CODEWITHSUNDEEP
c#linq-to-sql
adl
adl

Reputation: 2045

Is it possible to stop a query after the first time the where condition is true? (linq to sql)

Is it possible to stop a query after the first time the where condition is true?

I want to get only the first record which it's time is bigger than timeParameter. what I have now is:

var records = from rec in table
              where rec.time > timeParameter
              select rec;
return records.FirstOrDefault();

The time column in the database is ordered by ascending so the first time the where condition is true there is no need to keep querying. I have lots of rows in the database so I want the query to stop as soon as possible.

Upvotes: 4

Views: 2076

Answers (2)

Justin Pihony
Justin Pihony

Reputation: 67115

This should stop, actually. records is an IQueryable, so will not run until you request data (via FirstOrDefault), which will add the appropriate stop.

Basically, when you assign the records variable to the LINQ statement, it is merely loaded into memory as what the SQL WILL be. However, it does not evaluate anything until you actually make a call to access the data. At that point, the SQL will be modified appropriately based on whatever extension method you are using (FirstOrDefault in this case) and actually executed.

However, as Andrew Barber points out, your query needs to actually state the order by or else it will run without it. (Unless the table is truly sorted on the DB side as DangerMonkey counterpoints)

          from rec in table
          where rec.time > timeParameter
          order by rec.time
          select rec;

PS. If you want to really figure out how this works, then you can look into Expression Trees and deferred execution

Upvotes: 2

StriplingWarrior
StriplingWarrior

Reputation: 156624

Your current code will do what you're asking.

The .FirstOrDefault() method short-circuits. In this case, it produces an SQL query that starts with SELECT TOP 1 ..., which the SQL Server engine knows to mean it can stop evaluating after it finds the first item.

When you say:

The time column in the database is ordered by ascending...

Do you mean that the whole table is ordered ascending by the time column? If that's the case, then the natural order of these elements is probably going to be correct. Nevertheless, I would throw an order by statement in there to ensure you're getting the ordering you want. The worst case is that the query engine will discard it as unnecessary.

var records = from rec in table
              where rec.time > timeParameter
              order by rec.time
              select rec;
return records.FirstOrDefault();

Upvotes: 2

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