Query: Adds aggressive prefetching for scalar aggregates

Microsoft.Azure.Cosmos/src/ChangeFeed/Pagination/CrossPartitionChangeFeedAsyncEnumerator.cs

@@ -158,10 +158,11 @@ public static CrossPartitionChangeFeedAsyncEnumerator Create(
                     documentContainer,
                     changeFeedPaginationOptions,
                     cancellationToken),
-                comparer: default /* this uses a regular queue instead of prioirty queue */,
+                comparer: default /* this uses a regular queue instead of priority queue */,
                 maxConcurrency: default,
-                cancellationToken,
-                state);
+                prefetchPolicy: PrefetchPolicy.PrefetchSinglePage,
+                cancellationToken: cancellationToken,
+                state: state);
 
             CrossPartitionChangeFeedAsyncEnumerator enumerator = new CrossPartitionChangeFeedAsyncEnumerator(
                 crossPartitionEnumerator,

Microsoft.Azure.Cosmos/src/Pagination/BufferedPartitionRangePageAsyncEnumerator.cs

@@ -10,7 +10,7 @@ namespace Microsoft.Azure.Cosmos.Pagination
     using Microsoft.Azure.Cosmos.Query.Core.Monads;
     using Microsoft.Azure.Cosmos.Tracing;
 
-    internal sealed class BufferedPartitionRangePageAsyncEnumerator<TPage, TState> : PartitionRangePageAsyncEnumerator<TPage, TState>, IPrefetcher
+    internal sealed class BufferedPartitionRangePageAsyncEnumerator<TPage, TState> : BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState>
         where TPage : Page<TState>
         where TState : State
     {
@@ -23,7 +23,10 @@ public BufferedPartitionRangePageAsyncEnumerator(PartitionRangePageAsyncEnumerat
             this.enumerator = enumerator ?? throw new ArgumentNullException(nameof(enumerator));
         }
 
-        public override ValueTask DisposeAsync() => this.enumerator.DisposeAsync();
+        public override ValueTask DisposeAsync()
+        {
+            return this.enumerator.DisposeAsync();
+        }
 
         protected override async Task<TryCatch<TPage>> GetNextPageAsync(ITrace trace, CancellationToken cancellationToken)
         {
@@ -40,20 +43,22 @@ protected override async Task<TryCatch<TPage>> GetNextPageAsync(ITrace trace, Ca
             return returnValue;
         }
 
-        public async ValueTask PrefetchAsync(ITrace trace, CancellationToken cancellationToken)
+        public override async ValueTask PrefetchAsync(ITrace trace, CancellationToken cancellationToken)
         {
             if (trace == null)
             {
                 throw new ArgumentNullException(nameof(trace));
             }
 
-            using (ITrace prefetchTrace = trace.StartChild("Prefetch", TraceComponent.Pagination, TraceLevel.Info))
+            if (this.bufferedPage.HasValue)
             {
-                if (this.bufferedPage.HasValue)
-                {
-                    return;
-                }
+                return;
+            }
 
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using (ITrace prefetchTrace = trace.StartChild("Prefetch", TraceComponent.Pagination, TraceLevel.Info))
+            {
                 await this.enumerator.MoveNextAsync(prefetchTrace);
                 this.bufferedPage = this.enumerator.Current;
             }

Microsoft.Azure.Cosmos/src/Pagination/BufferedPartitionRangePageAsyncEnumeratorBase.cs

@@ -0,0 +1,22 @@
+// ------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+// ------------------------------------------------------------
+
+namespace Microsoft.Azure.Cosmos.Pagination
+{
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.Azure.Cosmos.Tracing;
+
+    internal abstract class BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState> : PartitionRangePageAsyncEnumerator<TPage, TState>, IPrefetcher
+        where TPage : Page<TState>
+        where TState : State
+    {
+        protected BufferedPartitionRangePageAsyncEnumeratorBase(FeedRangeState<TState> feedRangeState, CancellationToken cancellationToken)
+            : base(feedRangeState, cancellationToken)
+        {
+        }
+
+        public abstract ValueTask PrefetchAsync(ITrace trace, CancellationToken cancellationToken);
+    }
+}

Microsoft.Azure.Cosmos/src/Pagination/CrossPartitionRangePageAsyncEnumerator.cs

@@ -34,64 +34,24 @@ public CrossPartitionRangePageAsyncEnumerator(
             CreatePartitionRangePageAsyncEnumerator<TPage, TState> createPartitionRangeEnumerator,
             IComparer<PartitionRangePageAsyncEnumerator<TPage, TState>> comparer,
             int? maxConcurrency,
+            PrefetchPolicy prefetchPolicy,
             CancellationToken cancellationToken,
             CrossFeedRangeState<TState> state = default)
         {
             this.feedRangeProvider = feedRangeProvider ?? throw new ArgumentNullException(nameof(feedRangeProvider));
             this.createPartitionRangeEnumerator = createPartitionRangeEnumerator ?? throw new ArgumentNullException(nameof(createPartitionRangeEnumerator));
             this.cancellationToken = cancellationToken;
 
-            this.lazyEnumerators = new AsyncLazy<IQueue<PartitionRangePageAsyncEnumerator<TPage, TState>>>(async (ITrace trace, CancellationToken token) =>
-            {
-                ReadOnlyMemory<FeedRangeState<TState>> rangeAndStates;
-                if (state != default)
-                {
-                    rangeAndStates = state.Value;
-                }
-                else
-                {
-                    // Fan out to all partitions with default state
-                    List<FeedRangeEpk> ranges = await feedRangeProvider.GetFeedRangesAsync(trace, token);
-
-                    List<FeedRangeState<TState>> rangesAndStatesBuilder = new List<FeedRangeState<TState>>(ranges.Count);
-                    foreach (FeedRangeInternal range in ranges)
-                    {
-                        rangesAndStatesBuilder.Add(new FeedRangeState<TState>(range, default));
-                    }
-
-                    rangeAndStates = rangesAndStatesBuilder.ToArray();
-                }
-
-                List<BufferedPartitionRangePageAsyncEnumerator<TPage, TState>> bufferedEnumerators = new List<BufferedPartitionRangePageAsyncEnumerator<TPage, TState>>(rangeAndStates.Length);
-                for (int i = 0; i < rangeAndStates.Length; i++)
-                {
-                    FeedRangeState<TState> feedRangeState = rangeAndStates.Span[i];
-                    PartitionRangePageAsyncEnumerator<TPage, TState> enumerator = createPartitionRangeEnumerator(feedRangeState);
-                    BufferedPartitionRangePageAsyncEnumerator<TPage, TState> bufferedEnumerator = new BufferedPartitionRangePageAsyncEnumerator<TPage, TState>(enumerator, cancellationToken);
-                    bufferedEnumerators.Add(bufferedEnumerator);
-                }
-
-                if (maxConcurrency.HasValue)
-                {
-                    await ParallelPrefetch.PrefetchInParallelAsync(bufferedEnumerators, maxConcurrency.Value, trace, token);
-                }
-
-                IQueue<PartitionRangePageAsyncEnumerator<TPage, TState>> queue;
-                if (comparer == null)
-                {
-                    queue = new QueueWrapper<PartitionRangePageAsyncEnumerator<TPage, TState>>(
-                        new Queue<PartitionRangePageAsyncEnumerator<TPage, TState>>(bufferedEnumerators));
-                }
-                else
-                {
-                    queue = new PriorityQueueWrapper<PartitionRangePageAsyncEnumerator<TPage, TState>>(
-                        new PriorityQueue<PartitionRangePageAsyncEnumerator<TPage, TState>>(
-                            bufferedEnumerators,
-                            comparer));
-                }
-
-                return queue;
-            });
+            this.lazyEnumerators = new AsyncLazy<IQueue<PartitionRangePageAsyncEnumerator<TPage, TState>>>((ITrace trace, CancellationToken token) =>
+                InitializeEnumeratorsAsync(
+                    feedRangeProvider,
+                    createPartitionRangeEnumerator,
+                    comparer,
+                    maxConcurrency,
+                    prefetchPolicy,
+                    state,
+                    trace,
+                    token));
         }
 
         public TryCatch<CrossFeedRangePage<TPage, TState>> Current { get; private set; }
@@ -281,6 +241,79 @@ private static bool IsSplitException(Exception exeception)
             return enumerators.Peek()?.FeedRangeState;
         }
 
+        private static async Task<IQueue<PartitionRangePageAsyncEnumerator<TPage, TState>>> InitializeEnumeratorsAsync(
+            IFeedRangeProvider feedRangeProvider,
+            CreatePartitionRangePageAsyncEnumerator<TPage, TState> createPartitionRangeEnumerator,
+            IComparer<PartitionRangePageAsyncEnumerator<TPage, TState>> comparer,
+            int? maxConcurrency,
+            PrefetchPolicy prefetchPolicy,
+            CrossFeedRangeState<TState> state,
+            ITrace trace,
+            CancellationToken token)
+        {
+            ReadOnlyMemory<FeedRangeState<TState>> rangeAndStates;
+            if (state != default)
+            {
+                rangeAndStates = state.Value;
+            }
+            else
+            {
+                // Fan out to all partitions with default state
+                List<FeedRangeEpk> ranges = await feedRangeProvider.GetFeedRangesAsync(trace, token);
+
+                List<FeedRangeState<TState>> rangesAndStatesBuilder = new List<FeedRangeState<TState>>(ranges.Count);
+                foreach (FeedRangeInternal range in ranges)
+                {
+                    rangesAndStatesBuilder.Add(new FeedRangeState<TState>(range, default));
+                }
+
+                rangeAndStates = rangesAndStatesBuilder.ToArray();
+            }
+
+            IReadOnlyList<BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState>> bufferedEnumerators = CreateBufferedEnumerators(
+                prefetchPolicy,
+                createPartitionRangeEnumerator,
+                rangeAndStates,
+                token);
+
+            if (maxConcurrency.HasValue)
+            {
+                await ParallelPrefetch.PrefetchInParallelAsync(bufferedEnumerators, maxConcurrency.Value, trace, token);
+            }
+
+            IQueue<PartitionRangePageAsyncEnumerator<TPage, TState>> queue = comparer == null
+                ? new QueueWrapper<PartitionRangePageAsyncEnumerator<TPage, TState>>(
+                    new Queue<PartitionRangePageAsyncEnumerator<TPage, TState>>(bufferedEnumerators))
+                : new PriorityQueueWrapper<PartitionRangePageAsyncEnumerator<TPage, TState>>(
+                    new PriorityQueue<PartitionRangePageAsyncEnumerator<TPage, TState>>(
+                        bufferedEnumerators,
+                        comparer));
+            return queue;
+        }
+
+        private static IReadOnlyList<BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState>> CreateBufferedEnumerators(
+            PrefetchPolicy policy,
+            CreatePartitionRangePageAsyncEnumerator<TPage, TState> createPartitionRangeEnumerator, 
+            ReadOnlyMemory<FeedRangeState<TState>> rangeAndStates,
+            CancellationToken cancellationToken)
+        {
+            List<BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState>> bufferedEnumerators = new (rangeAndStates.Length);
+            for (int i = 0; i < rangeAndStates.Length; i++)
+            {
+                FeedRangeState<TState> feedRangeState = rangeAndStates.Span[i];
+                PartitionRangePageAsyncEnumerator<TPage, TState> enumerator = createPartitionRangeEnumerator(feedRangeState);
+                BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState> bufferedEnumerator = policy switch
+                {
+                    PrefetchPolicy.PrefetchSinglePage => new BufferedPartitionRangePageAsyncEnumerator<TPage, TState>(enumerator, cancellationToken),
+                    PrefetchPolicy.PrefetchAll => new FullyBufferedPartitionRangeAsyncEnumerator<TPage, TState>(enumerator, cancellationToken),
+                    _ => throw new ArgumentOutOfRangeException(nameof(policy)),
+                };
+                bufferedEnumerators.Add(bufferedEnumerator);
+            }
+
+            return bufferedEnumerators;
+        }
+
         private interface IQueue<T> : IEnumerable<T>
         {
             T Peek();

Microsoft.Azure.Cosmos/src/Pagination/FullyBufferedPartitionRangeAsyncEnumerator.cs

@@ -0,0 +1,96 @@
+// ------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+// ------------------------------------------------------------
+namespace Microsoft.Azure.Cosmos.Pagination
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.Azure.Cosmos.Query.Core.Monads;
+    using Microsoft.Azure.Cosmos.Tracing;
+
+    internal sealed class FullyBufferedPartitionRangeAsyncEnumerator<TPage, TState> : BufferedPartitionRangePageAsyncEnumeratorBase<TPage, TState>
+        where TPage : Page<TState>
+        where TState : State
+    {
+        private readonly PartitionRangePageAsyncEnumerator<TPage, TState> enumerator;
+        private readonly List<TPage> bufferedPages;
+        private int currentIndex;
+        private Exception exception;
+
+        private bool HasPrefetched => (this.exception != null) || (this.bufferedPages.Count > 0);
+
+        public FullyBufferedPartitionRangeAsyncEnumerator(PartitionRangePageAsyncEnumerator<TPage, TState> enumerator, CancellationToken cancellationToken)
+            : base(enumerator.FeedRangeState, cancellationToken)
+        {
+            this.enumerator = enumerator ?? throw new ArgumentNullException(nameof(enumerator));
+            this.bufferedPages = new List<TPage>();
+        }
+
+        public override ValueTask DisposeAsync()
+        {
+            return this.enumerator.DisposeAsync();
+        }
+
+        public override async ValueTask PrefetchAsync(ITrace trace, CancellationToken cancellationToken)
+        {
+            if (trace == null)
+            {
+                throw new ArgumentNullException(nameof(trace));
+            }
+
+            if (this.HasPrefetched)
+            {
+                return;
+            }
+
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using (ITrace prefetchTrace = trace.StartChild("Prefetch", TraceComponent.Pagination, TraceLevel.Info))
+            {
+                while (await this.enumerator.MoveNextAsync(prefetchTrace))
+                {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    TryCatch<TPage> current = this.enumerator.Current;
+                    if (current.Succeeded)
+                    {
+                        this.bufferedPages.Add(current.Result);
+                    }
+                    else
+                    {
+                        this.exception = current.Exception;
+                        break;
+                    }
+                }
+            }
+        }
+
+        protected override async Task<TryCatch<TPage>> GetNextPageAsync(ITrace trace, CancellationToken cancellationToken)
+        {
+            TryCatch<TPage> result;
+            if (this.currentIndex < this.bufferedPages.Count)
+            {
+                result = TryCatch<TPage>.FromResult(this.bufferedPages[this.currentIndex]);
+            }
+            else if (this.currentIndex == this.bufferedPages.Count && this.exception != null)
+            {
+                result = TryCatch<TPage>.FromException(this.exception);
+            }
+            else
+            {
+                await this.enumerator.MoveNextAsync(trace);
+                result = this.enumerator.Current;
+            }
+
+            ++this.currentIndex;
+            return result;
+        }
+
+        public override void SetCancellationToken(CancellationToken cancellationToken)
+        {
+            base.SetCancellationToken(cancellationToken);
+            this.enumerator.SetCancellationToken(cancellationToken);
+        }
+    }
+}

Microsoft.Azure.Cosmos/src/Pagination/PrefetchPolicy.cs

@@ -0,0 +1,11 @@
+// ------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+// ------------------------------------------------------------
+namespace Microsoft.Azure.Cosmos.Pagination
+{
+    internal enum PrefetchPolicy
+    {
+        PrefetchSinglePage,
+        PrefetchAll
+    }
+}

Microsoft.Azure.Cosmos/src/Query/Core/Pipeline/CosmosQueryExecutionContextFactory.cs

@@ -379,6 +379,7 @@ private static TryCatch<IQueryPipelineStage> TryCreatePassthroughQueryExecutionC
                 queryPaginationOptions: new QueryPaginationOptions(
                     pageSizeHint: inputParameters.MaxItemCount),
                 partitionKey: inputParameters.PartitionKey,
+                prefetchPolicy: PrefetchPolicy.PrefetchSinglePage,
                 maxConcurrency: inputParameters.MaxConcurrency,
                 cancellationToken: cancellationToken,
                 continuationToken: inputParameters.InitialUserContinuationToken);

Microsoft.Azure.Cosmos/src/Query/Core/Pipeline/CrossPartition/Parallel/ParallelCrossPartitionQueryPipelineStage.cs

@@ -137,6 +137,7 @@ public static TryCatch<IQueryPipelineStage> MonadicCreate(
             Cosmos.PartitionKey? partitionKey,
             QueryPaginationOptions queryPaginationOptions,
             int maxConcurrency,
+            PrefetchPolicy prefetchPolicy,
             CosmosElement continuationToken,
             CancellationToken cancellationToken)
         {
@@ -159,10 +160,11 @@ public static TryCatch<IQueryPipelineStage> MonadicCreate(
             CrossFeedRangeState<QueryState> state = monadicExtractState.Result;
 
             CrossPartitionRangePageAsyncEnumerator<QueryPage, QueryState> crossPartitionPageEnumerator = new CrossPartitionRangePageAsyncEnumerator<QueryPage, QueryState>(
-                documentContainer,
-                ParallelCrossPartitionQueryPipelineStage.MakeCreateFunction(documentContainer, sqlQuerySpec, queryPaginationOptions, partitionKey, cancellationToken),
+                feedRangeProvider: documentContainer,
+                createPartitionRangeEnumerator: ParallelCrossPartitionQueryPipelineStage.MakeCreateFunction(documentContainer, sqlQuerySpec, queryPaginationOptions, partitionKey, cancellationToken),
                 comparer: Comparer.Singleton,
-                maxConcurrency,
+                maxConcurrency: maxConcurrency,
+                prefetchPolicy: prefetchPolicy,
                 state: state,
                 cancellationToken: cancellationToken);