101K| 70 (2)| 00:00:01 |
----------------------------------------------------------------------------
Statistics
----------------------------------------------------------
2 recursive calls
2 db block gets
262 consistent gets
37 physical reads
176 redo size
526 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
#沒有建立索引的，查詢全表掃描
SQL> select count(*) from emp3 where dep_id = 2;
COUNT(*)
----------
85726
Execution Plan
----------------------------------------------------------
Plan hash value: 1396384608
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 | 104 (1)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 2 | | |
|* 2 | TABLE ACCESS FULL| EMP3 | 50541 | 98K| 104 (1)| 00:00:02 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("DEP_ID"=2)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
373 consistent gets
0 physical reads
0 redo size
528 bytes sent via SQL*Net to client
524 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 count(*) from emp3 where emp3_name like 'qa%';
COUNT(*)
----------
85726
Execution Plan
----------------------------------------------------------
Plan hash value: 3884997069
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
|* 2 | INDEX RANGE SCAN| EMP3_NAME_IX | 7 | 56 | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMP3_NAME" LIKE 'qa%')
filter("EMP3_NAME" LIKE 'qa%')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
395 consistent gets
0 physical reads
0 redo size
528 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
相關文章
- 1. Oracle基礎——索引
- 2. Oracle 基礎篇 --- 索引選項
- 3. Oracle 基礎篇 --- 索引訪問
- 4. Oracle 基礎篇 --- B樹索引內部結構
- 5. Oracle 基礎篇 --- 重建B樹索引對性能的影響
- 6. 基礎-索引
- 7. mongodb基礎-索引
- 8. ORACLE 數據庫索引篇
- 9. Elasticsearch 2.2.0 基礎篇：Lucene倒排索引
- 10. MySQL視圖與索引（基礎篇 下）
- 更多相關文章...
- • SQLite 索引（Index） - SQLite教程
- • Scala 基礎語法 - Scala教程
- • ☆基於Java Instrument的Agent實現
- • Kotlin學習（二）基本類型
相關標籤/搜索
每日一句
-
每一个你不满意的现在，都有一个你没有努力的曾经。
歡迎關注本站公眾號,獲取更多信息
