oracle table access by index rowid 说明，rowid是伪列（pseudocolumn），在查询结果输出时它被构造出来的。rowid并不会真正存在
一. 测试环境
sql> select * from v$version where rownum=1;
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oracle database 11g enterprise edition release11.2.0.3.0 - 64bit production
sql> create table dave as selectobject_id,object_name,object_type,created,timestamp,status from all_objects;
表已创建。
sql> create table dave2 as select * from dave;
表已创建。
--收集统计信息，这里没有收集直方图：
sql> exec dbms_stats.gather_table_stats(ownname=>'sys',tabname =>'dave',estimate_percent => 10 ,method_opt =>'forcolumns size 1',degree=>10,cascade => true);
pl/sql 过程已成功完成。
sql> exec dbms_stats.gather_table_stats(ownname=>'sys',tabname =>'dave2',estimate_percent => 10 ,method_opt =>'forcolumns size 1',degree=>10,cascade => true);
pl/sql 过程已成功完成。
sql> alter system flush buffer_cache;
系统已更改。
--查看全表扫描时的执行计划：
sql> set autot traceonly
sql> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id;
已选择72762行。
执行计划
----------------------------------------------------------
plan hash value: 3613449503
------------------------------------------------------------------------------------
| id |operation | name | rows | bytes |tempspc| cost (%cpu)| time |
------------------------------------------------------------------------------------
| 0 |select statement | | 72520 | 3824k| | 695 (1)| 00:00:09 |
|* 1 | hash join | | 72520 | 3824k| 2536k| 695 (1)| 00:00:09 |
| 2 | table access full| dave2 | 71990 | 1687k| | 213 (1)| 00:00:03 |
| 3 | table access full| dave | 72520 | 2124k| | 213 (1)| 00:00:03 |
------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
1 -access(d1.object_id=d2.object_id)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
6353 consistent gets
1558 physical reads
0 redo size
3388939 bytes sent via sql*net toclient
53874 bytes received via sql*netfrom client
4852 sql*net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
72762 rows processed
sql>
sql> create index idx_dave_object_idon dave(object_id);
索引已创建。
sql> create index idx_dave_object_id2 ondave2(object_id);
索引已创建。
--在次查看执行计划：
sql> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id;
已选择72762行。
执行计划
----------------------------------------------------------
plan hash value: 3613449503
------------------------------------------------------------------------------------
| id |operation | name | rows | bytes |tempspc| cost (%cpu)| time |
------------------------------------------------------------------------------------
| 0 |select statement | | 72520 | 3824k| | 695 (1)| 00:00:09 |
|* 1 | hash join | | 72520 | 3824k| 2536k| 695 (1)| 00:00:09 |
| 2 | table access full| dave2 | 71990 | 1687k| | 213 (1)| 00:00:03 |
| 3 | table access full| dave | 72520 | 2124k| | 213 (1)| 00:00:03 |
------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
1 -access(d1.object_id=d2.object_id)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
6353 consistent gets
0 physical reads
0 redo size
3388939 bytes sent via sql*net toclient
53874 bytes received via sql*netfrom client
4852 sql*net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
72762 rows processed
这里的物理读为0. 但是还是走的是全表扫描。
sql> alter system flush buffer_cache;
系统已更改。
sql> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id and d1.object_id
已选择98行。
执行计划
----------------------------------------------------------
plan hash value: 504164237
----------------------------------------------------------------------------------------------------
| id |operation | name | rows | bytes | cost (%cpu)| time |
----------------------------------------------------------------------------------------------------
| 0 |select statement | | 3600 | 189k| 23 (5)| 00:00:01 |
|* 1 | hash join | | 3600 | 189k| 23 (5)| 00:00:01 |
| 2 | table access by index rowid| dave2 | 3600 | 86400 | 11 (0)| 00:00:01 |
|* 3 | index range scan | idx_dave_object_id2 | 648 | | 3 (0)| 00:00:01 |
| 4 | table access by index rowid| dave | 3626 | 106k| 11 (0)| 00:00:01 |
|* 5 | index range scan | idx_dave_object_id | 653| | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
1 -access(d1.object_id=d2.object_id)
3 -access(d2.object_id
5 -access(d1.object_id
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
20 consistent gets
6 physical reads
0 redo size
3317 bytes sent via sql*net toclient
590 bytes received via sql*netfrom client
8 sql*net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
98 rows processed
sql>
走索引之后，物理读从1558降到6.
，