Fetch every nth row with LINQ

Question

We have a table in our SQL database with historical raw data I need to create charts from. We access the DB via Entity Framework and LINQ.

For smaller datetime inter

Answer 1

You can use the Select overload that includes the item index of enumerations. Something like this should do the trick --

var data = myDataLogEnumeration.
        Select((dt,i) => new { DataLog = dt, Index = i }).
        Where(x => x.Index % nth == 0).
        Select(x => x.DataLog);

If you need to limit the query with a Where or sort with OrderBy, you must do it before the first Select, otherwise the indexes will be all wrong --

var data = myDataLogEnumeration.
        Where(dt => dt.DateTime > dateLimit).
        OrderBy(dt => dt.SomeField).
        Select((dt,i) => new { DataLog = dt, Index = i }).
        Where(x => x.Index % nth == 0).
        Select(x => x.DataLog);

Unfortunately, as juharr commented, this overload is not supported in Entity Framework. One way to deal with this is to do something like this --

var data = entity.DataLogSet.
        Where(dt => dt.DateTime > dateLimit).
        OrderBy(dt => dt.SomeField).
        ToArray().
        Select((dt,i) => new { DataLog = dt, Index = i }).
        Where(x => x.Index % nth == 0).
        Select(x => x.DataLog);

Note the addition of a ToArray(). This isn't ideal though as it will force loading all the data that matches the initial query before selecting only every nth row.