Re: Worthwhile optimisation of position()?

Thomas Hallgren wrote:
> Tom Lane wrote:
>> Tim Allen <tim@proximity.com.au> writes:
>>  
>>> Thomas Hallgren wrote:
>>>    
>>>> The position function must look for 'ch' everywhere in the string so 
>>>> there's no way it can use an index.
>>>>       
>>
>>  
>>> I think the '= 0' bit is what Chris was suggesting could be the basis 
>>> for an optimisation.
>>>     
>>
>> Yeah.  AFAICS the transformation Chris suggested is valid.  I'm really
>> dubious that it's worth expending planner cycles to look for it though.
>> LIKE is something that everybody and his brother uses, but who uses this
>> position()=0 locution?
>>
>>             regards, tom lane
>>   
> The documentation says: position('om' in 'Thomas') == 3 so i assumed 
> that the returned index was 1-based and that a zero meant 'not found'. 
> If I'm wrong ,perhaps the docs need to be updated?
> 

The docs are correct so my initial point was correct. "position('ch' in user) = 0" is 
equivalent to "user NOT LIKE '%ch%'" and there's no way you can index that.

Regards,
Thomas Hallgren