Composing remote and local linq queries
One of the cool things with Linq is that queries are composable, i.e. you can add further query constraints by selecting from an existing query. Nothing is executed until you try to read from the query. This allows IQueryable to compose all the added constraints and transform it into the underlying query structure, most commonly SQL.
However it does come with the pitfall that there are a lot of things legal in Linq expressions that will die at run time. This happens because an expression may not have an equivalent syntax in the transformed language, like calling a method as part of a where clause.
This does not mean that you can't use linq for the portion of the query that is not executable by the provider. As long as you know what expression is affected, you can use query composition to build a query that executes some part remotely and some part against the object model in memory.
Let's suppose we wish to execute the following query:
But our query provider doesn't understand the extension method that allows us to check the list of Tags. In order for this query to work, that portion must be executed against the result set of the date range query. We could coerce the first portion to a list or array and then query that portion, but that would just force the date query to be materialized before we could prune the set. Instead we want to feed the stream of matching entries into a second query, composing a new query that contains both portions as a single query and won't access the database until we iterate over it.
To accomplish this I created an extension method that coerces the query into a sequence that yields each item as it is returned by the database query:
UPDATE: As Scott points out in the comments, my
AsSequence just re-implements what is already available in Ling as
AsEnumerable. So the above really just serves to explain how
AsEnumerable defers execution to enumeration rather than query definition.
AsEnumerable allows me to compose the query from server and local expressions like this:
When q is enumerated, the first expression is converted to SQL and executes against the database. Each item returned from the database is then fed into the second query, which checks its expression and yields the item to the caller, should the expression match. Since
q.AsSequence() is used as part of query composition, it does not force the first expression to execute at the time of query definition as
q.ToList() would. The additional benefit is that even when
q.AsSequence() is executed, it never builds the entire result set in memory as a list to iterate over, but rather just streams each database query result item through its own expression evaluation.
Of course, this still have the performance implications of sending data across the wire and filtering it locally. However, this is not an uncommon problem when SQL alone cannot provide all the filtering. The benefit of this approach is reduced memory pressure on execution, better control when execution occurs and the ability to use Linq syntax to do the secondary filtering.