sqlite single transaction without foreign key or triggers

Rob Williscroft rtw at freenet.co.uk
Thu May 14 00:46:16 CEST 2009


John Machin wrote in news:b722bd36-c8f1-4cdf-8625-2550cee21511
@i28g2000prd.googlegroups.com in comp.lang.python:

> On May 13, 11:46 am, a... at pythoncraft.com (Aahz) wrote:
>> In article <Xns9C09513903E8FrtwfreenetREMOVEc... at 216.196.109.145>,
>> Rob Williscroft  <r... at freenet.co.uk> wrote:
>>
>>
>>
>> >Aahz wrote innews:guao50$1jv$1 at panix3.panix.comin comp.lang.python:
>> >> In article <Xns9C08E179B66D8rtwfreenetREMOVEc... at 216.196.109.145>,
>> >> Rob Williscroft  <r... at freenet.co.uk> wrote:
>>
>> >>>db.execute( '''
>> >>>         update "sessions" set "uid" = ?
>> >>>         where "uid" = ?
>> >>>         and exists(
>> >>>                  select * from "users" where "uid" > = ?
>> >>>           )
>> >>>    ''',
>> >>>    (v['uid'],s.SID, v['uid'])
>> >>>  )
>>
>> >> This will be more efficient if you do "select uid from users".
>>
>> >What will be more efficient ?
>>
>> >Do you mean the "select * ..." or do you want to take the exists
>> >sub-query out and put it in a python if ?
>>
>> "select uid" will be more efficient than "select *", although I
>> suppose I could be wrong about that given how little I know about
>> current query optimizers.

It seems the usual advice about premeture optimisation should apply,
namely write clear code (*), then optimise the bottlenecks when you 
actualy find you need to.

*) for some definition of "clear code".

> My take is that it won't matter what you select if the optimiser is
> smart enough; something that requires minimal resources to produce is
> indicated in case the optimiser is dumb:
> 
>  ... exists (select 1 from ... )

I have to maintain some code writen by someone who thinks replacing 
"*" in queries with "1" is always a good idea (you know just in case), 
he wrote: 

    	select count(1) from ...

of course it didn't do what he thought it did.

Rob.
-- 
http://www.victim-prime.dsl.pipex.com/



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