[每日一题] OCP1z0-047 :2013-08-08 相关子查询中EXISTS的使用.....................................28

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正确答案是：AC

EXISTS谓词非常简单，它是对一个非空集的测试。如果在其子查询中存在任何行，则返回TRUE，否则为FALSE。该谓词不会返回UNKNOWN结果。EXIST（）谓词语法如下： <EXISTS谓词>：：＝[NOTEXISTS]<表子查询>

对exists的测试，看执行计划：

gyj@MYDB> create table t3(id number,name varchar2(100));

Table created.

gyj@MYDB> insert into t3  select level,'a'||level from dual connect by level<1000000;

999999 rows created.

gyj@MYDB> create index idx_t3 on t3(id);

Index created.

gyj@MYDB> create table t4(id number,name varchar2(100));

Table created.

gyj@MYDB> insert into t4  select level,'a'||level from dual connect by level<10;

9 rows created.

gyj@MYDB> commit;

Commit complete.

gyj@MYDB> exec dbms_stats.gather_table_stats(user,'t3',cascade => true);

PL/SQL procedure successfully completed.

gyj@MYDB> exec dbms_stats.gather_table_stats(user,'t4');

PL/SQL procedure successfully completed.

看完计行计划就明白了EXISTS的执行步骤:

gyj@MYDB> set autot traceonly;
gyj@MYDB> select * from t4 where id in (select id from t3);

9 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1092212754

-----------------------------------------------------------------------------
| Id  | Operation          | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |        |     9 |    99 |    21   (0)| 00:00:01 |
|   1 |  NESTED LOOPS SEMI |        |     9 |    99 |    21   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T4     |     9 |    54 |     3   (0)| 00:00:01 |
|*  3 |   INDEX RANGE SCAN | IDX_T3 |   999K|  4882K|     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

   3 - access("ID"="ID")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         20  consistent gets
          0  physical reads
          0  redo size
        723  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          9  rows processed

EXISTS半连接的伪代码

 open tab1
  while tab1 still has records
     fetch  record from tab1
     result = false
     open tab2
     while tab2 still has records
         fetch record from tab2
         if(tab1.record matches tab2.record) then
            result = true
            exit loop
          end if
     end loop
     close tab2
     if (result = true) return tab1 record
  end loop
  close tab1

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