Question

I have a database that stores dates broken into int’s. I.e. Day, Month and Year are stored separately in different columns.

In existing SQL queries, the method used is:

DATEADD(dd, – 1, DATEADD(mm, Z.LastMonth, DATEADD(yy, Z.LastYear – 1900, 0))) AS last_date

I cannot change the database to store dates instead.

Looking further into the dateadd() function, it’s converting from an integer (0).
Linq to SQL does not have a similar functionality i.e. Convert.ToDateTime(0). That results in an InvalidCastException.

I have tried concatenating strings to create a date, but it is WAYYYYY tooooo sllloowww. the time difference was about 10 minutes.

What else can I do? I don’t particularly want to start mixing in SQL queries into the project either.

Thanks.

Answer 1

Can you not just write:

table.Select(z => new DateTime(z.LastYear, z.LastMonth, z.LastDay));

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Alright, I tested this in Linqpad and it seems that Linq to SQL does decide to generate some weird character-conversion query. I’m not entirely convinced that this is the source of the perf issue, but you can force a projection like this:

var dates = 
    (from z in table
     select new { Year = z.LastYear, Month = z.LastMonth, Day = z.LastDay })
    .AsEnumerable()
    .Select(d => new 
        {
            Date = new DateTime(d.Year, d.Month, d.Day),
            NextDate = new DateTime(d.Year, d.Month, 1).AddMonths(2).AddDays(-1)
        });

This will also get you the last day of the next month.

If you’re really seeing such a huge performance, difference, though, I would venture a guess that you’re looking in the wrong place, and that there’s something else that’s different. 90% of all database-related performance problems are due to poor or nonexistent indexing or non-sargable queries.