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linq to sql - performance improvement

Posted on 2010-09-06
Medium Priority
Last Modified: 2013-11-11
I have 2 tables:


table name: table1




table name: table2







To query table2, I use the following code:


var result = (from t1 in table1

join t2 in table2 on t1.id eqauls t2.c1

where t2.id == (from t22 in table2          

                                where t22.c1 == t1.id

                                order by t22.date1 descending


                                select t22.id).first()


select t2)


return result.ToList()


To improve performance, I am thinking about using the following query instead:


var result = (from t1 in table1

join t2 on table2 on t1.id == t2.c1

where (!t2.c2.HasValue) || (t2.id == (from t22 in table2

                                                                                where t22.c1 == t1.id

                                                                                order by t22.date1 descending


                                                                                select t22.id).first()))


select t2)


return result.ToList()



My thought is, if condition !t2.c2.HasValue is true, then the second component of "or" statement will not be looked at. As a result, the subquery will not be run, improving the performance.


1. Is this correct? Or will the subquery be run anyway.

2. in the subquery, is the query submitted when .First() is called? Or when result.ToList() is executed?


Question by:fippif
LVL 19

Accepted Solution

Bardobrave earned 334 total points
ID: 33615768
Mmmm... as far as I know, linq to sql makes a direct translation from it's codification to SQL, if you pass mouse pointer over a linq to sql query on debug time you'll be the translated SQL query.

Although C derived languages are clever enough to stop evaluating conditions when it founds the first one that makes true or false a statement I don't think that SQL would be as clever (although maybe it depends on the engine executing the SQL...).

Try to see in debug time if the SQL generated by your linq statement is always the same or if it's truncated when first part of OR condition is true, this way you'll know if really the performance is going to improve.

Regarding your second question... linq to sql is made of classes and methods that execute on a straight way, so I supose that every query is submitted when it's line of code is executed. On your example you'll have your query executed and it's results stored on "result" variable (note that this variable is not able to be of any type, it has to be of a type that matches the result of your sql statement execution, this is because when this line of code runs, your variable is going to receive the result of the query execution). Afterwards, result.ToList() will only perform a type conversion, it doesn't fire any stored query.
LVL 64

Assisted Solution

by:Fernando Soto
Fernando Soto earned 334 total points
ID: 33617910
Does there exist a relationship between Table1 and Table2 in the database? If so what is it?
Can you explain in words what you want to achieve with the queries?

Assisted Solution

yamir2k earned 332 total points
ID: 33636688
Your queries look a bit weird. Table1 and table2 are of Table<T> type? I think you should be more specific about your tables and relations between them. Generally, you should not use join when writing a LINQ to SQL query. Instead, you should use associations between your LINQ to SQL classes.

Author Closing Comment

ID: 34141869
I did not really get my answer on this question. But thanks for the input.

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