CSHARP-5785: Optimize LINQ translation for First() and FirstOrDefault() methods with predicates #1817
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…() methods with predicates
adelinowona
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Pull Request Overview
This PR optimizes LINQ translation for First() and FirstOrDefault() methods with predicates by adding a limit: 1 parameter to the $filter operator in the generated MongoDB aggregation pipeline. This optimization prevents MongoDB from scanning the entire array once a matching element is found, improving query performance.
Key changes:
- Modified the translator to include
limit: 1in$filteroperations when translatingFirst()andFirstOrDefault()with predicates - Refactored to use a shared
isFirstMethodflag for consistency across conditional logic - Updated all test assertions to expect the new
limit: 1parameter in generated aggregation stages
Reviewed Changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| FirstOrLastMethodToAggregationExpressionTranslator.cs | Added logic to set limit: 1 for $filter when translating First/FirstOrDefault with predicates, and refactored conditional checks to use isFirstMethod flag |
| FirstOrLastMethodToAggregationExpressionTranslatorTests.cs | Updated test assertions to verify limit: 1 is included in $filter operations |
| CSharp5779Tests.cs | Updated test assertions for dictionary operations to expect limit: 1 in generated stages; also fixed a missing comma in line 95 |
| CSharp5258Tests.cs | Updated test assertion and removed extra space in expected stage output |
| CSharp4048Tests.cs | Updated test assertions for IGrouping operations and fixed missing comma in line 121 |
| CSharp3524Tests.cs | Updated test assertion for SelectMany to expect limit: 1 |
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| var expectedStages = new[] | ||
| { | ||
| "{ $group : { _id : '$_id', _elements : { $push: '$X' } } }", | ||
| "{ $project : { _id : '$_id' Result : { $arrayElemAt : ['$_elements', 0] } } }", |
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How was this test not failing?
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I have no idea. I just happened to catch it.
| var sourceAst = sourceTranslation.Ast; | ||
| var itemSerializer = ArraySerializerHelper.GetItemSerializer(sourceTranslation.Serializer); | ||
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| var isFirstMethod = method.Name.StartsWith("First"); |
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In general we prefer to test which method it is like this:
var isFirstMethod = method.IsOneOf(__firstMethods);
It is less likely to have false matches than string matching (partial or otherwise).
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I did it this way since I reasoned that if we passed the initial if statement:
if (method.IsOneOf(__firstOrLastMethods)) then we already know that the method has matched one of the first methods so no need to test the methods again.
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The statement you make is true, but even so we prefer to stick to method.IsOneOf instead of string matching.
The only time we don't use method.IsOneOf is when we are translating a method that might be implemented by classes we don't even know about (e.g. CompareTo).
| var sourceAst = sourceTranslation.Ast; | ||
| var itemSerializer = ArraySerializerHelper.GetItemSerializer(sourceTranslation.Serializer); | ||
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| var isFirstMethod = method.Name.StartsWith("First"); |
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The statement you make is true, but even so we prefer to stick to method.IsOneOf instead of string matching.
The only time we don't use method.IsOneOf is when we are translating a method that might be implemented by classes we don't even know about (e.g. CompareTo).
| if (isFirstMethod) | ||
| { | ||
| var compatibilityLevel = context.TranslationOptions.CompatibilityLevel; | ||
| if (compatibilityLevel is >= ServerVersion.Server60) |
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I would rather this line say:
if (Feature.FilterLimit.IsSupported(compatibilityLevel.ToWireVersion()))
| "{ $sort : { _id : 1 } }" | ||
| }; | ||
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| var expectedStages = FilterLimitIsSupported |
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I almost find the longer version more readable:
var expectedStages = Feature.FilterLimit.IsSupported(CoreTestConfiguration.MaxWireVersion)
because I don't have to chase down how FilterLimitIsSupported is computed.
In some other files you still use the long form.
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I would rather keep the FilterLimitIsSupported. I didn't want to have to keep repeating the longer form everywhere especially when it is the same information that we can get once and just reuse everywhere.
In some other files. there is only one use case, which is why I just used the longer form directly.
| var parameterSymbol = context.CreateSymbol(parameterExpression, itemSerializer, isCurrent: false); | ||
| var predicateTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, predicateLambda, parameterSymbol); | ||
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| // The $filter limit parameter was introduced in MongoDB 6.0 |
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The comment becomes redundant and can be removed if you accept the change recommended on line 112 as then the code becomes self documenting.
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