Linq2Sql: query optimisation

Question

I have the following query:

1. Get list of required team ids from DB:

           IList<int> teamIds =
               (from sector in DbContext.sectors
                where sector.Type=typeValue
                group sector by sector.TeamId into teamSectors
                select teamSectors.Key
               ).ToList();
   

2. Using this list fetch all sectors for required teams:

   IList<InfrStadSector> sectorsForAllTeams = (from sector in DbContext.sectors
                            where teamIds.Contains(sector.TeamId)
                            select sector
                           ).ToList();
   

3. Create list of Stadiums based on sectors:

   IList<InftStadium> stadiums = new List<InfrStadium>();
   foreach(int teamId in teamIds)
   {
       IList<InfrStadSector> teamSectors = 
           sectorsForAllTeams.Where(sect=>sect.TeamId==teamIds).ToList();
       stadiums.Add(new InfrStadium(teamId, teamSectors);
   }
   

My concern here is that for collection received from DB I need to apply Where/ToList on the client side once per each team

Is there any way to optimize that?

Thanks.

P.S. Potentially, I could sort items on server (using index), then sort teamIds and work with sectors collection without really 'querying' collection... but probably there is a better way to optimize that?

Answer 1

I think you can do this in one step.

var stadiums = DbContext.sectors
                        .Where( s => s.Type == typeValue )
                        .ToLookup( s => s.TeamId )
                        .Select( l => new InfrStadium( l.Key, l.ToList() )
                        .ToList();

Although, it would be better if you could have the constructor for InfrStadium take an IEnumerable<Sector> rather than requiring a List<Sector>, then you could omit the extra ToList.