PosgreSql: Cast to decimal(28, x) for some Math operations

This prevents System.OverflowExeptions for results of updated operation on results reading.

Npgsql does not provide any ways to get some native numeric value
and scale it down to .Net decimal, everything is done internally within client.
So we have cast results.

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Providers/Expressions/MemberCompilers/MathCompilers.cs

@@ -1,6 +1,6 @@
 // Copyright (C) 2003-2010 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
+// 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:    2009.02.14
 
@@ -130,7 +130,7 @@ public static SqlExpression MathSignFloat(
     private static SqlExpression Min(SqlExpression left, SqlExpression right)
     {
       var result = SqlDml.Case();
-      result.Add(left < right, left);
+      _ = result.Add(left < right, left);
       result.Else = right;
       return result;
     }
@@ -230,7 +230,7 @@ public static SqlExpression MathMinDecimal(
     private static SqlExpression Max(SqlExpression left, SqlExpression right)
     {
       var result = SqlDml.Case();
-      result.Add(left > right, left);
+      _ = result.Add(left > right, left);
       result.Else = right;
       return result;
     }
@@ -373,7 +373,7 @@ public static SqlExpression MathCeilingDouble(
     public static SqlExpression MathCeilingDecimal(
       [Type(typeof(decimal))] SqlExpression d)
     {
-      return SqlDml.Ceiling(d);
+      return TryCastToDecimalPS(SqlDml.Ceiling(d), 28, 0);
     }
 
     [Compiler(typeof(Math), "Cos", TargetKind.Static | TargetKind.Method)]
@@ -408,7 +408,7 @@ public static SqlExpression MathFloorDouble(
     public static SqlExpression MathFloorDecimal(
       [Type(typeof(decimal))] SqlExpression d)
     {
-      return SqlDml.Floor(d);
+      return TryCastToDecimalPS(SqlDml.Floor(d), 28, 0);
     }
 
     [Compiler(typeof(Math), "Log", TargetKind.Static | TargetKind.Method)]
@@ -560,28 +560,58 @@ public static SqlExpression MathTruncateDouble(
     public static SqlExpression MathTruncateDecimal(
       [Type(typeof(decimal))] SqlExpression d)
     {
-      return SqlDml.Truncate(d);
+      return TryCastToDecimalPS(SqlDml.Truncate(d), 28, 0);
     }
 
     #region Round helpers
 
     private static SqlExpression GenericRound(SqlExpression value, SqlExpression digits, bool isDecimal, SqlExpression mode)
     {
-      if (mode.NodeType!=SqlNodeType.Container)
+      if (mode.NodeType != SqlNodeType.Container) {
         throw new NotSupportedException();
+      }
+
       var container = (SqlContainer) mode;
-      if (!(container.Value is MidpointRounding))
+      if (!(container.Value is MidpointRounding midpointRounding)) {
+        throw new NotSupportedException();
+      }
+      if (!isDecimal) {
+        return SqlDml.Round(value, digits, TypeCode.Double, midpointRounding);
+      }
+      if (digits == null) {
+        return TryCastToDecimalPS(SqlDml.Round(value, digits, TypeCode.Decimal, midpointRounding), 28, 0);
+      }
+      if (!(digits is SqlLiteral<int> scale)) {
         throw new NotSupportedException();
-      return SqlDml.Round(value, digits,
-        isDecimal ? TypeCode.Decimal : TypeCode.Double,
-        (MidpointRounding) container.Value);
+      }
+      return TryCastToDecimalPS(SqlDml.Round(value, digits, TypeCode.Decimal, midpointRounding), 28, Convert.ToInt16(scale.Value));
     }
 
     private static SqlExpression BankersRound(SqlExpression value, SqlExpression digits, bool isDecimal)
     {
-      return SqlDml.Round(value, digits, isDecimal ? TypeCode.Decimal : TypeCode.Double, MidpointRounding.ToEven);
+      if (!isDecimal) {
+        return SqlDml.Round(value, digits, TypeCode.Double, MidpointRounding.ToEven);
+      }
+      if (digits == null) {
+        return TryCastToDecimalPS(SqlDml.Round(value, digits, TypeCode.Decimal, MidpointRounding.ToEven), 28, 0);
+      }
+      if (!(digits is SqlLiteral<int> scale)) {
+        throw new NotSupportedException();
+      }
+      return TryCastToDecimalPS(SqlDml.Round(value, digits, TypeCode.Decimal, MidpointRounding.ToEven), 28, Convert.ToInt16(scale.Value));
     }
 
     #endregion
+
+    private static SqlExpression TryCastToDecimalPS(SqlExpression value, short precision, short scale)
+    {
+      var context = ExpressionTranslationContext.Current;
+      var provider = context.ProviderInfo.ProviderName;
+      if (provider.Equals(WellKnown.Provider.PostgreSql, StringComparison.Ordinal)) {
+        // to fit result into .Net decimal since Npgsql client libarary does not provide a way to make in on reading
+        return SqlDml.Cast(value, SqlType.Decimal, precision, scale);
+      }
+      return value;
+    }
   }
 }