当您省略 into 或批量返回到子句时,Oracle 将仅解析 sql 而不会从中发出任何提取。文档中没有明确说明会发生这种情况,但是文档确实指定了当您有 1 行返回时应该使用 INTO,如果您有可能返回超过 1 行,则应该使用 bulk。
例如:
SQL> create sequence testseq;
Sequence created.
SQL> alter session set events '10046 trace name context forever';
Session altered.
SQL> exec execute immediate 'select testseq.nextval from dual';
PL/SQL procedure successfully completed.
SQL> alter session set events '10046 trace name context off';
Session altered.
SQL> exit
我们在跟踪中看到 Oracle 没有打扰 FETCH 阶段:
=====================
PARSING IN CURSOR #140341213531640 len=32 dep=1 uid=83 oct=3 lid=83 tim=1363260261727946 hv=956010684 ad='7ac66b58' sqlid='56jwk2hwgr45w'
select testseq.nextval from dual
END OF STMT
PARSE #140341213531640:c=4001,e=50473,p=0,cr=2,cu=0,mis=1,r=0,dep=1,og=1,plh=112670795,tim=1363260261727944
EXEC #140341213531640:c=0,e=219,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=1,plh=112670795,tim=1363260261728249
STAT #140341213531640 id=1 cnt=0 pid=0 pos=1 obj=79530 op='SEQUENCE TESTSEQ (cr=0 pr=0 pw=0 time=181 us)'
STAT #140341213531640 id=2 cnt=0 pid=1 pos=1 obj=0 op='FAST DUAL (cr=0 pr=0 pw=0 time=1 us cost=2 size=0 card=1)'
CLOSE #140341213531640:c=4001,e=24391,dep=1,type=3,tim=1363260261752736
EXEC #140341212444728:c=8002,e=75407,p=0,cr=2,cu=0,mis=0,r=1,dep=0,og=1,plh=0,tim=1363260261752783
*** 2013-03-14 11:24:29.866
CLOSE #140341212444728:c=0,e=37,dep=0,type=0,tim=1363260269866098
=====================
与:
SQL> alter session set events '10046 trace name context forever';
Session altered.
SQL> var a number
SQL> exec execute immediate 'select testseq.nextval from dual' into :a;
PL/SQL procedure successfully completed.
SQL> alter session set events '10046 trace name context off';
Session altered.
现在:
PARSING IN CURSOR #139830768042232 len=32 dep=1 uid=83 oct=3 lid=83 tim=1363260428931803 hv=956010684 ad='7ac66b58' sqlid='56jwk2hwgr45w'
select testseq.nextval from dual
END OF STMT
PARSE #139830768042232:c=0,e=38,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=1,plh=112670795,tim=1363260428931802
EXEC #139830768042232:c=0,e=86,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=1,plh=112670795,tim=1363260428931917
FETCH #139830768042232:c=0,e=22,p=0,cr=0,cu=0,mis=0,r=1,dep=1,og=1,plh=112670795,tim=1363260428931980
STAT #139830768042232 id=1 cnt=1 pid=0 pos=1 obj=79530 op='SEQUENCE TESTSEQ (cr=0 pr=0 pw=0 time=39 us)'
STAT #139830768042232 id=2 cnt=1 pid=1 pos=1 obj=0 op='FAST DUAL (cr=0 pr=0 pw=0 time=2 us cost=2 size=0 card=1)'
CLOSE #139830768042232:c=0,e=0,dep=1,type=3,tim=1363260428931980
EXEC #139830768045912:c=0,e=294,p=0,cr=0,cu=0,mis=0,r=1,dep=0,og=1,plh=0,tim=1363260428931980
*** 2013-03-14 11:27:13.138
CLOSE #139830768045912:c=0,e=45,dep=0,type=0,tim=1363260433138490
=====================
可以看到 FETCH。我认为理想情况下,如果用户在未定义 INTO / BULK INTO 的情况下发出选择,Oracle 应该抛出错误。