oracle11G时间类型格式转化

If you really care about performance, the real answer is to fix your data model.

Code and Test Cases:

--Function to convert a string to a date, or return null if the format is wrong.
create or replace function validate_date(p_string in string) return date is
begin
return to_date(p_string, 'MONTH DD, YYYY');
exception when others then
begin
return to_date(p_string, 'MM/DD/YYYY');
exception when others then
begin
return to_date(p_string, 'DD-MON-RR');
exception when others then
return null;
end;
end;
end;
/

--Test individual values
select validate_date('JULY 31, 2009') from dual;
2009-07-31
select validate_date('7/31/2009') from dual;
2009-07-31
select validate_date('31-JUL-09') from dual;
2009-07-31
select validate_date('2009-07-31') from dual;
Simple Performance Test:

--Create table to hold test data
create table test1(a_date varchar2(1000)) nologging;

--Insert 10 million rows
begin
for i in 1 .. 100 loop
insert /*+ append */ into test1
select to_char(sysdate+level, 'MM/DD/YYYY') from dual connect by level <= 100000;

commit;
end loop;
end;
/

--"Warm up" the database, run this a few times, see how long a count takes.
--Best case time to count: 2.3 seconds
select count(*) from test1;


--How long does it take to convert all those strings?
--6 minutes... ouch
select count(*)
from test1
where validate_date(a_date) is not null;