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Oracle 执行计划中的buffer sort实际上没有排序，只是把数据加载到内存，不扫描多次表。
--制造数据
drop table test purge;
drop table test1 purge;
create table test as select * from dba_objects where rownum<10;
create table test1 as select * from dba_objects where rownum<10;
create index ind_t_object_id on test(object_id);
create index ind_t1_object_id on test1(object_id);
exec dbms_stats.gather_table_stats(user,‘test‘,cascade => true);
exec dbms_stats.gather_table_stats(user,‘test1‘,cascade => true);
--执行SQL
select /*+gg_test*/* from test t, test1 t1 where t.object_name like ‘T%‘;
----------------------------------------------------------
Plan hash value: 702683263
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 9 | 1350 | 6 (0)| 00:00:01 |
| 1 | MERGE JOIN CARTESIAN| | 9 | 1350 | 6 (0)| 00:00:01 |
|* 2 | TABLE ACCESS FULL | TEST | 1 | 75 | 3 (0)| 00:00:01 |
| 3 | BUFFER SORT | | 9 | 675 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | TEST1 | 9 | 675 | 3 (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("T"."OBJECT_NAME" LIKE ‘T%‘)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
1991 bytes sent via SQL*Net to client
348 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
--#keys=0表示没有排序，Oracle中如何判断SQL是否真正做了排序
select projection
from v$sql_plan_statistics_all
where sql_id in(select sql_id from v$sql where
sql_text like‘select /*+gg_test*/* from test t, test1 t1 where t.object_name like ‘‘T%‘‘ ‘)
and operation=‘BUFFER‘ and options=‘SORT‘;
(#keys=0) "T1"."OWNER"[VARCHAR2,30], "T1"."OBJECT_NAME"[VARCHAR2,128], "T1"."SUBOBJECT_NAME"[VARCHAR2,30], "T1"."OBJECT_ID"[NUMBER,22], "T1"."DATA_OBJECT_ID"[NUMBER,22], "T1"."OBJECT_TYPE"[VARCHAR2,19],
"T1"."CREATED"[DATE,7], "T1"."LAST_DDL_TIME"[DATE,7], "T1"."TIMESTAMP"[VARCHAR2,19], "T1"."STATUS"[VARCHAR2,7], "T1"."TEMPORARY"[VARCHAR2,1], "T1"."GENERATED"[VARCHAR2,1], "T1"."SECONDARY"[VARCHAR2,1], "T1"."NAMESPACE"[NUMBER,22], "T1"."EDITION_NAME"[VARCHAR2,30]
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原文地址：http://blog.csdn.net/stevendbaguo/article/details/42173093
