Statically preallocate Comma TextNode as mostly used

Author: SergeiPavlov

Orm/Xtensive.Orm/Sql/Compiler/Internals/Nodes/ContainerNode.cs

@@ -164,11 +164,7 @@ internal override void AcceptVisitor(NodeVisitor visitor)
     internal void FlushBuffer()
     {
       if (stringBuilder.Length > 0) {
-        var s = stringBuilder.ToString();
-        if (s.Length < 3) {
-          s = string.Intern(s);
-        }
-        children.Add(new TextNode(s));
+        children.Add(TextNode.Create(stringBuilder.ToString()));
         lastNodeIsText = true;
         lastChar = stringBuilder[^1];
         _ = stringBuilder.Clear();

Orm/Xtensive.Orm/Sql/Compiler/Internals/Nodes/TextNode.cs

@@ -9,19 +9,30 @@ namespace Xtensive.Sql.Compiler
   [DebuggerDisplay("Text = {Text}")]
   internal class TextNode : Node
   {
+    private const string CommaString = ", ";
+    private static readonly TextNode CommaNode = new TextNode(CommaString);
+
     public readonly string Text;
 
-    internal override void AcceptVisitor(NodeVisitor visitor)
+    internal override void AcceptVisitor(NodeVisitor visitor) => visitor.Visit(this);
+
+    public static TextNode Create(string text)
     {
-      visitor.Visit(this);
+      if (text.Length < 3) {
+        text = string.Intern(text);
+        if (text == CommaString) {
+          return CommaNode;
+        }
+      }
+      return new TextNode(text);
     }
 
     // Constructor
 
-    public TextNode(string text)
+    private TextNode(string text)
       : base(true)
     {
       Text = text;
     }
   }
-}
+}