Postgresql TypeMapper changes

- TimeSpan reading assumes some values as MinValue/MaxValue, otherwise
  writing and then reading of MinValue/MaxValue will not be the same
  because of different number of fractional points in Postgre and .NET.
  We loose 18 particular values for comfort of comparison (we loose them
  anyway because of PG resolution). The odds of having MaxValue - 1-to-7
  ticks or MinValue + 1-to-8 ticks as true values are extremely low.
- some comments improved

Author: alex-kulakov

Orm/Xtensive.Orm.PostgreSql/Sql.Drivers.PostgreSql/v8_0/TypeMapper.cs

@@ -20,7 +20,11 @@ namespace Xtensive.Sql.Drivers.PostgreSql.v8_0
   internal class TypeMapper : Sql.TypeMapper
   {
     // 6 fractions instead of .NET's 7
-    private const long DateTimeMaxValueAdjustedTicks = 3155378975999999990; 
+    private const long DateTimeMaxValueAdjustedTicks = 3155378975999999990;
+
+    // 6 fractions instead of .NET's 7
+    private const long TimeSpanMinValueAdjustedTicks = -9223372036854775800;
+    private const long TimeSpanMaxValueAdjustedTicks = 9223372036854775800;
 
     protected readonly bool legacyTimestampBehaviorEnabled;
     protected readonly TimeSpan? defaultTimeZone;
@@ -236,9 +240,17 @@ public override object ReadTimeSpan(DbDataReader reader, int index)
       var nativeReader = (NpgsqlDataReader) reader;
       var nativeInterval = nativeReader.GetFieldValue<NpgsqlInterval>(index);
 
-      // support for full-range of Timespans required us to use raw type
+      // support for full-range of TimeSpans requires us to use raw type
       // and construct timespan from its' values.
-      return PostgreSqlHelper.ResurrectTimeSpanFromNpgsqlInterval(nativeInterval);
+      var result = PostgreSqlHelper.ResurrectTimeSpanFromNpgsqlInterval(nativeInterval);
+
+      // for confinience of comparison in .NET we lose 7th fractional point and treat several
+      // .Net values as one, Min or Max value.
+      if (result == TimeSpan.MinValue || result.Ticks == TimeSpanMinValueAdjustedTicks)
+        return TimeSpan.MinValue;
+      if (result == TimeSpan.MaxValue || result.Ticks == TimeSpanMaxValueAdjustedTicks)
+        return TimeSpan.MaxValue;
+      return result;
     }
 
     [SecuritySafeCritical]
@@ -259,10 +271,10 @@ public override object ReadDateTime(DbDataReader reader, int index)
       if (value == DateTime.MinValue || value == DateTime.MaxValue)
         return value;
       if (value.Ticks == DateTimeMaxValueAdjustedTicks) {
-        // When Infinity aliases are disabled.
+        // Applied when Infinity aliases are disabled.
         // To not ruin possible comparisons with defined value,
         // it is better to return definded value,
-        // not the 6-digit version from PostgreSQL
+        // not the 6-digit version from PostgreSQL.
         return DateTime.MaxValue;
       }
       return value;
@@ -273,11 +285,11 @@ public override object ReadDateTimeOffset(DbDataReader reader, int index)
     {
       var nativeReader = (NpgsqlDataReader) reader;
       var value = nativeReader.GetFieldValue<DateTimeOffset>(index);
-      if (value.Ticks == DateTimeMaxValueAdjustedTicks ) {
-        // When Infinity aliases are disabled.
+      if (value.Ticks == DateTimeMaxValueAdjustedTicks) {
+        // Applied when Infinity aliases are disabled.
         // To not ruin possible comparisons with defined values,
         // it is better to return definded value,
-        // not the 6-fractions version from PostgreSQL
+        // not the 6-fractions version from PostgreSQL.
         return DateTimeOffset.MaxValue;
       }
       if (value == DateTimeOffset.MaxValue || value == DateTimeOffset.MaxValue)
@@ -288,7 +300,7 @@ public override object ReadDateTimeOffset(DbDataReader reader, int index)
         return value;
       }
       else {
-        // Here we try to apply connection timezone.
+        // Here, we try to apply connection timezone to the values we read.
         // To not get it from internal connection of DbDataReader
         // we assume that time zone switch happens (if happens)
         // in DomainConfiguration.ConnectionInitializationSql and