Oracle 全表扫描及其执行计划 full table scan

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    全表扫描是Oracle访问数据库表是较为常见的访问方式之一。很多朋友一看到SQL语句执行计划中的全表扫描，就要考虑对其进行修理一番。全表扫描的存在，的确存在可能优化的余地。但事实上很多时候全表扫描也并非是最低效的，完全要看不同的情形与场合，任一方式都是有利有弊的，也就是具体情况要具体分析。本文描述了什么是全表扫描以及何时发生全表扫描，何时全表扫描才低效。
  本文涉及到的相关链接：
     高水位线和全表扫描
      启用 AUTOTRACE 功能
      Oracle 测试常用表BIG_TABLE
     Oracle db_file_mulitblock_read_count参数

1、什么是全表扫描?
    全表扫描就是扫表表中所有的行，实际上是扫描表中所有的数据块，因为Oracle中最小的存储单位是Oracle block。
    扫描所有的数据块就包括高水位线以内的数据块，即使是空数据块在没有被释放的情形下也会被扫描而导致I/O增加。
    在全表扫描期间，通常情况下，表上这些相邻的数据块被按顺序(sequentially)的方式访问以使得一次I/O可以读取多个数据块。
    一次读取更多的数据块有助于全表扫描使用更少的I/O，对于可读取的数据块被限制于参数DB_FILE_MULTIBLOCK_READ_COUNT。

2、何时发生全表扫描?
    a、表上的索引失效或无法被使用的情形(如对谓词使用函数、计算、NULL值、不等运算符、类型转换)
    b、查询条件返回了整个表的大部分数据
    c、使用了并行方式访问表
    d、使用full 提示
    e、统计信息缺失时使得Oracle认为全表扫描比索引扫描更高效
    f、表上的数据块小于DB_FILE_MULTIBLOCK_READ_COUNT值的情形可能产生全表扫描

3、演示全表扫描的情形

a、准备演示环境scott@ORA11G> select * from v$version where rownum<2;BANNER--------------------------------------------------------------------------------Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production--创建表tscott@ORA11G> CREATE TABLE t   2  AS  3  SELECT rownum AS n, rpad('*',100,'*') AS pad   4  FROM dual  5  CONNECT BY level <= 1000;Table created.--添加索引scott@ORA11G> create unique index t_pk on t(n);Index created.scott@ORA11G> alter table t add constraint t_pk primary key(n) using index t_pk;Table altered.--收集统计信息scott@ORA11G> execute dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);PL/SQL procedure successfully completed.scott@ORA11G> set autot trace exp;scott@ORA11G> select count(*) from t;   --->count(*)的时候使用了索引快速扫描Execution Plan----------------------------------------------------------Plan hash value: 454320086----------------------------------------------------------------------| Id  | Operation             | Name | Rows  | Cost (%CPU)| Time     |----------------------------------------------------------------------|   0 | SELECT STATEMENT      |      |     1 |     2   (0)| 00:00:01 ||   1 |  SORT AGGREGATE       |      |     1 |            |          ||   2 |   INDEX FAST FULL SCAN| T_PK |  1000 |     2   (0)| 00:00:01 |----------------------------------------------------------------------scott@ORA11G> set autot off;scott@ORA11G> alter table t move;  --->进行move tableTable altered.-->move 之后索引失效，如下所示scott@ORA11G> @idx_info           Enter value for owner: scottEnter value for table_name: tTable Name    INDEX_NAME     CL_NAM               CL_POS STATUS   IDX_TYP         DSCD------------- -------------- -------------------- ------ -------- --------------- ----T             T_PK           N                         1 UNUSABLE NORMAL          ASCb、索引失效导致全表扫描scott@ORA11G> set autot trace exp;scott@ORA11G> select count(*) from t;  Execution Plan----------------------------------------------------------Plan hash value: 2966233522-------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |-------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |     7   (0)| 00:00:01 ||   1 |  SORT AGGREGATE    |      |     1 |            |          ||   2 |   TABLE ACCESS FULL| T    |  1000 |     7   (0)| 00:00:01 |-------------------------------------------------------------------scott@ORA11G> set autot off;scott@ORA11G> alter index t_pk rebuild;   -->重建索引Index altered.scott@ORA11G> @idx_infoEnter value for owner: scottEnter value for table_name: tTable Name     INDEX_NAME       CL_NAM               CL_POS STATUS   IDX_TYP         DSCD-------------- ---------------- -------------------- ------ -------- --------------- ----T              T_PK             N                         1 VALID    NORMAL          ASCc、返回了整个表的大部分数据使用了全表扫描scott@ORA11G> select count(pad) from t where n<=990;Execution Plan----------------------------------------------------------Plan hash value: 2966233522---------------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |   105 |     7   (0)| 00:00:01 ||   1 |  SORT AGGREGATE    |      |     1 |   105 |            |          ||*  2 |   TABLE ACCESS FULL| T    |   991 |   101K|     7   (0)| 00:00:01 |---------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("N"<=990)--返回小部分数据时，使用的是索引扫描scott@ORA11G> select count(pad) from t where n<=10;Execution Plan----------------------------------------------------------Plan hash value: 4270555908-------------------------------------------------------------------------------------| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |-------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |      |     1 |   105 |     3   (0)| 00:00:01 ||   1 |  SORT AGGREGATE              |      |     1 |   105 |            |          ||   2 |   TABLE ACCESS BY INDEX ROWID| T    |    10 |  1050 |     3   (0)| 00:00:01 ||*  3 |    INDEX RANGE SCAN          | T_PK |    10 |       |     2   (0)| 00:00:01 |-------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   3 - access("N"<=10)d、使用并行方式访问表时使用了全表扫描scott@ORA11G> select /*+ parallel(3) */ count(pad) from t where n<=10;Execution Plan----------------------------------------------------------Plan hash value: 3126468333----------------------------------------------------------------------------------------------------------------| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |----------------------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT       |          |     1 |   105 |     3   (0)| 00:00:01 |        |      |            ||   1 |  SORT AGGREGATE        |          |     1 |   105 |            |          |        |      |            ||   2 |   PX COORDINATOR       |          |       |       |            |          |        |      |            ||   3 |    PX SEND QC (RANDOM) | :TQ10000 |     1 |   105 |            |          |  Q1,00 | P->S | QC (RAND)  ||   4 |     SORT AGGREGATE     |          |     1 |   105 |            |          |  Q1,00 | PCWP |            ||   5 |      PX BLOCK ITERATOR |          |    10 |  1050 |     3   (0)| 00:00:01 |  Q1,00 | PCWC |            ||*  6 |       TABLE ACCESS FULL| T        |    10 |  1050 |     3   (0)| 00:00:01 |  Q1,00 | PCWP |            |----------------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   6 - filter("N"<=10)Note-----   - Degree of Parallelism is 3 because of hint--Author : Robinson--Blog   : http://blog.csdn.net/robinson_0612e、使用full提示时使用了全表扫描scott@ORA11G> select /*+ full(t) */ count(pad) from t where n<=10;Execution Plan----------------------------------------------------------Plan hash value: 2966233522---------------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |   105 |     7   (0)| 00:00:01 ||   1 |  SORT AGGREGATE    |      |     1 |   105 |            |          ||*  2 |   TABLE ACCESS FULL| T    |    10 |  1050 |     7   (0)| 00:00:01 |---------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("N"<=10)         f、统计信息缺失导致全表扫描的情形scott@ORA11G> exec dbms_stats.delete_table_stats('SCOTT','T');PL/SQL procedure successfully completed.scott@ORA11G> select count(pad) from t where n<=10;Execution Plan----------------------------------------------------------Plan hash value: 2966233522---------------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |    65 |     7   (0)| 00:00:01 ||   1 |  SORT AGGREGATE    |      |     1 |    65 |            |          ||*  2 |   TABLE ACCESS FULL| T    |    10 |   650 |     7   (0)| 00:00:01 |---------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("N"<=10)Note-----   - dynamic sampling used for this statement (level=2)--上面的执行计划使用了全表扫描，而且提示使用了动态采样，也就是缺乏统计信息--表上的数据块小于DB_FILE_MULTIBLOCK_READ_COUNT值的情形可能产生全表扫描的情形不演示

4、全表扫描何时低效?

--先来做几个实验a、演示表上的相关信息scott@ORA11G> @idx_infoEnter value for owner: scottEnter value for table_name: big_tableTable Name                Index Name                CL_NAM    CL_POS Status   IDX_TYP         DSCD------------------------- ------------------------- --------- ------ -------- --------------- ----BIG_TABLE                 BIG_TABLE_PK              ID             1 VALID    NORMAL          ASCscott@ORA11G> @idx_statEnter value for input_table_name: big_tableEnter value for owner: scott                                                     AVG LEAF BLKS AVG DATA BLKSBLEV IDX_NAME        LEAF_BLKS   DST_KEYS       PER KEY       PER KEY CLUST_FACT LAST_ANALYZED         TB_BLKS    TB_ROWS---- -------------- ---------- ---------- ------------- ------------- ---------- ------------------ ---------- ----------   1 BIG_TABLE_PK          208     100000             1             1       1483 20130524 10:45:51        1515     100000--数据库参数设置scott@ORA11G> show parameter optimizer_index_NAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------optimizer_index_caching              integer     0optimizer_index_cost_adj             integer     100scott@ORA11G> show parameter optimizer_modeNAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------optimizer_mode                       string      ALL_ROWSb、查询返回20%数据行的情形scott@ORA11G> alter system flush buffer_cache;                                                  scott@ORA11G> select sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;Execution Plan----------------------------------------------------------Plan hash value: 3098837282                             -- 执行计划中，使用了索引范围扫描---------------------------------------------------------------------------------------------| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |              |     1 |    18 |   341   (0)| 00:00:05 ||   1 |  SORT AGGREGATE              |              |     1 |    18 |            |          ||   2 |   TABLE ACCESS BY INDEX ROWID| BIG_TABLE    | 20046 |   352K|   341   (0)| 00:00:05 ||*  3 |    INDEX RANGE SCAN          | BIG_TABLE_PK | 20046 |       |    43   (0)| 00:00:01 |---------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   3 - access("ID">=20000 AND "ID"<=40000)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets        351  consistent gets        351  physical reads          0  redo size        427  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processedscott@ORA11G> alter system flush buffer_cache; scott@ORA11G> select /*+ full(big_table) */ sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;Execution Plan----------------------------------------------------------Plan hash value: 599409829                ---- 使用了提示执行为全表扫描--------------------------------------------------------------------------------| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------------|   0 | SELECT STATEMENT   |           |     1 |    18 |   413   (1)| 00:00:05 ||   1 |  SORT AGGREGATE    |           |     1 |    18 |            |          ||*  2 |   TABLE ACCESS FULL| BIG_TABLE | 20046 |   352K|   413   (1)| 00:00:05 |--------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("ID"<=40000 AND "ID">=20000)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets       1486  consistent gets       1484  physical reads          0  redo size        427  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processed--注意对比上面两次操作中的consistent gets与physical readsc、查询返回30%数据行的情形scott@ORA11G> alter system flush buffer_cache;scott@ORA11G> select sum(object_id),avg(object_id) from big_table where id between 20000 and 50000;Execution Plan----------------------------------------------------------Plan hash value: 599409829             --->尽管返回数据的总行数为30%，而此时优化器使用了全表扫描--------------------------------------------------------------------------------| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------------|   0 | SELECT STATEMENT   |           |     1 |    18 |   413   (1)| 00:00:05 ||   1 |  SORT AGGREGATE    |           |     1 |    18 |            |          ||*  2 |   TABLE ACCESS FULL| BIG_TABLE | 30012 |   527K|   413   (1)| 00:00:05 |--------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("ID"<=50000 AND "ID">=20000)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets       1486  consistent gets       1484  physical reads          0  redo size        427  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processed--下面使用提示来强制优化器走索引扫描scott@ORA11G> alter system flush buffer_cache;scott@ORA11G> select /*+ index(big_table big_table_pk) */ sum(object_id),avg(object_id)   2  from big_table where id between 20000 and 50000;Execution Plan----------------------------------------------------------Plan hash value: 3098837282---------------------------------------------------------------------------------------------| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |              |     1 |    18 |   511   (1)| 00:00:07 ||   1 |  SORT AGGREGATE              |              |     1 |    18 |            |          ||   2 |   TABLE ACCESS BY INDEX ROWID| BIG_TABLE    | 30012 |   527K|   511   (1)| 00:00:07 ||*  3 |    INDEX RANGE SCAN          | BIG_TABLE_PK | 30012 |       |    64   (0)| 00:00:01 |---------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   3 - access("ID">=20000 AND "ID"<=50000)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets        526  consistent gets        526  physical reads          0  redo size        427  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processed          --注意观察每一次测试时所耗用的物理读与逻辑读--从上面的测试可以看出，当表上所返回的数据行数接近于表上的30%时，Oracle 倾向于使用全表扫描--而对于表上所返回的数据行数接近于表上的30%的情形，我们给与索引提示，此时比全表扫描更高效，即全表扫描是低效的--笔者同时测试了数据返回总行数接近80%的情形以及创建了一个百万记录的进行对比测试--大致结论，如果查询所返回的数据的总行数仅仅是表上数据的百分之八十以下，而使用了全表扫描，即可认为该全表扫描是低效的--注：--具体情况需要具体分析，如果你的表是千万级的，返回总数据的百分之零点几都会导致很大的差异--其次，表上的索引应具有良好的聚簇因子，如不然，测试的结果可能有天壤之别--最后，上面所描述的返回总行数应与执行结果返回的行数有差异，是指多少行参与了sum(object_id)

5、小表的全表扫描是否高效?

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--使用scott下dept表，仅有4行数据scott@ORA11G> select * from dept where deptno>10;3 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2985873453            --->执行计划选择了索引扫描---------------------------------------------------------------------------------------| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------------|   0 | SELECT STATEMENT            |         |     3 |    60 |     2   (0)| 00:00:01 ||   1 |  TABLE ACCESS BY INDEX ROWID| DEPT    |     3 |    60 |     2   (0)| 00:00:01 ||*  2 |   INDEX RANGE SCAN          | PK_DEPT |     3 |       |     1   (0)| 00:00:01 |---------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - access("DEPTNO">10)Statistics----------------------------------------------------------          0  recursive calls          0  db block gets          4  consistent gets                      -->使用了4次逻辑读          0  physical reads          0  redo size        515  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          3  rows processed-->下面强制使用全表扫描scott@ORA11G> select /*+ full(dept) */ * from dept where deptno>10;3 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 3383998547 --------------------------------------------------------------------------| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------|   0 | SELECT STATEMENT  |      |     3 |    60 |     3   (0)| 00:00:01 ||*  1 |  TABLE ACCESS FULL| DEPT |     3 |    60 |     3   (0)| 00:00:01 |--------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - filter("DEPTNO">10)Statistics----------------------------------------------------------          1  recursive calls          0  db block gets          4  consistent gets         -->此时的逻辑读同样为4次          0  physical reads          0  redo size        515  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          3  rows processed--下面来看看count(*)的情形scott@ORA11G> select count(*) from dept;1 row selected.Execution Plan----------------------------------------------------------Plan hash value: 3051237957               --->执行计划选择了索引全扫描--------------------------------------------------------------------| Id  | Operation        | Name    | Rows  | Cost (%CPU)| Time     |--------------------------------------------------------------------|   0 | SELECT STATEMENT |         |     1 |     1   (0)| 00:00:01 ||   1 |  SORT AGGREGATE  |         |     1 |            |          ||   2 |   INDEX FULL SCAN| PK_DEPT |     4 |     1   (0)| 00:00:01 |--------------------------------------------------------------------Statistics----------------------------------------------------------          0  recursive calls          0  db block gets          1  consistent gets            -->逻辑读仅为1次          0  physical reads          0  redo size        335  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processed-->下面强制使用全表扫描scott@ORA11G> select /*+ full(dept) */ count(*) from dept;1 row selected.Execution Plan----------------------------------------------------------Plan hash value: 315352865-------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |-------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |     3   (0)| 00:00:01 ||   1 |  SORT AGGREGATE    |      |     1 |            |          ||   2 |   TABLE ACCESS FULL| DEPT |     4 |     3   (0)| 00:00:01 |-------------------------------------------------------------------Statistics----------------------------------------------------------          0  recursive calls          0  db block gets          3  consistent gets     -->使用了3次逻辑读          0  physical reads          0  redo size        335  bytes sent via SQL*Net to client        349  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)          1  rows processed                    --对于小表，从上面的情形可以看出，使用索引扫描也是比全表扫描高效--因此，建议始终为小表建立索引