That's question which I am asking to myself for some time now. I am afraid that it is strongly dependant on the case (db engine, size of parent and child, how many children has parent). Using n+1 killer (groupby in ibatis) is much cleaner in Java code, co I think it's better, unless developing some mission critical application. Michal Malecki ps one question - do we need to have sorted result by the grouped by column?
> Ok, the flip-side of this approach is that you increase the size of the > returned result by N*(size of the lookup). > > So if the lookup is 10 bytes and you are fetching 10 records of the > child, your payload size is 100 bytes > instead of 10 bytes. ( I wonder if JDBC makes some internal > optimisations - keeps a pointer or something > to same record if duplicates exist?) > > That's not a problem per se, but which one is better: > > 1. This approach or ... > 2. Using an "in clause" and making another round-trip? > > Cheers > > Abdullah > > > Michal Malecki wrote: > > >Well, I think you need only simple join in your sql, no extra features of > >ibatis needed: > >select * from CHILD, PARENT where parent_id=PARENT.id; > >am I right? > >Michal Malecki > > > > > > > >>What about relationships from the child up to the parent? Are they also > >>classified as N+1? > >> > >>Suppose we have child table with a fk of parent_id. > >> > >>if I do: > >> > >> SELECT * from CHILD > >> > >>I'll get a column with parent_id with values. > >> > >>Now suppose I also want details pertaining to the parent_id along with > >>the recordset of Child, I need not > >>do 10 separate selects for the parent ... is this also an N+1 problem? > >> > >>What is the preferred iBatis model for this? > >> > >>Cheers > >> > >>Abdullah > >> > >> > >>Kris Jenkins wrote: > >> > >> > >> > >>>friendVU admin wrote: > >>> > >>> > >>> > >>>>What is n+1? > >>>>.V > >>>> > >>>> > >>> > >>>n++! :-P > >>> > >>>It's when each child object in a complex query has to be selected > >>>seperately. If 'parent' has 10 'children', you end up with 10+1 > >>>select statements being issued. See the developer's guide under > >>>'Avoiding N+1 selects' for the full lowdown. Version 2.0.9 comes with > >>>new groupBy goodness to banish the N+1 problem. > >>> > >>>Kris > >>> > >>> > >>> > > > > > > > > > > >
