diff --git a/Orm/Xtensive.Orm/Orm/Providers/Expressions/ExpressionProcessor.cs b/Orm/Xtensive.Orm/Orm/Providers/Expressions/ExpressionProcessor.cs
index ae10b97d18..a003318926 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/Expressions/ExpressionProcessor.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/Expressions/ExpressionProcessor.cs
@@ -28,13 +28,13 @@ internal sealed partial class ExpressionProcessor : ExpressionVisitor<SqlExpress
     private readonly StorageDriver driver;
     private readonly BooleanExpressionConverter booleanExpressionConverter;
     private readonly IMemberCompilerProvider<SqlExpression> memberCompilerProvider;
-    private readonly List<SqlExpression>[] sourceColumns;
+    private readonly IReadOnlyList<SqlExpression>[] sourceColumns;
     private readonly ExpressionEvaluator evaluator;
     private readonly ParameterExtractor parameterExtractor;
     private readonly LambdaExpression lambda;
     private readonly HashSet<QueryParameterBinding> bindings;
     private readonly List<ParameterExpression> activeParameters;
-    private readonly Dictionary<ParameterExpression, List<SqlExpression>> sourceMapping;
+    private readonly Dictionary<ParameterExpression, IReadOnlyList<SqlExpression>> sourceMapping;
     private readonly SqlCompiler compiler;
 
     private readonly Dictionary<QueryParameterIdentity, QueryParameterBinding> bindingsWithIdentity
@@ -448,7 +448,7 @@ private SqlExpression TryUnwrapEnum(SqlContainer container)
     // Constructors
 
     public ExpressionProcessor(
-      LambdaExpression lambda, HandlerAccessor handlers, SqlCompiler compiler, params List<SqlExpression>[] sourceColumns)
+      LambdaExpression lambda, HandlerAccessor handlers, SqlCompiler compiler, params IReadOnlyList<SqlExpression>[] sourceColumns)
     {
       ArgumentValidator.EnsureArgumentNotNull(lambda, "lambda");
       ArgumentValidator.EnsureArgumentNotNull(handlers, "handlers");
@@ -478,7 +478,7 @@ public ExpressionProcessor(
         throw Exceptions.InternalError(Strings.ExParametersCountIsNotSameAsSourceColumnListsCount, OrmLog.Instance);
       if (sourceColumns.Any(list => list.Any(c => c.IsNullReference())))
         throw Exceptions.InternalError(Strings.ExSourceColumnListContainsNullValues, OrmLog.Instance);
-      sourceMapping = new Dictionary<ParameterExpression, List<SqlExpression>>();
+      sourceMapping = new Dictionary<ParameterExpression, IReadOnlyList<SqlExpression>>();
     }
   }
 }
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Apply.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Apply.cs
index 5405a0e1f0..ccaff33c69 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Apply.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Apply.cs
@@ -92,10 +92,11 @@ private SqlSelect ProcessApplyViaSubqueries(ApplyProvider provider, SqlProvider
       if (shouldUseQueryReference) {
         var leftTable = left.PermanentReference;
         query = SqlDml.Select(leftTable);
-        query.Columns.AddRange(leftTable.Columns.Cast<SqlColumn>());
+        query.Columns.AddRange(leftTable.Columns);
       }
-      else
+      else {
         query = left.Request.Statement.ShallowClone();
+      }
 
       var isApplyExistence =
         provider.Right.Type==ProviderType.Existence ||
@@ -166,7 +167,7 @@ private SqlSelect ProcessApplyViaCrossApply(ApplyProvider provider, SqlProvider
         ? left.PermanentReference
         : left.Request.Statement.From;
       var leftColumns = leftShouldUseReference
-        ? leftTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) leftTable.Columns
         : left.Request.Statement.Columns;
 
       var rightShouldUseReference = ShouldUseQueryReference(provider, right);
@@ -174,7 +175,7 @@ private SqlSelect ProcessApplyViaCrossApply(ApplyProvider provider, SqlProvider
         ? right.PermanentReference
         : right.Request.Statement.From;
       var rightColumns = rightShouldUseReference
-        ? rightTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) rightTable.Columns
         : right.Request.Statement.Columns;
 
       var joinType = provider.ApplyType==JoinType.LeftOuter
@@ -185,8 +186,8 @@ private SqlSelect ProcessApplyViaCrossApply(ApplyProvider provider, SqlProvider
         joinType,
         leftTable,
         rightTable,
-        leftColumns.ToList(),
-        rightColumns.ToList());
+        leftColumns,
+        rightColumns);
 
       var query = SqlDml.Select(joinedTable);
       if (!leftShouldUseReference)
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Helpers.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Helpers.cs
index 8517876a2d..acac4f16b3 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Helpers.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Helpers.cs
@@ -67,7 +67,7 @@ protected virtual string ProcessAliasedName(string name)
     }
 
     protected Pair<SqlExpression, IEnumerable<QueryParameterBinding>> ProcessExpression(LambdaExpression le,
-      params List<SqlExpression>[] sourceColumns)
+      params IReadOnlyList<SqlExpression>[] sourceColumns)
     {
       var processor = new ExpressionProcessor(le, Handlers, this, sourceColumns);
       var result = new Pair<SqlExpression, IEnumerable<QueryParameterBinding>>(
@@ -81,7 +81,7 @@ protected static SqlSelect ExtractSqlSelect(CompilableProvider origin, SqlProvid
       if (ShouldUseQueryReference(origin, compiledSource)) {
         var queryRef = compiledSource.PermanentReference;
         var query = SqlDml.Select(queryRef);
-        query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+        query.Columns.AddRange(queryRef.Columns);
         return query;
       }
       return sourceSelect.ShallowClone();
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Index.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Index.cs
index 3931d89cc5..87ed0f2282 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Index.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Index.cs
@@ -94,7 +94,7 @@ private SqlSelect BuildUnionQuery(IndexInfo index)
 
       var unionRef = SqlDml.QueryRef(result);
       var query = SqlDml.Select(unionRef);
-      query.Columns.AddRange(unionRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(unionRef.Columns);
       return query;
     }
 
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Paging.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Paging.cs
index b8ecaa6316..96263dcaa8 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Paging.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Paging.cs
@@ -88,7 +88,7 @@ private SqlProvider VisitSkipRowNumber(SkipProvider provider)
       var source = compiledSource.Request.Statement;
       var queryRef = SqlDml.QueryRef(source);
       var query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
       query.Where = queryRef.Columns.Last() > skipParameterBinding.ParameterReference;
       return CreateProvider(query, bindings, provider, compiledSource);
     }
@@ -101,7 +101,7 @@ private SqlProvider VisitTakeRowNumber(TakeProvider provider)
       var source = compiledSource.Request.Statement;
       var queryRef = SqlDml.QueryRef(source);
       var query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
       query.Where = queryRef.Columns.Last() <= takeParameterBinding.ParameterReference;
       return CreateProvider(query, bindings, provider, compiledSource);
     }
@@ -116,7 +116,7 @@ private SqlProvider VisitPagingRowNumber(PagingProvider provider)
       var queryRef = SqlDml.QueryRef(source);
       var query = SqlDml.Select(queryRef);
       var rowNumberColumn = queryRef.Columns.Last();
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
       query.Where = SqlDml.Between(rowNumberColumn,
         fromParameterBinding.ParameterReference,
         toParameterBinding.ParameterReference);
diff --git a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.cs b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.cs
index 7fd4abeb09..5ab7e64028 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.cs
@@ -129,10 +129,12 @@ protected override SqlProvider VisitDistinct(DistinctProvider provider)
       if (!sourceSelect.Limit.IsNullReference() || !sourceSelect.Offset.IsNullReference()) {
         var queryRef = SqlDml.QueryRef(sourceSelect);
         query = SqlDml.Select(queryRef);
-        query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+        query.Columns.AddRange(queryRef.Columns);
       }
-      else
+      else {
         query = sourceSelect.ShallowClone();
+      }
+
       query.Distinct = true;
       return CreateProvider(query, provider, source);
     }
@@ -173,7 +175,7 @@ protected override SqlProvider VisitJoin(JoinProvider provider)
         ? left.PermanentReference
         : left.Request.Statement.From;
       var leftColumns = leftShouldUseReference
-        ? leftTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) leftTable.Columns
         : left.Request.Statement.Columns;
       var leftExpressions = leftShouldUseReference
         ? leftTable.Columns.Cast<SqlExpression>().ToList()
@@ -184,7 +186,7 @@ protected override SqlProvider VisitJoin(JoinProvider provider)
         ? right.PermanentReference
         : right.Request.Statement.From;
       var rightColumns = rightShouldUseReference
-        ? rightTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) rightTable.Columns
         : right.Request.Statement.Columns;
       var rightExpressions = rightShouldUseReference
         ? rightTable.Columns.Cast<SqlExpression>().ToList()
@@ -205,8 +207,8 @@ protected override SqlProvider VisitJoin(JoinProvider provider)
         joinType,
         leftTable,
         rightTable,
-        leftColumns.ToList(),
-        rightColumns.ToList(),
+        leftColumns,
+        rightColumns,
         joinExpression);
 
       var query = SqlDml.Select(joinedTable);
@@ -229,10 +231,10 @@ protected override SqlProvider VisitPredicateJoin(PredicateJoinProvider provider
         ? left.PermanentReference
         : left.Request.Statement.From;
       var leftColumns = leftShouldUseReference
-        ? leftTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) leftTable.Columns
         : left.Request.Statement.Columns;
       var leftExpressions = leftShouldUseReference
-        ? leftTable.Columns.Cast<SqlExpression>().ToList()
+        ? (IReadOnlyList<SqlExpression>) leftTable.Columns
         : ExtractColumnExpressions(left.Request.Statement);
 
       var rightShouldUseReference = ShouldUseQueryReference(provider, right);
@@ -240,10 +242,10 @@ protected override SqlProvider VisitPredicateJoin(PredicateJoinProvider provider
         ? right.PermanentReference
         : right.Request.Statement.From;
       var rightColumns = rightShouldUseReference
-        ? rightTable.Columns.Cast<SqlColumn>()
+        ? (IReadOnlyList<SqlColumn>) rightTable.Columns
         : right.Request.Statement.Columns;
       var rightExpressions = rightShouldUseReference
-        ? rightTable.Columns.Cast<SqlExpression>().ToList()
+        ? (IReadOnlyList<SqlExpression>) rightTable.Columns
         : ExtractColumnExpressions(right.Request.Statement);
 
 
@@ -257,8 +259,8 @@ protected override SqlProvider VisitPredicateJoin(PredicateJoinProvider provider
         joinType,
         leftTable,
         rightTable,
-        leftColumns.ToList(),
-        rightColumns.ToList(),
+        leftColumns,
+        rightColumns,
         joinExpression);
 
       var query = SqlDml.Select(joinedTable);
@@ -308,10 +310,18 @@ protected override SqlProvider VisitSelect(SelectProvider provider)
     {
       var compiledSource = Compile(provider.Source);
 
-      SqlSelect query = ExtractSqlSelect(provider, compiledSource);
-      var originalColumns = query.Columns.ToList();
-      query.Columns.Clear();
-      query.Columns.AddRange(provider.ColumnIndexes.Select(i => originalColumns[i]));
+      var query = ExtractSqlSelect(provider, compiledSource);
+      var queryColumns = query.Columns;
+      var columnIndexes = provider.ColumnIndexes;
+
+      var newIndex = 0;
+      var newColumns = new SqlColumn[columnIndexes.Count];
+      foreach (var index in columnIndexes) {
+        newColumns[newIndex++] = queryColumns[index];
+      }
+
+      queryColumns.Clear();
+      queryColumns.AddRange(newColumns);
 
       return CreateProvider(query, provider, compiledSource);
     }
@@ -407,7 +417,7 @@ protected override SqlProvider VisitIntersect(IntersectProvider provider)
       var queryRef = SqlDml.QueryRef(result);
 
       SqlSelect query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
 
       return CreateProvider(query, provider, left, right);
     }
@@ -431,7 +441,7 @@ protected override SqlProvider VisitExcept(ExceptProvider provider)
       var result = SqlDml.Except(leftSelect, rightSelect);
       var queryRef = SqlDml.QueryRef(result);
       SqlSelect query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
 
       return CreateProvider(query, provider, left, right);
     }
@@ -455,7 +465,7 @@ protected override SqlProvider VisitConcat(ConcatProvider provider)
       var result = SqlDml.UnionAll(leftSelect, rightSelect);
       var queryRef = SqlDml.QueryRef(result);
       SqlSelect query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
 
       return CreateProvider(query, provider, left, right);
     }
@@ -479,7 +489,7 @@ protected override SqlProvider VisitUnion(UnionProvider provider)
       var result = SqlDml.Union(leftSelect, rightSelect);
       var queryRef = SqlDml.QueryRef(result);
       SqlSelect query = SqlDml.Select(queryRef);
-      query.Columns.AddRange(queryRef.Columns.Cast<SqlColumn>());
+      query.Columns.AddRange(queryRef.Columns);
 
       return CreateProvider(query, provider, left, right);
     }
diff --git a/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlColumnCollection.cs b/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlColumnCollection.cs
index f5061db42b..73b1866dea 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlColumnCollection.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlColumnCollection.cs
@@ -3,9 +3,8 @@
 // For conditions of distribution and use, see license.
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
-using System.Linq;
 using Xtensive.Core;
 
 
@@ -15,74 +14,123 @@ namespace Xtensive.Sql.Dml
   /// Represents collection of <see cref="SqlColumn"/>s.
   /// </summary>
   [Serializable]
-  public class SqlColumnCollection : Collection<SqlColumn>
+  public class SqlColumnCollection : ICollection<SqlColumn>, IReadOnlyList<SqlColumn>
   {
     private static readonly StringComparer Comparer = StringComparer.OrdinalIgnoreCase;
+    private readonly List<SqlColumn> columnList;
 
-    public SqlColumn this[string name]
-    {
-      get
-      {
-        if (string.IsNullOrEmpty(name))
-          return null;
-        return this.FirstOrDefault(column => Comparer.Equals(column.Name, name));
-      }
-      set { throw new NotSupportedException(); }
-    }
+    /// <summary>
+    /// Gets the number of elements contained in the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    public int Count => columnList.Count;
 
-    public bool IsReadOnly
-    {
-      get { return false; }
-    }
+    /// <inheritdoc cref="ICollection{T}.IsReadOnly"/>>
+    bool ICollection<SqlColumn>.IsReadOnly => false;
 
-    public void Add(SqlColumn item, string alias)
-    {
-      base.Add(SqlDml.ColumnRef(item, alias));
-    }
+    /// <inheritdoc cref="IEnumerable.GetEnumerator"/>>
+    IEnumerator IEnumerable.GetEnumerator() => columnList.GetEnumerator();
+
+    /// <inheritdoc cref="IEnumerable{T}.GetEnumerator"/>>
+    IEnumerator<SqlColumn> IEnumerable<SqlColumn>.GetEnumerator() => columnList.GetEnumerator();
+
+    /// <summary>
+    /// Returns a <see cref="List{T}.Enumerator"/> that iterates through the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    public List<SqlColumn>.Enumerator GetEnumerator() => columnList.GetEnumerator();
 
-    public void Add(SqlExpression expression)
+    /// <summary>
+    /// Gets or sets the column at the specified <paramref name="index"/>.
+    /// </summary>
+    public SqlColumn this[int index]
     {
-      if (expression is SqlColumn)
-        base.Add(expression as SqlColumn);
-      else
-        base.Add(SqlDml.ColumnRef(SqlDml.Column(expression)));
+      get => columnList[index];
+      set => columnList[index] = value;
     }
 
-    public void Add(SqlExpression expression, string alias)
+    /// <summary>
+    /// Gets the column with the specified <paramref name="name"/>
+    /// or <c>null</c> if collection doesn't contain such a column.
+    /// </summary>
+    public SqlColumn this[string name] =>
+      string.IsNullOrEmpty(name) ? null : columnList.Find(column => Comparer.Equals(column.Name, name));
+
+    /// <summary>
+    /// Adds a specified <paramref name="column"/> to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    public void Add(SqlColumn column) => columnList.Add(column);
+
+    /// <summary>
+    /// Builds a <see cref="SqlColumnRef"/> to the specified <paramref name="column"/> using
+    /// the provided <paramref name="alias"/> and then adds it to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    /// <exception cref="ArgumentNullException"><paramref name="alias"/> is null.</exception>
+    public void Add(SqlColumn column, string alias)
     {
-      ArgumentValidator.EnsureArgumentNotNull(alias, "alias");
-      base.Add(SqlDml.ColumnRef(SqlDml.Column(expression), alias));
+      ArgumentValidator.EnsureArgumentNotNull(alias, nameof(alias));
+      columnList.Add(SqlDml.ColumnRef(column, alias));
     }
 
-    public void Add(SqlColumnRef columnReference)
+    /// <summary>
+    /// Builds a <see cref="SqlColumnRef"/> by the specified <paramref name="expression"/> and
+    /// then adds it to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    public void Add(SqlExpression expression) =>
+      columnList.Add(expression is SqlColumn column ? column : SqlDml.ColumnRef(SqlDml.Column(expression)));
+
+    /// <summary>
+    /// Builds a <see cref="SqlColumnRef"/> by the specified <paramref name="expression"/> and
+    /// <paramref name="alias"/>; then adds it to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    /// <exception cref="ArgumentNullException"><paramref name="alias"/> is <c>null</c>.</exception>
+    public void Add(SqlExpression expression, string alias)
     {
-      ArgumentValidator.EnsureArgumentNotNull(columnReference, "columnReference");
-      base.Add(columnReference);
+      ArgumentValidator.EnsureArgumentNotNull(alias, nameof(alias));
+      columnList.Add(SqlDml.ColumnRef(SqlDml.Column(expression), alias));
     }
 
+    /// <summary>
+    /// Builds a <see cref="SqlColumnRef"/> by the specified <paramref name="expression"/> and <paramref name="alias"/>
+    /// then inserts it into <see cref="SqlColumnCollection"/> at the specified <paramref name="index"/>.
+    /// </summary>
+    /// <exception cref="ArgumentNullException"><paramref name="alias"/> is <c>null</c>.</exception>
+    /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is less than <c>0</c>.
+    /// -or- <paramref name="index"/> is greater than <see cref="Count"/>.</exception>
     public void Insert(int index, SqlExpression expression, string alias)
     {
-      ArgumentValidator.EnsureArgumentNotNull(alias, "alias");
-      Insert(index, SqlDml.ColumnRef(SqlDml.Column(expression), alias));
+      ArgumentValidator.EnsureArgumentNotNull(alias, nameof(alias));
+      columnList.Insert(index, SqlDml.ColumnRef(SqlDml.Column(expression), alias));
     }
 
+    /// <summary>
+    /// Adds <paramref name="columns"/> to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    /// <param name="columns">Columns to be added.</param>
     public void AddRange(params SqlColumn[] columns)
     {
-      ArgumentValidator.EnsureArgumentNotNull(columns, "columns");
-      foreach (SqlColumn c in columns) {
-        ArgumentValidator.EnsureArgumentNotNull(c, "column");
-        base.Add(c);
-      }
+      ArgumentValidator.EnsureArgumentNotNull(columns, nameof(columns));
+      columnList.AddRange(columns);
     }
 
-    public void AddRange<TColumn>(IEnumerable<TColumn> columns)
-      where TColumn : SqlColumn
-    {
-      ArgumentValidator.EnsureArgumentNotNull(columns, "columns");
-      foreach (TColumn c in columns)
-        base.Add(c);
-    }
+    /// <summary>
+    /// Adds <paramref name="columns"/> to the end of the <see cref="SqlColumnCollection"/>.
+    /// </summary>
+    /// <param name="columns">Columns to be added.</param>
+    /// <typeparam name="TColumn">A type of the columns in the specified <paramref name="columns"/>
+    /// collection; it must be <see cref="SqlColumn"/> or its inheritor.</typeparam>
+    public void AddRange<TColumn>(IEnumerable<TColumn> columns) where TColumn : SqlColumn =>
+      columnList.AddRange(columns);
+
+    /// <inheritdoc cref="ICollection{T}.Contains"/>
+    public bool Contains(SqlColumn item) => columnList.Contains(item);
+
+    /// <inheritdoc cref="ICollection{T}.CopyTo"/>
+    public void CopyTo(SqlColumn[] array, int arrayIndex) => columnList.CopyTo(array, arrayIndex);
+
+    /// <inheritdoc cref="ICollection{T}.Remove"/>
+    public bool Remove(SqlColumn item) => columnList.Remove(item);
 
+    /// <inheritdoc cref="ICollection{T}.Clear"/>
+    public void Clear() => columnList.Clear();
 
     // Constructors
 
@@ -91,14 +139,24 @@ public void AddRange<TColumn>(IEnumerable<TColumn> columns)
     /// </summary>
     public SqlColumnCollection()
     {
+      columnList = new List<SqlColumn>();
     }
 
     /// <summary>
     /// Initializes new instance of this type.
     /// </summary>
-    public SqlColumnCollection(IList<SqlColumn> list)
-      : base(list)
+    public SqlColumnCollection(IEnumerable<SqlColumn> columns)
+    {
+      columnList = new List<SqlColumn>(columns);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SqlColumnCollection"/> class.
+    /// This is special version it uses provided list as is.
+    /// </summary>
+    internal SqlColumnCollection(List<SqlColumn> columns)
     {
+      columnList = columns;
     }
   }
 }
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlTableColumnCollection.cs b/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlTableColumnCollection.cs
index a07a4105a9..e359ae0090 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlTableColumnCollection.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/Collections/SqlTableColumnCollection.cs
@@ -3,8 +3,9 @@
 // For conditions of distribution and use, see license.
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
+using System.Runtime.CompilerServices;
 
 namespace Xtensive.Sql.Dml
 {
@@ -12,36 +13,98 @@ namespace Xtensive.Sql.Dml
   /// Represents collection of <see cref="SqlColumn"/>s.
   /// </summary>
   [Serializable]
-  public class SqlTableColumnCollection
-    : ReadOnlyCollection<SqlTableColumn>
+  public class SqlTableColumnCollection : IReadOnlyList<SqlTableColumn>
   {
     private static readonly StringComparer Comparer = StringComparer.OrdinalIgnoreCase;
+    private readonly List<SqlTableColumn> columnList;
+    private Dictionary<string, SqlTableColumn> columnLookup;
 
     /// <summary>
-    /// An indexer that provides access to collection items by their names.
-    /// Returns <see langword="null"/> if there is no such item.
+    /// Gets the number of elements contained in the <see cref="SqlTableColumnCollection"/>.
+    /// </summary>
+    public int Count => columnList.Count;
+
+    /// <inheritdoc cref="IEnumerable.GetEnumerator"/>>
+    IEnumerator IEnumerable.GetEnumerator() => columnList.GetEnumerator();
+
+    /// <inheritdoc cref="IEnumerable{T}.GetEnumerator"/>>
+    IEnumerator<SqlTableColumn> IEnumerable<SqlTableColumn>.GetEnumerator() => columnList.GetEnumerator();
+
+    /// <summary>
+    /// Returns a <see cref="List{T}.Enumerator"/> that iterates through the <see cref="SqlTableColumnCollection"/>.
+    /// </summary>
+    public List<SqlTableColumn>.Enumerator GetEnumerator() => columnList.GetEnumerator();
+
+    /// <summary>
+    /// Gets the column at the specified <paramref name="index"/>.
+    /// </summary>
+    public SqlTableColumn this[int index] => columnList[index];
+
+    /// <summary>
+    /// Gets the column with the specified <paramref name="name"/>
+    /// or <see langword="null"/> if collection doesn't contain such a column.
     /// </summary>
     public SqlTableColumn this[string name]
     {
-      get
-      {
-        if (string.IsNullOrEmpty(name))
+      get {
+        if (string.IsNullOrEmpty(name)) {
           return null;
-        foreach (SqlTableColumn column in this)
-          if (Comparer.Equals(column.Name, name))
-            return column;
-        return null;
+        }
+
+        var count = columnList.Count;
+        return count <= 16 ? FindColumnInList(name) : FindColumnInDictionaryLookup(name, count);
+      }
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private SqlTableColumn FindColumnInList(string name)
+    {
+      foreach (var column in columnList) {
+        if (Comparer.Equals(column.Name, name)) {
+          return column;
+        }
+      }
+
+      return null;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private SqlTableColumn FindColumnInDictionaryLookup(string name, int count)
+    {
+      if (columnLookup != null) {
+        return columnLookup.TryGetValue(name, out var column) ? column : null;
+      }
+
+      SqlTableColumn result = null;
+      columnLookup = new Dictionary<string, SqlTableColumn>(count, Comparer);
+      for (var index = count - 1; index >= 0; index--) {
+        var column = columnList[index];
+        var columnName = column.Name;
+        columnLookup[columnName] = column;
+        if (Comparer.Equals(columnName, name)) {
+          result = column;
+        }
       }
+
+      return result;
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SqlTableColumnCollection"/> class.
+    /// </summary>
+    /// <param name="columns">A collection of <see cref="SqlTableColumn"/>s to be wrapped.</param>
+    public SqlTableColumnCollection(IEnumerable<SqlTableColumn> columns)
+    {
+      columnList = new List<SqlTableColumn>(columns);
     }
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SqlTableColumnCollection"/> class.
+    /// This is special version it uses provided list as is.
     /// </summary>
-    /// <param name="list">The list to wrap.</param>
-    /// <exception cref="T:System.ArgumentNullException">list is null.</exception>
-    public SqlTableColumnCollection(IList<SqlTableColumn> list)
-      : base(list)
+    internal SqlTableColumnCollection(List<SqlTableColumn> columns)
     {
+      columnList = columns;
     }
   }
 }
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Sql/Dml/SqlContainsTable.cs b/Orm/Xtensive.Orm/Sql/Dml/SqlContainsTable.cs
index 97bc7db3f6..4c377d939f 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/SqlContainsTable.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/SqlContainsTable.cs
@@ -66,35 +66,40 @@ public SqlQueryExpression UnionAll(ISqlQueryExpression operand)
     // Constructors
 
 
-    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columnNames)
+    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames)
       : this(dataTable, freeText, columnNames, ArrayUtils<string>.EmptyArray, null)
     {
     }
 
-    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columnNames, ICollection<string> targetColumnNames)
+    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames, ICollection<string> targetColumnNames)
       : this(dataTable, freeText, columnNames, targetColumnNames, null)
     {
     }
 
-    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columNames, SqlExpression topN)
-      : this(dataTable, freeText, columNames, ArrayUtils<string>.EmptyArray, topN)
+    internal SqlContainsTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames, SqlExpression topN)
+      : this(dataTable, freeText, columnNames, ArrayUtils<string>.EmptyArray, topN)
     {
     }
 
-    internal SqlContainsTable(DataTable dataTable, SqlExpression searchCondition, IEnumerable<string> columnNames, ICollection<string> targetColumnNames, SqlExpression topNByRank)
+    internal SqlContainsTable(DataTable dataTable, SqlExpression searchCondition, ICollection<string> columnNames, ICollection<string> targetColumnNames, SqlExpression topNByRank)
       : base(string.Empty)
     {
       TargetTable = SqlDml.TableRef(dataTable);
       SearchCondition = searchCondition;
       TopNByRank = topNByRank;
-      var targetColumns = new List<SqlTableColumn>();
-      if (targetColumnNames.Count == 0)
-        targetColumns.Add(Asterisk);
-      else
-        targetColumns = targetColumnNames.Select(cn => SqlDml.TableColumn(this, cn)).ToList();
-      TargetColumns = new SqlTableColumnCollection(targetColumns);
-
-      columns = new SqlTableColumnCollection(columnNames.Select(column=>SqlDml.TableColumn(this, column)).ToList());
+      var targetColumnCount = targetColumnNames.Count;
+      if (targetColumnCount == 0) {
+        TargetColumns = new SqlTableColumnCollection(new List<SqlTableColumn>(1) {Asterisk});
+      }
+      else {
+        var targetColumns = new List<SqlTableColumn>(targetColumnCount);
+        targetColumns.AddRange(targetColumnNames.Select(columnName => SqlDml.TableColumn(this, columnName)));
+        TargetColumns = new SqlTableColumnCollection(targetColumns);
+      }
+
+      var columnList = new List<SqlTableColumn>(columnNames.Count);
+      columnList.AddRange(columnNames.Select(columnName => SqlDml.TableColumn(this, columnName)));
+      columns = new SqlTableColumnCollection(columnList);
     }
   }
 }
diff --git a/Orm/Xtensive.Orm/Sql/Dml/SqlFreeTextTable.cs b/Orm/Xtensive.Orm/Sql/Dml/SqlFreeTextTable.cs
index 8bf1535cad..b0f0274ef5 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/SqlFreeTextTable.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/SqlFreeTextTable.cs
@@ -71,35 +71,40 @@ public SqlQueryExpression UnionAll(ISqlQueryExpression operand)
 
     // Constructors
 
-    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columnNames)
+    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames)
       : this(dataTable, freeText, columnNames, ArrayUtils<string>.EmptyArray, null)
     {
     }
 
-    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columnNames, ICollection<string> targetColumnNames)
+    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames, ICollection<string> targetColumnNames)
       : this(dataTable, freeText, columnNames, targetColumnNames, null)
     {
     }
 
-    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columNames, SqlExpression topN)
-      : this(dataTable, freeText, columNames, ArrayUtils<string>.EmptyArray, topN)
+    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames, SqlExpression topN)
+      : this(dataTable, freeText, columnNames, ArrayUtils<string>.EmptyArray, topN)
     {
     }
 
-    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, IEnumerable<string> columnNames, ICollection<string> targetColumnNames, SqlExpression topNByRank)
+    internal SqlFreeTextTable(DataTable dataTable, SqlExpression freeText, ICollection<string> columnNames, ICollection<string> targetColumnNames, SqlExpression topNByRank)
       : base(string.Empty)
     {
       TargetTable = SqlDml.TableRef(dataTable);
       FreeText = freeText;
       TopNByRank = topNByRank;
-      var targetColumns = new List<SqlTableColumn>();
-      if (targetColumnNames.Count == 0)
-        targetColumns.Add(Asterisk);
-      else
-        targetColumns = targetColumnNames.Select(cn => SqlDml.TableColumn(this, cn)).ToList();
-      TargetColumns = new SqlTableColumnCollection(targetColumns);
-
-      columns = new SqlTableColumnCollection(columnNames.Select(column=>SqlDml.TableColumn(this, column)).ToList());
+      var targetColumnCount = targetColumnNames.Count;
+      if (targetColumnCount == 0) {
+        TargetColumns = new SqlTableColumnCollection(new List<SqlTableColumn>(1) {Asterisk});
+      }
+      else {
+        var targetColumns = new List<SqlTableColumn>(targetColumnCount);
+        targetColumns.AddRange(targetColumnNames.Select(columnName => SqlDml.TableColumn(this, columnName)));
+        TargetColumns = new SqlTableColumnCollection(targetColumns);
+      }
+
+      var columnList = new List<SqlTableColumn>(columnNames.Count);
+      columnList.AddRange(columnNames.Select(columnName => SqlDml.TableColumn(this, columnName)));
+      columns = new SqlTableColumnCollection(columnList);
     }
   }
 }
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Sql/Dml/SqlJoinedTable.cs b/Orm/Xtensive.Orm/Sql/Dml/SqlJoinedTable.cs
index 4d19e98647..36de650ecb 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/SqlJoinedTable.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/SqlJoinedTable.cs
@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 
 namespace Xtensive.Sql.Dml
 {
@@ -30,11 +29,13 @@ public SqlJoinExpression JoinExpression
 
     internal override object Clone(SqlNodeCloneContext context)
     {
-      if (context.NodeMapping.ContainsKey(this))
+      if (context.NodeMapping.ContainsKey(this)) {
         return context.NodeMapping[this];
+      }
 
-      var clone = new SqlJoinedTable((SqlJoinExpression)joinExpression.Clone(context));
-      clone.AliasedColumns = new SqlColumnCollection(AliasedColumns);
+      var clone = new SqlJoinedTable((SqlJoinExpression) joinExpression.Clone(context)) {
+        AliasedColumns = new SqlColumnCollection(new List<SqlColumn>(AliasedColumns))
+      };
       context.NodeMapping[this] = clone;
       return clone;
     }
@@ -54,17 +55,25 @@ public override IEnumerator<SqlTable> GetEnumerator()
     // Constructor
 
     internal SqlJoinedTable(SqlJoinExpression joinExpression)
+      : this(joinExpression, joinExpression.Left.Columns, joinExpression.Right.Columns)
     {
-      this.joinExpression = joinExpression;
-      var joinedColumns = joinExpression.Left.Columns.Concat(joinExpression.Right.Columns).ToList();
-      columns = new SqlTableColumnCollection(joinedColumns);
-      AliasedColumns = new SqlColumnCollection(columns.Cast<SqlColumn>().ToList());
     }
 
-    internal SqlJoinedTable(SqlJoinExpression joinExpression, IEnumerable<SqlColumn> leftColumns, IEnumerable<SqlColumn> rightColumns)
-      : this (joinExpression)
+    internal SqlJoinedTable(SqlJoinExpression joinExpression, IReadOnlyList<SqlColumn> leftColumns, IReadOnlyList<SqlColumn> rightColumns)
     {
-      AliasedColumns = new SqlColumnCollection(leftColumns.Concat(rightColumns).ToList());
+      this.joinExpression = joinExpression;
+      var allLeftColumns = joinExpression.Left.Columns;
+      var allRightColumns = joinExpression.Right.Columns;
+
+      var joinedColumns = new List<SqlTableColumn>(allLeftColumns.Count + allRightColumns.Count);
+      joinedColumns.AddRange(allLeftColumns);
+      joinedColumns.AddRange(allRightColumns);
+      columns = new SqlTableColumnCollection(joinedColumns);
+
+      var aliasedColumns = new List<SqlColumn>(leftColumns.Count + rightColumns.Count);
+      aliasedColumns.AddRange(leftColumns);
+      aliasedColumns.AddRange(rightColumns);
+      AliasedColumns = new SqlColumnCollection(aliasedColumns);
     }
   }
 }
diff --git a/Orm/Xtensive.Orm/Sql/Dml/SqlQueryRef.cs b/Orm/Xtensive.Orm/Sql/Dml/SqlQueryRef.cs
index 68fe18299e..ae110c9514 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/SqlQueryRef.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/SqlQueryRef.cs
@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.Linq;
 
 namespace Xtensive.Sql.Dml
@@ -57,36 +56,36 @@ internal SqlQueryRef(ISqlQueryExpression query, string name)
       this.query = query;
       var queryColumns = new List<SqlTableColumn>();
       foreach (var queryExpression in query) {
-        
-        var select = queryExpression as SqlSelect;
-        if (select!=null) {
-          var selectColumns = select.Columns.ToList();
-          select.Columns.Clear();
-          foreach (var originalColumn in selectColumns) {
+        if (queryExpression is SqlSelect sqlSelect) {
+          var selectColumns = sqlSelect.Columns;
+          for (int index = 0, count = selectColumns.Count; index < count; index++) {
+            var originalColumn = selectColumns[index];
             var column = originalColumn;
-            var stubColumn = column as SqlColumnStub;
-            if (!ReferenceEquals(null, stubColumn))
+            if (column is SqlColumnStub stubColumn) {
               column = stubColumn.Column;
-            var columnRef = column as SqlColumnRef;
-            if (!ReferenceEquals(null, columnRef)) {
+            }
+
+            if (column is SqlColumnRef columnRef) {
               stubColumn = columnRef.SqlColumn as SqlColumnStub;
-              if (!ReferenceEquals(null, stubColumn))
+              if (!ReferenceEquals(null, stubColumn)) {
                 column = stubColumn.Column;
+              }
             }
-            select.Columns.Add(column);
+
+            selectColumns[index] = column;
             queryColumns.Add(SqlDml.TableColumn(this, originalColumn.Name));
           }
         }
 
-        var freeTextTable = queryExpression as SqlFreeTextTable;
-        if (freeTextTable!=null )
-          foreach (var originalColumn in freeTextTable.Columns)
-            queryColumns.Add(SqlDml.TableColumn(this, originalColumn.Name));
+        if (queryExpression is SqlFreeTextTable freeTextTable) {
+          queryColumns.AddRange(
+            freeTextTable.Columns.Select(originalColumn => SqlDml.TableColumn(this, originalColumn.Name)));
+        }
 
-        var containsTable = queryExpression as SqlContainsTable;
-        if (containsTable!=null )
-          foreach (var originalColumn in containsTable.Columns)
-            queryColumns.Add(SqlDml.TableColumn(this, originalColumn.Name));
+        if (queryExpression is SqlContainsTable containsTable) {
+          queryColumns.AddRange(
+            containsTable.Columns.Select(originalColumn => SqlDml.TableColumn(this, originalColumn.Name)));
+        }
 
         break;
       }
diff --git a/Orm/Xtensive.Orm/Sql/Dml/SqlTableRef.cs b/Orm/Xtensive.Orm/Sql/Dml/SqlTableRef.cs
index c3fba85062..f5930a3b35 100644
--- a/Orm/Xtensive.Orm/Sql/Dml/SqlTableRef.cs
+++ b/Orm/Xtensive.Orm/Sql/Dml/SqlTableRef.cs
@@ -17,75 +17,67 @@ namespace Xtensive.Sql.Dml
   [Serializable]
   public class SqlTableRef : SqlTable
   {
-    private DataTable dataTable;
-
     /// <summary>
     /// Gets the name of the instance.
     /// </summary>
     /// <value>The name.</value>
-    public override string Name
-    {
-      get { return string.IsNullOrEmpty(base.Name) ? dataTable.DbName : base.Name; }
-    }
+    public override string Name => string.IsNullOrEmpty(base.Name) ? DataTable.DbName : base.Name;
 
     /// <summary>
     /// Gets the referenced table.
     /// </summary>
     /// <value>The table.</value>
-    public DataTable DataTable
-    {
-      get { return dataTable; }
-    }
+    public DataTable DataTable { get; private set; }
 
     internal override object Clone(SqlNodeCloneContext context)
     {
-      if (context.NodeMapping.ContainsKey(this))
+      if (context.NodeMapping.ContainsKey(this)) {
         return context.NodeMapping[this];
+      }
 
-      var clone = new SqlTableRef {Name = Name, dataTable = DataTable};
+      return CreateClone(context);
+    }
+
+    private SqlTableRef CreateClone(SqlNodeCloneContext context)
+    {
+      var clone = new SqlTableRef {Name = Name, DataTable = DataTable};
       context.NodeMapping[this] = clone;
-      var columnClones = new Collection<SqlTableColumn>();
-      foreach (var column in columns)
-        columnClones.Add((SqlTableColumn) column.Clone(context));
+      var columnClones = new List<SqlTableColumn>(columns.Count);
+      columnClones.AddRange(columns.Select(column => (SqlTableColumn) column.Clone(context)));
+
       clone.columns = new SqlTableColumnCollection(columnClones);
 
       return clone;
     }
 
-    public override void AcceptVisitor(ISqlVisitor visitor)
-    {
-      visitor.Visit(this);
-    }
-
+    public override void AcceptVisitor(ISqlVisitor visitor) => visitor.Visit(this);
 
     // Constructors
 
     private SqlTableRef()
-    {
-    }
+    { }
 
     internal SqlTableRef(DataTable dataTable)
-      : this(dataTable, string.Empty)
-    {
-    }
+      : this(dataTable, string.Empty, Array.Empty<string>())
+    { }
 
     internal SqlTableRef(DataTable dataTable, string name)
-      : this(dataTable, name, ArrayUtils<string>.EmptyArray)
-    {
-      this.dataTable = dataTable;
-      var tableColumns = new List<SqlTableColumn>();
-      foreach (DataTableColumn c in dataTable.Columns)
-        tableColumns.Add(SqlDml.TableColumn(this, c.Name));
-      columns = new SqlTableColumnCollection(tableColumns);
-    }
+      : this(dataTable, name, Array.Empty<string>())
+    { }
 
     internal SqlTableRef(DataTable dataTable, string name, params string[] columnNames)
       : base(name)
     {
-      this.dataTable = dataTable;
-      var tableColumns = columnNames.Length == 0 
-        ? dataTable.Columns.Select(c => SqlDml.TableColumn(this, c.Name)).ToList() 
-        : columnNames.Select(cn => SqlDml.TableColumn(this, cn)).ToList();
+      DataTable = dataTable;
+      List<SqlTableColumn> tableColumns;
+      if (columnNames.Length == 0) {
+        tableColumns = new List<SqlTableColumn>(dataTable.Columns.Count);
+        tableColumns.AddRange(dataTable.Columns.Select(column => SqlDml.TableColumn(this, column.Name)));
+      }
+      else {
+        tableColumns = new List<SqlTableColumn>(columnNames.Length);
+        tableColumns.AddRange(columnNames.Select(columnName => SqlDml.TableColumn(this, columnName)));
+      }
       columns = new SqlTableColumnCollection(tableColumns);
     }
   }
diff --git a/Orm/Xtensive.Orm/Sql/SqlDml.cs b/Orm/Xtensive.Orm/Sql/SqlDml.cs
index 8cab506da3..ebe100165a 100644
--- a/Orm/Xtensive.Orm/Sql/SqlDml.cs
+++ b/Orm/Xtensive.Orm/Sql/SqlDml.cs
@@ -840,7 +840,8 @@ public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable
       return Join(joinType, left, right, null);
     }
 
-    public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable right, IList<SqlColumn> leftColumns, IList<SqlColumn> rightColumns)
+    public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable right,
+      IReadOnlyList<SqlColumn> leftColumns, IReadOnlyList<SqlColumn> rightColumns)
     {
       return Join(joinType, left, right, leftColumns, rightColumns, null);
     }
@@ -854,7 +855,8 @@ public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable
       return new SqlJoinedTable(new SqlJoinExpression(joinType, left, right, expression));
     }
 
-    public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable right, IList<SqlColumn> leftColumns, IList<SqlColumn> rightColumns, SqlExpression expression)
+    public static SqlJoinedTable Join(SqlJoinType joinType, SqlTable left, SqlTable right,
+      IReadOnlyList<SqlColumn> leftColumns, IReadOnlyList<SqlColumn> rightColumns, SqlExpression expression)
     {
       ArgumentValidator.EnsureArgumentNotNull(left, "left");
       ArgumentValidator.EnsureArgumentNotNull(right, "right");