sqlite single transaction without foreign key or triggers

John Machin sjmachin at lexicon.net
Wed May 13 04:44:10 CEST 2009


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.

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 ... )




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