EFCacheContains

A library to help cache query plans using the 'Contains' method for Entity Framework

About

Entity Framework translates queries written in Linq to SQL, and this is an expensive compilation. To help with performance, Entity Framework caches its "query plans" after compiling them (not to be confused with the SQL database's query plan). However, there are several cases where EF does not perform caching. One is the case where the IEnumerable.Contains() method is invoked. This library solves the problem by intercepting the expression trees before they get to EF and rewriting the contains method so that it is cached -- at least for small lists. By default it will rewrite xs.Contains(x) as xs[0] == x || xs[1] == x || ... || xs[n] == x. However, it only performs this for lists up to a configurable length, by default 5. In principle, EF should create one query plan for each different array length. When no rewriting is done (i.e. for long lists), no query plan is cached.

Installation

This is distributed as a NuGet package on nuget.org:

Install-Package EFCacheContains -Version 1.0.40

Usage

using berkeleychurchill.CacheContains;

...

var myQuery = from r in myContext.Records.CacheContains()
              where myList.Contains(r.Id)
              select r;

Typically, myQuery will reference an expression tree that has a call to Contains in it. But, so long as myList contains 5 elements or fewer, the call to CacheContains() will rewrite the expression tree as a cascade of boolean checks. To change the cutoff size from 5 to another value, pass it as a parameter to CacheContains:

var myQuery = from r in myContext.Records.CacheContains(10)
              where myList.Contains(r.Id)
              select r;

One can also change the default maximum list size to rewrite the expression tree:

CacheContains.QueryableExtensions.DefaultMaxSize = 20;

Troubleshooting

It's very important that before you use this library you perform profiling to ensure that EF query plan caching is a bottleneck. However, it's usually not enough to just rewrite your queries to invoke CacheContains. There are a number of other reasons that EF might not cache your query. The only way to be sure that things are working properly is to verify that you're obtaining the expected performance speedup. It's good to profile again and compare to make sure there aren't any lingering queries which aren't being cached. You may find it helpful to log the database queries and make sure that they're properly parameterized.

If you find a situation where this library is not rewriting a query that has a Contains method call, please file a bug report. A reproducible test case is very helpful (and it's even better if you can reproduce the problem with just Linq but no Entity Framework).

Release Notes

1.0.40

• Fixing CI so that packages aren't improperly marked as prereleases.

1.0.36

• Fix for environments that do not allow optional parameters in expression trees.
• New way to set default maximum list size.

1.0.31

• Important bug fix for lists containing more than 2 items.
• Performance improvements
• Additional testing.

1.0.26

• Initial release.