diff --git a/Orm/Xtensive.Orm.Tests.Core/Collections/PriorityQueueTest.cs b/Orm/Xtensive.Orm.Tests.Core/Collections/PriorityQueueTest.cs
deleted file mode 100644
index 6e728f2398..0000000000
--- a/Orm/Xtensive.Orm.Tests.Core/Collections/PriorityQueueTest.cs
+++ /dev/null
@@ -1,100 +0,0 @@
-// Copyright (C) 2003-2010 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
-// Created by: Alex Gamzov
-// Created:    2007.06.15
-
-using System;
-using NUnit.Framework;
-using Xtensive.Collections;
-using Xtensive.Core;
-
-
-namespace Xtensive.Orm.Tests.Core.Collections
-{
-  [TestFixture]
-  public class PriorityQueueTest
-  {
-    [Test]
-    public void FastEnqueeDequeTest()
-    {
-      PriorityQueue<string, string> pq = new PriorityQueue<string, string>();
-      int count = 10000;
-      using (new Measurement("Fast Enqueue test", count)) {
-        for (int i = 0; i < count; i++)
-        {
-          pq.Enqueue(DateTime.Now.ToString(), Guid.NewGuid().ToString());
-        }
-      }
-      Assert.AreEqual(pq.Count, count);
-      using (new Measurement("Fast Dequeue test", count)) {
-        while (pq.Count > 0)
-          pq.Dequeue();
-      }
-    }
-
-    [Test]
-    public void EnqueueDequeTest()
-    {
-      PriorityQueue<string, string> pqAsc = new PriorityQueue<string, string>(Direction.Negative);
-      for (int i = 0; i < 10000; i++)
-      {
-        string data = Guid.NewGuid().ToString();
-        pqAsc.Enqueue(data, data);
-      }
-      for (int i = 0; i < pqAsc.Count - 1; i++)
-      {
-        Assert.Greater(pqAsc[i], pqAsc[i + 1]);
-      }
-
-      PriorityQueue<string, string> pqDesc = new PriorityQueue<string, string>(Direction.Positive);
-      for (int i = 0; i < 10000; i++)
-      {
-        string data = Guid.NewGuid().ToString();
-        pqDesc.Enqueue(data, data);
-      }
-      for (int i = 0; i < pqDesc.Count - 1; i++)
-      {
-        Assert.Less(pqDesc[i], pqDesc[i + 1]);
-      }
-    
-    }
-
-    [Test]
-    public void RemoveTest()
-    {
-      PriorityQueue<string, string> pq = new PriorityQueue<string, string>();
-      string item = "item";
-      Assert.Throws<InvalidOperationException>(() => {
-        pq.Enqueue("1", "s1");
-        pq.Enqueue(item, "s2");
-        pq.Enqueue("3", "s3");
-        pq.Remove("fff");
-      });
-    }
-
-    [Test]
-    public void DequeRangeTest()
-    {
-      PriorityQueue<string, string> pq = new PriorityQueue<string, string>(Direction.Positive);
-      pq.Enqueue("item1.1", "s1");
-      pq.Enqueue("item1.2", "s1");
-      pq.Enqueue("item2.1", "s2");
-      pq.Enqueue("item2.2", "s2");
-      pq.Enqueue("item3", "s3");
-      string[] result = pq.DequeueRange("s2");
-      Assert.AreEqual(5, pq.Count + result.Length);
-      Assert.AreEqual(4, result.Length);
-
-      PriorityQueue<string, string> pqDesc = new PriorityQueue<string, string>(Direction.Negative);
-      pqDesc.Enqueue("item1.1", "s1");
-      pqDesc.Enqueue("item1.2", "s1");
-      pqDesc.Enqueue("item2.1", "s2");
-      pqDesc.Enqueue("item2.2", "s2");
-      pqDesc.Enqueue("item3", "s3");
-      string[] resultDesc = pqDesc.DequeueRange("s2");
-      Assert.AreEqual(5, pqDesc.Count + resultDesc.Length);
-      Assert.AreEqual(3, resultDesc.Length);
-    }
-  }
-}
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Collections/PriorityQueue.cs b/Orm/Xtensive.Orm/Collections/PriorityQueue.cs
deleted file mode 100644
index 6a9a3321d5..0000000000
--- a/Orm/Xtensive.Orm/Collections/PriorityQueue.cs
+++ /dev/null
@@ -1,359 +0,0 @@
-// Copyright (C) 2003-2010 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
-// Created by: Alex Gamzov
-// Created:    2007.06.13
-
-using System;
-using System.Collections.Generic;
-using System.Collections;
-using System.Diagnostics;
-using System.Runtime.Serialization;
-using System.Security;
-using System.Security.Permissions;
-using Xtensive.Comparison;
-using Xtensive.Core;
-
-
-
-namespace Xtensive.Collections
-{
-  /// <summary>
-  /// Priority queue.
-  /// </summary>
-  /// <typeparam name="T"><see cref="Type"/> of objects to be stored in queue.</typeparam>
-  /// <typeparam name="TPriority"><see cref="Type"/> of priority value.</typeparam>
-  [Serializable]
-  [DebuggerDisplay("Count = {Count}")]
-  public class PriorityQueue<T, TPriority> : IPriorityQueue<T, TPriority>, 
-    ISerializable
-    where TPriority: IComparable<TPriority>
-  {
-    // Consts
-    private const int minimalCapacity = 16;
-    private const float defaultGrowFactor = 1.4f;
-    private const float trimThresholdFactor = 0.9f;
-
-    // Fields
-    [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
-    private Pair<T, TPriority>[] items;   // Actual data buffer
-    private int version; // Version to detect consistency while enumeration executes
-    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
-    private int count; // Count of items in queue
-    private readonly float growFactor = defaultGrowFactor;
-    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
-    private readonly Direction direction;
-    private readonly PriorityQueueItemComparer<T, TPriority> comparer;
-
-    #region IPriorityQueue<T,TPriority> Members
-
-    /// <inheritdoc/>
-    public int Capacity
-    {
-      [DebuggerStepThrough]
-      get {
-        return items.Length;
-      }
-      set {
-        if (value != items.Length) {
-          if (value < count)
-            throw new ArgumentOutOfRangeException("value", Strings.ExInvalidCapacity);
-          if (value < minimalCapacity) {
-            value = minimalCapacity;
-          }
-          Pair<T, TPriority>[] newItems = new Pair<T, TPriority>[value];
-          Array.Copy(items, newItems, Count);
-          items = newItems;
-          version++;
-        }
-      }
-    }
-
-    /// <inheritdoc/>
-    public Direction Direction
-    {
-      [DebuggerStepThrough]
-      get { return direction; }
-    }
-
-    /// <inheritdoc/>
-    public long Count
-    {
-      [DebuggerStepThrough]
-      get { return count; }
-    }
-
-    /// <inheritdoc/>
-    public T this[int index]
-    {
-      get {
-        if (index < 0)
-          throw new ArgumentOutOfRangeException(Strings.ExArgumentValueMustBeGreaterThanOrEqualToZero);
-        if (index >= count)
-          throw new ArgumentOutOfRangeException(string.Format(Strings.ExIndexShouldBeInNMRange, 0, count-1));
-        return items[count-index-1].First;
-      }
-    }
-
-    /// <inheritdoc/>
-    public bool Contains(T item)
-    {
-      Predicate<Pair<T, TPriority>> predicate = GetEqualityPredicate(item);
-      return Array.FindIndex(items, 0, count, predicate) >= 0;
-    }
-
-    /// <inheritdoc/>
-    public void Remove(T item)
-    {
-      Predicate<Pair<T, TPriority>> predicate = GetEqualityPredicate(item);
-      int index = Array.FindIndex(items, 0, count, predicate);
-      if (index<0) {
-        throw new InvalidOperationException(Strings.ExItemNotFound);
-      }
-      Array.Copy(items, index+1, items, index, count-index-1);
-      Array.Clear(items, count-1, 1);
-      count--;
-    }
-
-    /// <inheritdoc/>
-    public void TrimExcess()
-    {
-      int trimThreshold = (int)(items.Length * trimThresholdFactor);
-      if (count < trimThreshold)
-        Capacity = count;
-    }
-
-    /// <inheritdoc/>
-    public void Clear()
-    {
-      int capacity = items.Length;
-      Array.Clear(items, 0, capacity);
-      count = 0;
-      version++;
-    }
-
-    /// <inheritdoc/>
-    public void Enqueue(T item, TPriority priority)
-    {
-      int insertIndex = GetIndex(item, priority);
-      EnsureCapacity((int) (Count+1));
-      if (insertIndex < Count)
-      {
-        Array.Copy(items, insertIndex, items, insertIndex + 1, Count - insertIndex);
-      }
-      items[insertIndex] = new Pair<T, TPriority>(item, priority);
-      count++;
-      version++;
-    }
-
-    /// <inheritdoc/>
-    public T Dequeue()
-    {
-      if (Count == 0){
-        throw new InvalidOperationException(Strings.ExCollectionIsEmpty);
-      }
-      T result = items[Count - 1].First;
-      Array.Clear(items, (int) (Count - 1), 1);
-      count--;
-      TrimExcess();
-      version++;
-      return result;
-    }
-
-    /// <inheritdoc/>
-    public T[] DequeueRange(TPriority priority)
-    {
-      int index = GetIndex(default(T), priority);
-      if (index < count){
-        T[] result = new T[count - index];
-        for (int i = index; i < count;i++ ) {
-          result[i - index] = items[i].First;
-        }
-        Array.Clear(items, index, count-index);
-        count = index;
-        return result;
-      }
-      else {
-        return new T[0];
-      }
-
-    }
-
-    /// <inheritdoc/>
-    public T Peek()
-    {
-      if (Count==0) {
-        throw new InvalidOperationException(Strings.ExCollectionIsEmpty);
-      }
-      version++;
-      return items[Count - 1].First;
-    }
-
-    #endregion
-
-    #region IEnumerable<...> Members
-
-    /// <inheritdoc/>
-    [DebuggerStepThrough]
-    IEnumerator IEnumerable.GetEnumerator()
-    {
-      return GetEnumerator();
-    }
-
-    /// <inheritdoc/>
-    public IEnumerator<T> GetEnumerator()
-    {
-      int initialVersion = version;
-      for (int i = 0; i < Count;i++ ) {
-        if (version != initialVersion)
-          throw new InvalidOperationException(Strings.ExCollectionHasBeenChanged);
-        yield return items[i].First;
-      }
-    }
-
-    #endregion
-
-    #region ICloneable Members
-
-    /// <inheritdoc/>
-    public object Clone()
-    {
-      return new PriorityQueue<T, TPriority>(this);
-    }
-
-    #endregion
-
-    #region Private \ internal members
-
-    private int GetIndex(T item, TPriority priority)
-    {
-      Pair<T,TPriority> element = new Pair<T, TPriority>(item, priority);
-      int insertIndex = Array.BinarySearch(items, 0, (int)Count, element, comparer);
-      if (insertIndex < 0) {
-        insertIndex ^= -1;
-      }
-      while (insertIndex < Count && insertIndex > 0 && comparer.Compare(items[insertIndex-1], element) == 0)
-      {
-        insertIndex--;
-      }
-      return insertIndex;
-    }
-
-    private static Predicate<Pair<T, TPriority>> GetEqualityPredicate(T item)
-    {
-      return delegate(Pair<T, TPriority> innerItem)
-      {
-        return AdvancedComparerStruct<T>.System.Equals(innerItem.First, item);
-      };
-    }
-
-    private void EnsureCapacity(int requiredCapacity)
-    {
-      int currentCapacity = items.Length;
-      if (currentCapacity < requiredCapacity)
-      {
-        int newCapacity = currentCapacity == 0
-          ? minimalCapacity
-          : Convert.ToInt32(currentCapacity * growFactor);
-        if (newCapacity < requiredCapacity)
-          newCapacity = requiredCapacity;
-        Capacity = newCapacity;
-      }
-    }
-
-    #endregion
-
-
-    // Constructors
-
-    private PriorityQueue(PriorityQueue<T, TPriority> source)
-    {
-      items = new Pair<T, TPriority>[source.Count];
-      Array.Copy(source.items, items, source.Count);
-      count = items.Length;
-      direction = source.direction;
-    }
-
-    /// <summary>
-    /// Initializes new instance of this type.
-    /// </summary>
-    public PriorityQueue()
-      : this(Direction.Positive)
-    {
-    }
-
-    /// <summary>
-    /// Initializes new instance of this type.
-    /// </summary>
-    /// <param name="direction">Initial <see cref="Direction"/> property value.</param>
-    public PriorityQueue(Direction direction)
-      : this(direction, minimalCapacity)
-    {
-    }
-
-
-    /// <summary>
-    /// Initializes new instance of this type.
-    /// </summary>
-    /// <param name="direction">Initial <see cref="Direction"/> property value.</param>
-    /// <param name="initialCapacity">Initial <see cref="Capacity"/> property value.</param>
-    public PriorityQueue(Direction direction, int initialCapacity) 
-      : this(direction, initialCapacity, defaultGrowFactor)
-    {
-    }
- 
-    /// <summary>
-    /// Initializes new instance of this type.
-    /// </summary>
-    /// <param name="direction">Initial <see cref="Direction"/> property value.</param>
-    /// <param name="initialCapacity">Initial <see cref="Capacity"/> property value.</param>
-    /// <param name="growFactor">Capacity grow factor.</param>
-    public PriorityQueue(Direction direction, int initialCapacity, float growFactor)
-    {
-      if (direction==Direction.None)
-        throw Exceptions.InvalidArgument(direction, "direction");
-      this.direction = direction;
-      if (initialCapacity < minimalCapacity) {
-        initialCapacity = minimalCapacity;
-      }
-      items = new Pair<T, TPriority>[initialCapacity]; 
-      count = 0;
-      this.growFactor = growFactor;
-      comparer = new PriorityQueueItemComparer<T, TPriority>(direction);
-    }
-
-    #region ISerializable Members
-
-    /// <summary>
-    /// Serializes instance of this type.
-    /// </summary>
-    /// <param name="info"></param>
-    /// <param name="context"></param>
-    [SecurityCritical]
-    public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
-    {
-      Pair<T, TPriority>[] arrItems = new Pair<T, TPriority>[count];
-      Array.Copy(items, arrItems, count);
-      info.AddValue("Items", arrItems);
-      info.AddValue("GrowFactor", growFactor);
-      info.AddValue("Direction", direction);
-    }
-
-    /// <summary>
-    /// Deserializes instance of this type.
-    /// </summary>
-    /// <param name="info"></param>
-    /// <param name="context"></param>
-    protected PriorityQueue(SerializationInfo info, StreamingContext context)
-    {
-      items = (Pair<T, TPriority>[])info.GetValue("Items", typeof(Pair<T, TPriority>[]));
-      growFactor = info.GetSingle("GrowFactor");
-      direction = (Direction)info.GetValue("Direction", typeof(Direction));
-      if (direction==Direction.None)
-        throw Exceptions.InvalidArgument(direction, "direction");
-      count = items.Length;
-    }
-
-    #endregion
-  }
-}
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Orm/Building/Builders/AttributeProcessor.cs b/Orm/Xtensive.Orm/Orm/Building/Builders/AttributeProcessor.cs
index c6a24c455e..645331f053 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Builders/AttributeProcessor.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Builders/AttributeProcessor.cs
@@ -471,7 +471,7 @@ private LambdaExpression GetExpressionFromProvider(Type providerType, string pro
           string.Format(Strings.ExMemberXShouldReturnValueThatIsAssignableToLambdaExpression, memberName));
       }
 
-      var expression = (LambdaExpression) method.Invoke(null, new object[0]);
+      var expression = (LambdaExpression) method.Invoke(null, Array.Empty<object>());
       if (expression.Parameters.Count != 1 || !expression.Parameters[0].Type.IsAssignableFrom(parameterType)) {
         throw new DomainBuilderException(string.Format(
           Strings.ExLambdaExpressionReturnedByXShouldTakeOneParameterOfTypeYOrAnyBaseTypeOfIt, memberName,
diff --git a/Orm/Xtensive.Orm/Orm/Linq/ExpressionExtensions.cs b/Orm/Xtensive.Orm/Orm/Linq/ExpressionExtensions.cs
index 9da5e7637d..96179b1fba 100644
--- a/Orm/Xtensive.Orm/Orm/Linq/ExpressionExtensions.cs
+++ b/Orm/Xtensive.Orm/Orm/Linq/ExpressionExtensions.cs
@@ -220,7 +220,7 @@ public static ParameterInfo[] GetConstructorParameters(this NewExpression expres
       // ReSharper disable ConditionIsAlwaysTrueOrFalse
       // ReSharper disable HeuristicUnreachableCode
       if (expression.Constructor == null) {
-        return new ParameterInfo[0];
+        return Array.Empty<ParameterInfo>();
       }
       // ReSharper restore ConditionIsAlwaysTrueOrFalse
       // ReSharper restore HeuristicUnreachableCode
diff --git a/Orm/Xtensive.Orm/Orm/Linq/Translator.Expressions.cs b/Orm/Xtensive.Orm/Orm/Linq/Translator.Expressions.cs
index 1fb83fa002..abe3097ac4 100644
--- a/Orm/Xtensive.Orm/Orm/Linq/Translator.Expressions.cs
+++ b/Orm/Xtensive.Orm/Orm/Linq/Translator.Expressions.cs
@@ -406,9 +406,12 @@ protected override Expression VisitMethodCall(MethodCallExpression mc)
           return Visit(customCompiler.Invoke(mc.Object, mc.Arguments.ToArray()));
         }
 
+        var methodDeclaringType = method.DeclaringType;
+        var methodName = method.Name;
+
         // Visit Query. Deprecated.
 #pragma warning disable 612,618
-        if (method.DeclaringType == WellKnownOrmTypes.Query) {
+        if (methodDeclaringType == WellKnownOrmTypes.Query) {
           // Query.All<T>
           if (method.IsGenericMethodSpecificationOf(WellKnownMembers.Query.All)) {
             return ConstructQueryable(mc);
@@ -439,7 +442,7 @@ protected override Expression VisitMethodCall(MethodCallExpression mc)
           throw new InvalidOperationException(String.Format(Strings.ExMethodCallExpressionXIsNotSupported, mc.ToString(true)));
         }
         // Visit QueryEndpoint.
-        if (method.DeclaringType == typeof(QueryEndpoint)) {
+        if (methodDeclaringType == typeof(QueryEndpoint)) {
           // Query.All<T>
           if (method.IsGenericMethodSpecificationOf(WellKnownMembers.QueryEndpoint.All)) {
             return ConstructQueryable(mc);
@@ -476,35 +479,32 @@ protected override Expression VisitMethodCall(MethodCallExpression mc)
 #pragma warning restore 612,618
 
         // Visit Queryable extensions.
-        if (method.DeclaringType==typeof (QueryableExtensions))
-          if (method.Name==WellKnownMembers.Queryable.ExtensionLeftJoin.Name)
-            return VisitLeftJoin(mc);
-          else if (method.Name=="In")
-            return VisitIn(mc);
-          else if (method.Name==WellKnownMembers.Queryable.ExtensionLock.Name)
-            return VisitLock(mc);
-          else if (method.Name==WellKnownMembers.Queryable.ExtensionTake.Name)
-            return VisitTake(mc.Arguments[0], mc.Arguments[1]);
-          else if (method.Name==WellKnownMembers.Queryable.ExtensionSkip.Name)
-            return VisitSkip(mc.Arguments[0], mc.Arguments[1]);
-          else if (method.Name==WellKnownMembers.Queryable.ExtensionElementAt.Name)
-            return VisitElementAt(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.ReturnType, false);
-          else if (method.Name==WellKnownMembers.Queryable.ExtensionElementAtOrDefault.Name)
-            return VisitElementAt(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.ReturnType, true);
-          else if (method.Name == WellKnownMembers.Queryable.ExtensionCount.Name)
-            return VisitAggregate(mc.Arguments[0], method, null, context.IsRoot(mc), mc);
-          else if (method.Name == WellKnownMembers.Queryable.ExtensionTag.Name)
-            return VisitTag(mc);
-          else
-            throw new InvalidOperationException(String.Format(Strings.ExMethodCallExpressionXIsNotSupported, mc.ToString(true)));
+        if (methodDeclaringType == typeof(QueryableExtensions)) {
+          return methodName switch {
+            nameof(QueryableExtensions.LeftJoin) => VisitLeftJoin(mc),
+            "In" => VisitIn(mc),
+            nameof(QueryableExtensions.Lock) => VisitLock(mc),
+            nameof(QueryableExtensions.Take) => VisitTake(mc.Arguments[0], mc.Arguments[1]),
+            nameof(QueryableExtensions.Skip) => VisitSkip(mc.Arguments[0], mc.Arguments[1]),
+            nameof(QueryableExtensions.ElementAt) => VisitElementAt(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.ReturnType, false),
+            nameof(QueryableExtensions.ElementAtOrDefault) => VisitElementAt(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.ReturnType, true),
+            nameof(QueryableExtensions.Count) => VisitAggregate(mc.Arguments[0], method, null, context.IsRoot(mc), mc),
+            nameof(QueryableExtensions.Tag) => VisitTag(mc),
+            _ => throw new InvalidOperationException(String.Format(Strings.ExMethodCallExpressionXIsNotSupported, mc.ToString(true)))
+          };
+        }
         // Visit Collection extensions
-        if (method.DeclaringType==typeof(CollectionExtensionsEx))
-          if (method.Name==WellKnownMembers.Collection.ExtensionContainsAny.Name)
-            return VisitContainsAny(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
-          else if (method.Name==WellKnownMembers.Collection.ExtensionContainsAll.Name)
-            return VisitContainsAll(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
-          else if (method.Name==WellKnownMembers.Collection.ExtensionContainsNone.Name)
-            return VisitContainsNone(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
+        if (methodDeclaringType == typeof(CollectionExtensionsEx)) {
+          switch (methodName) {
+            case nameof(CollectionExtensionsEx.ContainsAny):
+              return VisitContainsAny(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
+            case nameof(CollectionExtensionsEx.ContainsAll):
+              return VisitContainsAll(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
+            case nameof(CollectionExtensionsEx.ContainsNone):
+              return VisitContainsNone(mc.Arguments[0], mc.Arguments[1], context.IsRoot(mc), method.GetGenericArguments()[0]);
+          }
+        }
+
 
         // Process local collections
         if (mc.Object.IsLocalCollection(context)) {
@@ -512,12 +512,12 @@ protected override Expression VisitMethodCall(MethodCallExpression mc)
           // List.Contains
           // Array.Contains
           ParameterInfo[] parameters = method.GetParameters();
-          if (method.Name=="Contains" && parameters.Length==1)
+          if (methodName=="Contains" && parameters.Length==1)
             return VisitContains(mc.Object, mc.Arguments[0], false);
         }
 
         var result = base.VisitMethodCall(mc);
-        if (result!=mc && result.NodeType==ExpressionType.Call) {
+        if (result != mc && result.NodeType == ExpressionType.Call) {
           var visitedMethodCall = (MethodCallExpression) result;
           if (visitedMethodCall.Arguments.Any(arg => arg.IsProjection()))
             throw new InvalidOperationException(String.Format(Strings.ExMethodCallExpressionXIsNotSupported, mc.ToString(true)));
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlProvider.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlProvider.cs
index 73e7bf0e61..227990d31b 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlProvider.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlProvider.cs
@@ -102,7 +102,7 @@ public SqlProvider(HandlerAccessor handlers, QueryRequest request,
     /// <param name="provider">The provider.</param>
     /// <param name="permanentReference">The permanent reference.</param>
     public SqlProvider(SqlProvider provider, SqlTable permanentReference)
-      : base(provider.Origin, provider.Sources.Cast<ExecutableProvider>().ToArray())
+      : base(provider.Origin, provider.Sources.Cast<ExecutableProvider>().ToArray(provider.Sources.Count))
     {
       this.permanentReference = permanentReference;
       handlers = provider.handlers;
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlProviderPreparer.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlProviderPreparer.cs
index b62fcf0997..8504615137 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlProviderPreparer.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlProviderPreparer.cs
@@ -1,9 +1,10 @@
-// Copyright (C) 2012 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
+// Copyright (C) 2012-2022 Xtensive LLC.
+// This code is distributed under MIT license terms.
+// See the License.txt file in the project root for more information.
 // Created by: Denis Krjuchkov
 // Created:    2012.01.29
 
+using System;
 using Xtensive.Core;
 using Xtensive.Orm.Rse;
 using Xtensive.Orm.Rse.Compilation;
@@ -25,7 +26,7 @@ public ExecutableProvider Process(ExecutableProvider rootProvider)
       request.Prepare();
       // Optional part - remove all underlying providers to save memory.
       return request.CheckOptions(QueryRequestOptions.AllowOptimization)
-        ? new SqlProvider(handlers, request, sqlProvider.Origin.MakeVoid(), new ExecutableProvider[0])
+        ? new SqlProvider(handlers, request, sqlProvider.Origin.MakeVoid(), Array.Empty<ExecutableProvider>())
         : rootProvider;
     }
 
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlSelectProcessor.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlSelectProcessor.cs
index 2f85f051ad..9266a08475 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlSelectProcessor.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlSelectProcessor.cs
@@ -15,8 +15,7 @@ internal class SqlSelectProcessor : ISqlVisitor
 
     public void Visit(SqlAggregate node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
+      VisitNullable(node.Expression);
     }
 
     public void Visit(SqlAlterDomain node)
@@ -47,8 +46,7 @@ public void Visit(SqlAssignment node)
     {
       if (node.Left!=null)
         Visit(node.Left);
-      if (node.Right is not null)
-        Visit(node.Right);
+      VisitNullable(node.Right);
     }
 
     public void Visit(SqlBatch node)
@@ -57,20 +55,15 @@ public void Visit(SqlBatch node)
 
     public void Visit(SqlBetween node)
     {
-      if (node.Left is not null)
-        Visit(node.Left);
-      if (node.Right is not null)
-        Visit(node.Right);
-      if (!node.Expression)
-        Visit(node.Expression);
+      VisitNullable(node.Left);
+      VisitNullable(node.Right);
+      VisitNullable(node.Expression);
     }
 
     public void Visit(SqlBinary node)
     {
-      if (node.Left is not null)
-        Visit(node.Left);
-      if (node.Right is not null)
-        Visit(node.Right);
+      VisitNullable(node.Left);
+      VisitNullable(node.Right);
     }
 
     public void Visit(SqlBreak node)
@@ -79,16 +72,13 @@ public void Visit(SqlBreak node)
 
     public void Visit(SqlCase node)
     {
-      if (node.Value is not null)
-        Visit(node.Value);
-      if (node.Else is not null)
-        Visit(node.Else);
+      VisitNullable(node.Value);
+      VisitNullable(node.Else);
     }
 
     public void Visit(SqlCast node)
     {
-      if (node.Operand is not null)
-        Visit(node.Operand);
+      VisitNullable(node.Operand);
     }
 
     public void Visit(SqlCloseCursor node)
@@ -97,14 +87,12 @@ public void Visit(SqlCloseCursor node)
 
     public void Visit(SqlCollate node)
     {
-      if (node.Operand is not null)
-        Visit(node.Operand);
+      VisitNullable(node.Operand);
     }
 
     public void Visit(SqlColumnRef node)
     {
-      if (node.SqlColumn is not null)
-        Visit(node.SqlColumn);
+      VisitNullable(node.SqlColumn);
     }
 
     public void Visit(SqlConcat node)
@@ -197,8 +185,7 @@ public void Visit(SqlDelete node)
     {
       if (node.Delete!=null)
         Visit(node.Delete);
-      if (node.Where is not null)
-        Visit(node.Where);
+      VisitNullable(node.Where);
     }
 
     public void Visit(SqlDropAssertion node)
@@ -259,8 +246,7 @@ public void Visit(SqlPlaceholder node)
 
     public void Visit(SqlExtract node)
     {
-      if (node.Operand is not null)
-        Visit(node.Operand);
+      VisitNullable(node.Operand);
     }
 
     public void Visit(SqlFastFirstRowsHint node)
@@ -303,8 +289,7 @@ public void Visit(SqlIf node)
         Visit(node.True);
       if (node.False!=null)
         Visit(node.False);
-      if (node.Condition is not null)
-        Visit(node.Condition);
+      VisitNullable(node.Condition);
     }
 
     public void Visit(SqlInsert node)
@@ -319,11 +304,10 @@ public void Visit(SqlInsert node)
 
     public void Visit(SqlJoinExpression node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
-      if (node.Left!=null)
+      VisitNullable(node.Expression);
+      if (node.Left != null)
         Visit(node.Left);
-      if (node.Right!=null)
+      if (node.Right != null)
         Visit(node.Right);
     }
 
@@ -333,12 +317,9 @@ public void Visit(SqlJoinHint node)
 
     public void Visit(SqlLike node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
-      if (node.Escape is not null)
-        Visit(node.Escape);
-      if (node.Pattern is not null)
-        Visit(node.Pattern);
+      VisitNullable(node.Expression);
+      VisitNullable(node.Escape);
+      VisitNullable(node.Pattern);
     }
 
     public void Visit(SqlLiteral node)
@@ -347,8 +328,7 @@ public void Visit(SqlLiteral node)
 
     public void Visit(SqlMatch node)
     {
-      if (node.Value is not null)
-        Visit(node.Value);
+      VisitNullable(node.Value);
       if (node.SubQuery is not null)
         Visit(node.SubQuery);
     }
@@ -375,8 +355,7 @@ public void Visit(SqlOpenCursor node)
 
     public void Visit(SqlOrder node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
+      VisitNullable(node.Expression);
     }
 
     public void Visit(SqlParameterRef node)
@@ -385,10 +364,8 @@ public void Visit(SqlParameterRef node)
 
     public void Visit(SqlRound node)
     {
-      if (node.Argument is not null)
-        Visit(node.Argument);
-      if (node.Length is not null)
-        Visit(node.Length);
+      VisitNullable(node.Argument);
+      VisitNullable(node.Length);
     }
 
     public void Visit(SqlQueryExpression node)
@@ -439,8 +416,7 @@ public void Visit(SqlTableRef node)
 
     public void Visit(SqlTrim node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
+      VisitNullable(node.Expression);
     }
 
     public void Visit(SqlSelect node)
@@ -451,16 +427,12 @@ public void Visit(SqlSelect node)
         Visit(column);
       foreach (var column in node.OrderBy)
         Visit(column);
-      if (node.From!=null)
+      if (node.From != null)
         Visit(node.From);
-      if (node.Having is not null)
-        Visit(node.Having);
-      if (node.Limit is not null)
-        Visit(node.Limit);
-      if (node.Offset is not null)
-        Visit(node.Offset);
-      if (node.Where is not null)
-        Visit(node.Where);
+      VisitNullable(node.Having);
+      VisitNullable(node.Limit);
+      VisitNullable(node.Offset);
+      VisitNullable(node.Where);
       foreach (var hint in node.Hints)
         Visit(hint);
 
@@ -478,7 +450,7 @@ public void Visit(SqlSelect node)
         rootSelect.Comment = SqlComment.Join(rootSelect.Comment, node.Comment);
         node.Comment = null;
       }
-      
+
       var addOrderBy = hasPaging
         && node.OrderBy.Count==0
         && providerInfo.Supports(ProviderFeatures.PagingRequiresOrderBy);
@@ -495,8 +467,7 @@ public void Visit(SqlSubQuery node)
 
     public void Visit(SqlUnary node)
     {
-      if (node.Operand is not null)
-        Visit(node.Operand);
+      VisitNullable(node.Operand);
     }
 
     public void Visit(SqlMetadata node)
@@ -510,8 +481,7 @@ public void Visit(SqlUpdate node)
         Visit(node.From);
       if (node.Update!=null)
         Visit(node.Update);
-      if (node.Where is not null)
-        Visit(node.Where);
+      VisitNullable(node.Where);
       foreach (var value in node.Values.Values)
         Visit(value);
       foreach (var hint in node.Hints)
@@ -520,8 +490,7 @@ public void Visit(SqlUpdate node)
 
     public void Visit(SqlUserColumn node)
     {
-      if (node.Expression is not null)
-        Visit(node.Expression);
+      VisitNullable(node.Expression);
     }
 
     public void Visit(SqlUserFunctionCall node)
@@ -544,8 +513,7 @@ public void Visit(SqlVariant node)
 
     public void Visit(SqlWhile node)
     {
-      if (node.Condition is not null)
-        Visit(node.Condition);
+      VisitNullable(node.Condition);
       if (node.Statement!=null)
         Visit(node.Statement);
     }
@@ -572,6 +540,13 @@ public void Visit(SqlTable sqlTable)
       sqlTable.AcceptVisitor(this);
     }
 
+    private void VisitNullable(SqlExpression sqlExpression)
+    {
+      if (sqlExpression is not null) {
+        Visit(sqlExpression);
+      }
+    }
+
     private void Visit(ISqlQueryExpression queryExpression)
     {
       queryExpression.AcceptVisitor(this);
@@ -592,9 +567,9 @@ private void Visit(SqlHint sqlExpression)
 
     public void Visit(SqlComment comment)
     {
-      
+
     }
-    
+
     public static void Process(SqlSelect select, ProviderInfo providerInfo)
     {
       ArgumentValidator.EnsureArgumentNotNull(select, "select");
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Providers/CompilableProvider.cs b/Orm/Xtensive.Orm/Orm/Rse/Providers/CompilableProvider.cs
index 7e45c17828..5d631a298e 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Providers/CompilableProvider.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Providers/CompilableProvider.cs
@@ -1,10 +1,11 @@
-// Copyright (C) 2003-2010 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
+// Copyright (C) 2003-2023 Xtensive LLC.
+// This code is distributed under MIT license terms.
+// See the License.txt file in the project root for more information.
 // Created by: Alexey Kochetov
 // Created:    2008.07.03
 
 using System;
+using System.Collections.Generic;
 
 namespace Xtensive.Orm.Rse.Providers
 {
@@ -22,5 +23,10 @@ protected CompilableProvider(ProviderType type, params Provider[] sources)
       : base(type, sources)
     {
     }
+
+    protected CompilableProvider(ProviderType type)
+      : this(type, Array.Empty<Provider>())
+    {
+    }
   }
-}
\ No newline at end of file
+}
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Providers/ExecutableProvider.cs b/Orm/Xtensive.Orm/Orm/Rse/Providers/ExecutableProvider.cs
index 44dab7161b..3fdfd41e48 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Providers/ExecutableProvider.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Providers/ExecutableProvider.cs
@@ -173,7 +173,7 @@ public async Task<RecordSetReader> GetRecordSetReaderAsync(
     /// </summary>
     /// <param name="origin">The <see cref="Origin"/> property value.</param>
     /// <param name="sources">The <see cref="Provider.Sources"/> property value.</param>
-    protected ExecutableProvider(CompilableProvider origin, params ExecutableProvider[] sources)
+    protected ExecutableProvider(CompilableProvider origin, IReadOnlyList<ExecutableProvider> sources)
       : base(origin.Type, sources)
     {
       Origin = origin;
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Providers/Provider.cs b/Orm/Xtensive.Orm/Orm/Rse/Providers/Provider.cs
index 9d4aa1db99..b402ada76f 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Providers/Provider.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Providers/Provider.cs
@@ -1,10 +1,11 @@
-// Copyright (C) 2008-2020 Xtensive LLC.
+// Copyright (C) 2008-2022 Xtensive LLC.
 // This code is distributed under MIT license terms.
 // See the License.txt file in the project root for more information.
 // Created by: Alexey Kochetov
 // Created:    2008.07.03
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Text;
 using Xtensive.Core;
@@ -23,28 +24,18 @@ public abstract class Provider
     private const string ToString_Parameters = " ({0})";
     private const int    ToString_IndentSize = 2;
 
-    private Provider[] sources;
     private RecordSetHeader header;
 
     /// <summary>
     /// Gets <see cref="ProviderType"/> of the current instance.
     /// </summary>
-    public ProviderType Type { get; private set; }
+    public ProviderType Type { get; }
 
     /// <summary>
     /// Gets or sets the source providers 
     /// "consumed" by this provider to produce results of current provider.
     /// </summary>
-    public Provider[] Sources {
-      [DebuggerStepThrough]
-      get { return sources; }
-      [DebuggerStepThrough]
-      private set {
-        if (sources!=null)
-          throw Exceptions.AlreadyInitialized("Sources");
-        sources = value;
-      }
-    }
+    public IReadOnlyList<Provider> Sources { [DebuggerStepThrough] get; }
 
     /// <summary>
     /// Gets or sets the header of the record sequence this provide produces.
@@ -136,7 +127,7 @@ protected virtual string ParametersToString()
     /// </summary>
     /// <param name="type">The type of the provider.</param>
     /// <param name="sources"><see cref="Sources"/> property value.</param>
-    protected Provider(ProviderType type, params Provider[] sources)
+    protected Provider(ProviderType type, IReadOnlyList<Provider> sources)
     {
       Type = type;
       Sources = sources;