RE: What does LIKE '%%' actually do in a WHERE clause?

  • From: "Dunbar, Norman (Capgemini)" <norman.dunbar.capgemini@xxxxxxxxxxxxxxxxxxxxxxxxx>
  • To: Stephane Faroult <sfaroult@xxxxxxxxxxxx>
  • Date: Wed, 5 Oct 2011 15:20:58 +0000


>>     Hey, something new. I assume that 'id' is the PK (eg not 
>> null) and that it's the right type (varchar2 or char).
I'm not 100% sure if it is the PK, but it is indeed varchar2 and it is indexed.

>> I suspect that the optimizer:
>>    1) is smart enough to see that LIKE '%' does nothing and remove it
>>    2) NOT smart enough to see that LIKE '%%' does nothing more
>>    3) and as you noticed, goes for indexes instead of 
>> scanning tables with possibly a large number of rows.
Sounds reasonable.

>> Kind of trick based on a side-effect that is a potential 
>> time-bomb ... The fact that LIKE '%something' hits an index 
>> is just a matter of relative cost of scanning the index vs 
>> scanning the table. Such things can change.
Indeed, a test on 11g shows the same plan (and execution time) for % and %% - 
so it's looking like it's 9i related, maybe also 10g.

This is something that the developer/users run manually through a front end to 
the system (No, I didn't write it!) and they found this trick and found that it 
works fine (under 9i) - eventually, the developer's curiosity got the better of 
him and he asked a DBA.



Norman Dunbar
Contract Senior Oracle DBA
Capgemini Database Build Team
Internal : 7 28 2051
External : 0113 231 2051

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