Query: Generate a warning for multiple collection Includes

src/EFCore.Relational/Diagnostics/RelationalEventId.cs

@@ -77,6 +77,7 @@ private enum Id
             QueryPossibleUnintendedUseOfEqualsWarning,
             QueryPossibleExceptionWithAggregateOperatorWarning,
             ValueConversionSqlLiteralWarning, // This warning has been removed.
+            MultipleCollectionIncludeWarning,
 
             // Model validation events
             ModelValidationKeyDefaultValueWarning = CoreEventId.RelationalBaseId + 600,
@@ -611,6 +612,16 @@ private enum Id
         public static readonly EventId QueryPossibleExceptionWithAggregateOperatorWarning =
             MakeQueryId(Id.QueryPossibleExceptionWithAggregateOperatorWarning);
 
+        /// <summary>
+        ///     <para>
+        ///         A query is loading multiple related collections without configuring a <see cref="QuerySplittingBehavior"/>.
+        ///     </para>
+        ///     <para>
+        ///         This event is in the <see cref="DbLoggerCategory.Query" /> category.
+        ///     </para>
+        /// </summary>
+        public static readonly EventId MultipleCollectionIncludeWarning = MakeQueryId(Id.MultipleCollectionIncludeWarning);
+
         private static readonly string _validationPrefix = DbLoggerCategory.Model.Validation.Name + ".";
         private static EventId MakeValidationId(Id id) => new EventId((int)id, _validationPrefix + id);
 

src/EFCore.Relational/Diagnostics/RelationalLoggerExtensions.cs

@@ -4183,6 +4183,30 @@ public static void QueryPossibleExceptionWithAggregateOperatorWarning(
             }
         }
 
+        /// <summary>
+        ///     Logs for the <see cref="RelationalEventId.MultipleCollectionIncludeWarning" /> event.
+        /// </summary>
+        /// <param name="diagnostics"> The diagnostics logger to use. </param>
+        public static void MultipleCollectionIncludeWarning(
+            [NotNull] this IDiagnosticsLogger<DbLoggerCategory.Query> diagnostics)
+        {
+            var definition = RelationalResources.LogMultipleCollectionIncludeWarning(diagnostics);
+
+            if (diagnostics.ShouldLog(definition))
+            {
+                definition.Log(diagnostics);
+            }
+
+            if (diagnostics.NeedsEventData(definition, out var diagnosticSourceEnabled, out var simpleLogEnabled))
+            {
+                var eventData = new EventData(
+                    definition,
+                    (d, p) => ((EventDefinition)d).GenerateMessage());
+
+                diagnostics.DispatchEventData(definition, eventData, diagnosticSourceEnabled, simpleLogEnabled);
+            }
+        }
+
         /// <summary>
         ///     Logs for the <see cref="RelationalEventId.ModelValidationKeyDefaultValueWarning" /> event.
         /// </summary>

src/EFCore.Relational/Diagnostics/RelationalLoggingDefinitions.cs

@@ -474,5 +474,14 @@ public abstract class RelationalLoggingDefinitions : LoggingDefinitions
         /// </summary>
         [EntityFrameworkInternal]
         public EventDefinitionBase LogUnnamedIndexPropertiesMappedToNonOverlappingTables;
+
+        /// <summary>
+        ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+        ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+        ///     any release. You should only use it directly in your code with extreme caution and knowing that
+        ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+        /// </summary>
+        [EntityFrameworkInternal]
+        public EventDefinitionBase LogMultipleCollectionIncludeWarning;
     }
 }

src/EFCore.Relational/Properties/RelationalStrings.resx

@@ -660,4 +660,8 @@
   <data name="IncompatibleTableExcludedMismatch" xml:space="preserve">
     <value>Cannot use table '{table}' for entity type '{entityType}' since it is being used for entity type '{otherEntityType}' and it excluded from migration on one entity type, but not the other. Exclude the table from migrations on all entity types mapped to the table.</value>
   </data>
+  <data name="LogMultipleCollectionIncludeWarning" xml:space="preserve">
+    <value>Compiling a query which loads related collections for more than one collection navigation property either via 'Include' or through projection but no 'QuerySplittingBehavior' has been configured. By default EF Core will use 'QuerySplittingBehavior.SingleQuery' which can potentially result in slow query performance. See https://go.microsoft.com/fwlink/?linkid=2134277 for more information. To identify the query that's triggering this warning call .ConfigureWarnings(w =&gt; w.Throw(RelationalEventId.LogTooManyIncludesWarning))</value>
+    <comment>Warning RelationalEventId.MultipleCollectionIncludeWarning</comment>
+  </data>
 </root>

src/EFCore.Relational/Query/Internal/CollectionJoinApplyingExpressionVisitor.cs

@@ -3,6 +3,7 @@
 
 using System.Linq.Expressions;
 using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
 using Microsoft.EntityFrameworkCore.Utilities;
@@ -18,7 +19,8 @@ namespace Microsoft.EntityFrameworkCore.Query.Internal
     public class CollectionJoinApplyingExpressionVisitor : ExpressionVisitor
     {
         private readonly bool _splitQuery;
-        private readonly bool _userConfiguredBehavior;
+        private readonly bool _noConfiguredBehavior;
+        private readonly IDiagnosticsLogger<DbLoggerCategory.Query> _logger;
         private int _collectionId;
 
         /// <summary>
@@ -32,7 +34,8 @@ public CollectionJoinApplyingExpressionVisitor([NotNull] RelationalQueryCompilat
             Check.NotNull(queryCompilationContext, nameof(queryCompilationContext));
 
             _splitQuery = queryCompilationContext.QuerySplittingBehavior == QuerySplittingBehavior.SplitQuery;
-            _userConfiguredBehavior = RelationalOptionsExtension.Extract(queryCompilationContext.ContextOptions).QuerySplittingBehavior.HasValue;
+            _noConfiguredBehavior = queryCompilationContext.QuerySplittingBehavior == null;
+            _logger = queryCompilationContext.Logger;
         }
 
         /// <summary>
@@ -48,6 +51,13 @@ protected override Expression VisitExtension(Expression extensionExpression)
             if (extensionExpression is CollectionShaperExpression collectionShaperExpression)
             {
                 var collectionId = _collectionId++;
+
+                if (_noConfiguredBehavior
+                    && _collectionId == 2)
+                {
+                    _logger.MultipleCollectionIncludeWarning();
+                }
+
                 var projectionBindingExpression = (ProjectionBindingExpression)collectionShaperExpression.Projection;
                 var selectExpression = (SelectExpression)projectionBindingExpression.QueryExpression;
                 // Do pushdown beforehand so it updates all pending collections first

src/EFCore.Relational/Query/RelationalQueryCompilationContext.cs

@@ -33,8 +33,7 @@ public RelationalQueryCompilationContext(
             Check.NotNull(relationalDependencies, nameof(relationalDependencies));
 
             RelationalDependencies = relationalDependencies;
-            QuerySplittingBehavior = RelationalOptionsExtension.Extract(ContextOptions).QuerySplittingBehavior
-                ?? QuerySplittingBehavior.SingleQuery;
+            QuerySplittingBehavior = RelationalOptionsExtension.Extract(ContextOptions).QuerySplittingBehavior;
         }
 
         /// <summary>
@@ -43,8 +42,9 @@ public RelationalQueryCompilationContext(
         protected virtual RelationalQueryCompilationContextDependencies RelationalDependencies { get; }
 
         /// <summary>
-        ///     A value indicating the <see cref="EntityFrameworkCore.QuerySplittingBehavior"/> of the query.
+        ///     A value indicating the <see cref="EntityFrameworkCore.QuerySplittingBehavior"/> configured for the query.
+        ///     If no value has been configured then <see cref="QuerySplittingBehavior.SingleQuery"/> will be used.
         /// </summary>
-        public virtual QuerySplittingBehavior QuerySplittingBehavior { get; internal set; }
+        public virtual QuerySplittingBehavior? QuerySplittingBehavior { get; internal set; }
     }
 }