Oracle 12CR2查詢轉換教程之表擴充套件詳解

阿新 • • 發佈：2020-01-09

前言

在表擴充套件中，對於讀取一個分割槽表部分資料時優化器會生成使用索引的執行計劃。基於索引執行計劃可以提高效能，但索引維護會增加開鎖。在許多資料庫中，DML隻影響小部分資料。對於頻繁更新的表表擴充套件使用基於索引的執行計劃。你可以在以讀取為主的資料上建立一個索引，在以頻繁變化的資料上消除索引開銷。通過這種方式，表擴充套件在避免索引維護的同時提高了效能。

下面話不多說了，來一起看看詳細的介紹吧

表擴充套件工作原理

表分割槽使用表擴充套件成為可能。如果在一個分割槽表上建立一個本地索引，那麼優化器可能會標記索引對於特定的分割槽不可使用。實際有些分割槽沒有建立索引。在表擴充套件中，優化器將查詢轉換為一個union all語句，讓一些子查詢訪問建立索引的分割槽，一些子查詢訪問沒有建立索引的分割槽。優化器可以為每個分割槽選擇最有效的訪問路徑，而不管它是否存在於查詢所要訪問的所有分割槽中。

優化器不總是會選擇表擴充套件

.表擴充套件是基於成本

當資料庫訪問擴充套件表的每個分割槽只會跨越union all的所有分支一次，資料庫所連線的任何表都是在分支中被訪問。

.語義問題可能導致表擴充套件無效

例如，一個表出現在一個外連線的右邊對於表擴充套件來說是無效的。

可以使用expand_table hint來控制表擴充套件。這個hint會覆蓋基於成本的決策，但不會覆蓋語義檢查。

表擴充套件使用場景

優化器基於查詢中出現的謂詞條件對每個表必須被訪問的分割槽保持跟蹤。分割槽裁剪能讓優化器使用表擴充套件來生成更有效的執行計劃。

下面的例子假設滿足以下條件:

.想要對sh.sales表執行星型查詢，表sh.sales是基於time_id列進行範圍分割槽的一個分割槽表。

.想要禁用特定分割槽上的索引來查看錶擴充套件的優點。

操作步驟如下:

1.以sh使用者登入資料庫

[oracle@jytest1 ~]$ sqlplus sh/*****@jypdb

SQL*Plus: Release 12.2.0.1.0 Production on Wed Oct 31 18:09:54 2018

Copyright (c) 1982,2016,Oracle. All rights reserved.

Last Successful login time: Wed Oct 24 2018 17:00:11 +08:00

Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

SQL>

2.執行以下查詢

SQL> select * from sales where time_id >= to_date('2000-01-01 00:00:00','syyyy-mm-dd hh24:mi:ss') and prod_id = 38;
...........
 38 2470 24-DEC-01  2 999  1 31.47
 38 13440 24-DEC-01  2 999  1 31.47
 38 490 28-DEC-01  2 999  1 31.47
 38 8406 28-DEC-01  2 999  1 31.47
 38 1466 31-DEC-01  3 351  1 31.47
 38 4340 31-DEC-01  3 351  1 31.47
 38 10658 31-DEC-01  3 351  1 31.47
 38 11390 31-DEC-01  3 351  1 31.47
 38 23226 31-DEC-01  3 351  1 31.47

4224 rows selected.

3.查詢執行計劃

SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced allstats last runstats_last peeked_binds'));
SQL_ID 214qgysqqz0k8,child number 0
-------------------------------------
select * from sales where time_id >= to_date('2000-01-01 00:00:00','syyyy-mm-dd hh24:mi:ss') and prod_id = 38

Plan hash value: 2342444420

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation     | Name  | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | Pstart| Pstop | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT    |  | 1 | | | 224 (100)|  | | | 4224 |00:00:00.03 | 334 |
| 1 | PARTITION RANGE ITERATOR   |  | 1 | 5078 | 143K| 224 (0)| 00:00:01 | 13 | 28 | 4224 |00:00:00.03 | 334 |
| 2 | TABLE ACCESS BY LOCAL INDEX ROWID BATCHED| SALES  | 16 | 5078 | 143K| 224 (0)| 00:00:01 | 13 | 28 | 4224 |00:00:00.02 | 334 |
| 3 | BITMAP CONVERSION TO ROWIDS  |  | 8 | | |  |  | | | 4224 |00:00:00.01 | 24 |
|* 4 | BITMAP INDEX SINGLE VALUE  | SALES_PROD_BIX | 8 | | |  |  | 13 | 28 | 8 |00:00:00.01 | 24 |
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

 1 - SEL$1
 2 - SEL$1 / SALES@SEL$1

Outline Data
-------------

 /*+
 BEGIN_OUTLINE_DATA
 IGNORE_OPTIM_EMBEDDED_HINTS
 OPTIMIZER_FEATURES_ENABLE('12.2.0.1')
 DB_VERSION('12.2.0.1')
 ALL_ROWS
 NO_PARALLEL
 OUTLINE_LEAF(@"SEL$1")
 BITMAP_TREE(@"SEL$1" "SALES"@"SEL$1" AND(("SALES"."PROD_ID")))
 BATCH_TABLE_ACCESS_BY_ROWID(@"SEL$1" "SALES"@"SEL$1")
 END_OUTLINE_DATA
 */

Predicate Information (identified by operation id):
---------------------------------------------------

 4 - access("PROD_ID"=38)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

 1 - "PROD_ID"[NUMBER,22],"SALES"."CUST_ID"[NUMBER,"SALES"."TIME_ID"[DATE,7],"SALES"."CHANNEL_ID"[NUMBER,"SALES"."PROMO_ID"[NUMBER,"SALES"."QUANTITY_SOLD"[NUMBER,"SALES"."AMOUNT_SOLD"[NUMBER,22]
 2 - "PROD_ID"[NUMBER,22]
 3 - "SALES".ROWID[ROWID,10],"PROD_ID"[NUMBER,22]
 4 - STRDEF[BM VAR,STRDEF[BM VAR,7920],22]

Note
-----
 - automatic DOP: Computed Degree of Parallelism is 1 because of parallel threshold


58 rows selected.

在執行計劃中的Pstart與Pstop列，顯示了優化器判斷只需要訪問表的13到28分割槽。在優化器已經判斷了被訪問的分割槽之後，它將考慮所有這些分割槽上可以使用的索引。在上面的執行計劃中，優化器選擇使用sales_prod_bix點陣圖索引

4.禁用sales表中sales_1995分割槽上的索引;

SQL> alter index sales_prod_bix modify partition sales_1995 unusable;

Index altered.

5.再次執行之前的查詢語句，然後顯示執行計劃，可以看到執行計劃變成了由兩個子查詢組成的union all語句，第一個子查詢還是對13-28分割槽使用索引，第二個子查詢步驟對應的Pstart與Pstop為invalid，id=11的過濾條件為”PROD_ID”=38,id=9的過濾條件為”SALES”.”TIME_ID”=TO_DATE(‘ 2000-01-01 00:00:00',‘syyyy-mm-dd hh24:mi:ss')))這個過濾條件是為否的，所以過濾後的記錄為0，從對應的A-Rows列也可以看到記錄為0

SQL> select * from table(dbms_xplan.display_cursor(null,'syyyy-mm-dd hh24:mi:ss') and prod_id = 38

Plan hash value: 238952339

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation          | Name   | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | Pstart| Pstop | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT        |    |  1 |  |  | 224 (100)|   |  |  | 4224 |00:00:00.05 |  334 |
| 1 | VIEW           | VW_TE_2  |  1 | 5079 | 431K| 224 (0)| 00:00:01 |  |  | 4224 |00:00:00.05 |  334 |
| 2 | UNION-ALL         |    |  1 |  |  |   |   |  |  | 4224 |00:00:00.05 |  334 |
| 3 | PARTITION RANGE ITERATOR     |    |  1 | 5078 | 143K| 224 (0)| 00:00:01 | 13 | 28 | 4224 |00:00:00.03 |  334 |
| 4 |  TABLE ACCESS BY LOCAL INDEX ROWID BATCHED | SALES   |  16 | 5078 | 143K| 224 (0)| 00:00:01 | 13 | 28 | 4224 |00:00:00.02 |  334 |
| 5 |  BITMAP CONVERSION TO ROWIDS    |    |  8 |  |  |   |   |  |  | 4224 |00:00:00.01 |  24 |
|* 6 |  BITMAP INDEX SINGLE VALUE    | SALES_PROD_BIX |  8 |  |  |   |   | 13 | 28 |  8 |00:00:00.01 |  24 |
|* 7 | FILTER          |    |  1 |  |  |   |   |  |  |  0 |00:00:00.01 |  0 |
| 8 |  PARTITION RANGE EMPTY      |    |  0 |  1 | 29 |  1 (0)| 00:00:01 |INVALID|INVALID|  0 |00:00:00.01 |  0 |
|* 9 |  TABLE ACCESS BY LOCAL INDEX ROWID BATCHED| SALES   |  0 |  1 | 29 |  1 (0)| 00:00:01 |INVALID|INVALID|  0 |00:00:00.01 |  0 |
| 10 |  BITMAP CONVERSION TO ROWIDS    |    |  0 |  |  |   |   |  |  |  0 |00:00:00.01 |  0 |
|* 11 |  BITMAP INDEX SINGLE VALUE    | SALES_PROD_BIX |  0 |  |  |   |   |INVALID|INVALID|  0 |00:00:00.01 |  0 |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

 1 - SET$D0A14387 / VW_TE_2@SEL$0A5B0FFE
 2 - SET$D0A14387
 3 - SET$D0A14387_1
 4 - SET$D0A14387_1 / SALES@SEL$1
 7 - SET$D0A14387_2
 9 - SET$D0A14387_2 / SALES@SEL$1

Outline Data
-------------

 /*+
  BEGIN_OUTLINE_DATA
  IGNORE_OPTIM_EMBEDDED_HINTS
  OPTIMIZER_FEATURES_ENABLE('12.2.0.1')
  DB_VERSION('12.2.0.1')
  ALL_ROWS
  NO_PARALLEL
  OUTLINE_LEAF(@"SET$D0A14387_2")
  OUTLINE_LEAF(@"SET$D0A14387_1")
  OUTLINE_LEAF(@"SET$D0A14387")
  EXPAND_TABLE(@"SEL$1" "SALES"@"SEL$1")
  OUTLINE_LEAF(@"SEL$0A5B0FFE")
  OUTLINE(@"SET$D0A14387")
  EXPAND_TABLE(@"SEL$1" "SALES"@"SEL$1")
  OUTLINE(@"SEL$1")
  NO_ACCESS(@"SEL$0A5B0FFE" "VW_TE_2"@"SEL$0A5B0FFE")
  BITMAP_TREE(@"SET$D0A14387_1" "SALES"@"SEL$1" AND(("SALES"."PROD_ID")))
  BATCH_TABLE_ACCESS_BY_ROWID(@"SET$D0A14387_1" "SALES"@"SEL$1")
  BITMAP_TREE(@"SET$D0A14387_2" "SALES"@"SEL$1" AND(("SALES"."PROD_ID")))
  BATCH_TABLE_ACCESS_BY_ROWID(@"SET$D0A14387_2" "SALES"@"SEL$1")
  END_OUTLINE_DATA
 */

Predicate Information (identified by operation id):
---------------------------------------------------

 6 - access("PROD_ID"=38)
 7 - filter(NULL IS NOT NULL)
 9 - filter(("SALES"."TIME_ID"=TO_DATE(' 2000-01-01 00:00:00','syyyy-mm-dd
    hh24:mi:ss')))
 11 - access("PROD_ID"=38)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

 1 - "ITEM_1"[NUMBER,"ITEM_2"[NUMBER,"ITEM_3"[DATE,"ITEM_4"[NUMBER,"ITEM_5"[NUMBER,"ITEM_6"[NUMBER,"ITEM_7"[NUMBER,22]
 2 - STRDEF[22],STRDEF[22],STRDEF[7],STRDEF[22]
 3 - "SALES"."PROD_ID"[NUMBER,22]
 4 - "SALES"."PROD_ID"[NUMBER,22]
 5 - "SALES".ROWID[ROWID,"SALES"."PROD_ID"[NUMBER,22]
 6 - STRDEF[BM VAR,22]
 7 - "SALES"."PROD_ID"[NUMBER,22]
 8 - "SALES"."PROD_ID"[NUMBER,22]
 9 - "SALES"."PROD_ID"[NUMBER,22]
 10 - "SALES".ROWID[ROWID,22]
 11 - STRDEF[BM VAR,22]

Note
-----
 - automatic DOP: Computed Degree of Parallelism is 1 because of parallel threshold


93 rows selected.

6.禁用分割槽28上的索引(sales_q4_2003),它是查詢需要訪問的一個分割槽:

SQL> alter index sales_prod_bix modify partition sales_q4_2003 unusable;

Index altered.

SQL> alter index sales_time_bix modify partition sales_q4_2003 unusable;

Index altered.

通過禁用查詢需要訪問分割槽上的索引，查詢將不能再使用這些索引。

7.再次執行查詢語句，其執行計劃如下,執行計劃變成了由三個子查詢組成的union all語句，相比之前查詢多的第三個子查詢對錶sales的第28個分割槽執行全表掃描，這裡沒有索引可用，因為已經禁用28分割槽上的索引了。

SQL> select * from table(dbms_xplan.display_cursor(null,'syyyy-mm-dd hh24:mi:ss') and prod_id = 38

Plan hash value: 3857158179

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation          | Name   | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | Pstart| Pstop | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT        |    |  1 |  |  | 225 (100)|   |  |  | 4224 |00:00:00.20 |  334 |  44 |
| 1 | VIEW           | VW_TE_2  |  1 | 5080 | 431K| 225 (0)| 00:00:01 |  |  | 4224 |00:00:00.20 |  334 |  44 |
| 2 | UNION-ALL         |    |  1 |  |  |   |   |  |  | 4224 |00:00:00.19 |  334 |  44 |
| 3 | PARTITION RANGE ITERATOR     |    |  1 | 5078 | 143K| 223 (0)| 00:00:01 | 13 | 27 | 4224 |00:00:00.17 |  334 |  44 |
| 4 |  TABLE ACCESS BY LOCAL INDEX ROWID BATCHED | SALES   |  15 | 5078 | 143K| 223 (0)| 00:00:01 | 13 | 27 | 4224 |00:00:00.16 |  334 |  44 |
| 5 |  BITMAP CONVERSION TO ROWIDS    |    |  8 |  |  |   |   |  |  | 4224 |00:00:00.03 |  24 |  16 |
|* 6 |  BITMAP INDEX SINGLE VALUE    | SALES_PROD_BIX |  8 |  |  |   |   | 13 | 27 |  8 |00:00:00.03 |  24 |  16 |
|* 7 | FILTER          |    |  1 |  |  |   |   |  |  |  0 |00:00:00.01 |  0 |  0 |
| 8 |  PARTITION RANGE EMPTY      |    |  0 |  1 | 29 |  1 (0)| 00:00:01 |INVALID|INVALID|  0 |00:00:00.01 |  0 |  0 |
|* 9 |  TABLE ACCESS BY LOCAL INDEX ROWID BATCHED| SALES   |  0 |  1 | 29 |  1 (0)| 00:00:01 |INVALID|INVALID|  0 |00:00:00.01 |  0 |  0 |
| 10 |  BITMAP CONVERSION TO ROWIDS    |    |  0 |  |  |   |   |  |  |  0 |00:00:00.01 |  0 |  0 |
|* 11 |  BITMAP INDEX SINGLE VALUE    | SALES_PROD_BIX |  0 |  |  |   |   |INVALID|INVALID|  0 |00:00:00.01 |  0 |  0 |
| 12 | PARTITION RANGE SINGLE      |    |  1 |  1 | 87 |  2 (0)| 00:00:01 | 28 | 28 |  0 |00:00:00.01 |  0 |  0 |
|* 13 |  TABLE ACCESS FULL       | SALES   |  1 |  1 | 87 |  2 (0)| 00:00:01 | 28 | 28 |  0 |00:00:00.01 |  0 |  0 |
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

 1 - SET$D0A14387 / VW_TE_2@SEL$0A5B0FFE
 2 - SET$D0A14387
 3 - SET$D0A14387_1
 4 - SET$D0A14387_1 / SALES@SEL$1
 7 - SET$D0A14387_2
 9 - SET$D0A14387_2 / SALES@SEL$1
 12 - SET$D0A14387_3
 13 - SET$D0A14387_3 / SALES@SEL$1

Outline Data
-------------

 /*+
  BEGIN_OUTLINE_DATA
  IGNORE_OPTIM_EMBEDDED_HINTS
  OPTIMIZER_FEATURES_ENABLE('12.2.0.1')
  DB_VERSION('12.2.0.1')
  ALL_ROWS
  NO_PARALLEL
  OUTLINE_LEAF(@"SET$D0A14387_3")
  OUTLINE_LEAF(@"SET$D0A14387_2")
  OUTLINE_LEAF(@"SET$D0A14387_1")
  OUTLINE_LEAF(@"SET$D0A14387")
  EXPAND_TABLE(@"SEL$1" "SALES"@"SEL$1")
  OUTLINE_LEAF(@"SEL$0A5B0FFE")
  OUTLINE(@"SET$D0A14387")
  EXPAND_TABLE(@"SEL$1" "SALES"@"SEL$1")
  OUTLINE(@"SEL$1")
  NO_ACCESS(@"SEL$0A5B0FFE" "VW_TE_2"@"SEL$0A5B0FFE")
  BITMAP_TREE(@"SET$D0A14387_1" "SALES"@"SEL$1" AND(("SALES"."PROD_ID")))
  BATCH_TABLE_ACCESS_BY_ROWID(@"SET$D0A14387_1" "SALES"@"SEL$1")
  BITMAP_TREE(@"SET$D0A14387_2" "SALES"@"SEL$1" AND(("SALES"."PROD_ID")))
  BATCH_TABLE_ACCESS_BY_ROWID(@"SET$D0A14387_2" "SALES"@"SEL$1")
  FULL(@"SET$D0A14387_3" "SALES"@"SEL$1")
  END_OUTLINE_DATA
 */

Predicate Information (identified by operation id):
---------------------------------------------------

 6 - access("PROD_ID"=38)
 7 - filter(NULL IS NOT NULL)
 9 - filter(("SALES"."TIME_ID"=TO_DATE(' 2000-01-01 00:00:00','syyyy-mm-dd hh24:mi:ss')))
 11 - access("PROD_ID"=38)
 13 - filter("PROD_ID"=38)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

 1 - "ITEM_1"[NUMBER,22]
 12 - "SALES"."PROD_ID"[NUMBER,22]
 13 - "SALES"."PROD_ID"[NUMBER,22]

Note
-----
 - automatic DOP: Computed Degree of Parallelism is 1 because of parallel threshold


103 rows selected.

總結

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