Added retrieval of precision and scale information for decimal types (#189)

* Added retrieval of precision and scale information for decimal types to the datareader schema

* Transformed GetScale and GetPrecision methods into readonly properties. Also changed the name of a variable to make it more understandable.

Author: magneto-81

DuckDB.NET.Data/DuckDBDataReader.cs

@@ -294,11 +294,13 @@ public override DataTable GetSchemaTable()
                 { "ColumnName", typeof(string) },
                 { "DataType", typeof(Type) },
                 { "ColumnSize", typeof(int) },
-                { "AllowDBNull", typeof(bool) }
+                { "AllowDBNull", typeof(bool) },
+                { "NumericScale", typeof(byte) },
+                { "NumericPrecision", typeof(byte) }
             }
         };
 
-        var rowData = new object[5];
+        var rowData = new object[7];
 
         for (var i = 0; i < FieldCount; i++)
         {
@@ -307,6 +309,18 @@ public override DataTable GetSchemaTable()
             rowData[2] = GetFieldType(i);
             rowData[3] = -1;
             rowData[4] = true;
+
+            if (vectorReaders[i] is DecimalVectorDataReader decimalVectorDataReader) 
+            {
+                rowData[5] = decimalVectorDataReader.Scale;
+                rowData[6] = decimalVectorDataReader.Precision;
+            } 
+            else 
+            {
+                rowData[5] = DBNull.Value;
+                rowData[6] = DBNull.Value;
+            }
+
             table.Rows.Add(rowData);
         }
 

DuckDB.NET.Data/Internal/Reader/DecimalVectorDataReader.cs

@@ -1,22 +1,33 @@
 using System;
 using System.Numerics;
-using DuckDB.NET.Data.Extensions;
 using DuckDB.NET.Native;
 
 namespace DuckDB.NET.Data.Internal.Reader;
 
 internal sealed class DecimalVectorDataReader : NumericVectorDataReader
 {
     private readonly byte scale;
+    private readonly byte precision;
     private readonly DuckDBType decimalType;
 
     internal unsafe DecimalVectorDataReader(IntPtr vector, void* dataPointer, ulong* validityMaskPointer, DuckDBType columnType, string columnName) : base(dataPointer, validityMaskPointer, columnType, columnName)
     {
         using var logicalType = NativeMethods.Vectors.DuckDBVectorGetColumnType(vector);
         scale = NativeMethods.LogicalType.DuckDBDecimalScale(logicalType);
+        precision = NativeMethods.LogicalType.DuckDBDecimalWidth(logicalType);
         decimalType = NativeMethods.LogicalType.DuckDBDecimalInternalType(logicalType);
     }
 
+    internal byte Scale 
+    {
+        get => scale;
+    }
+
+    internal byte Precision
+    {
+        get => precision;
+    }
+
     protected override T GetValidValue<T>(ulong offset, Type targetType)
     {
         if (DuckDBType!= DuckDBType.Decimal)
@@ -61,4 +72,5 @@ private decimal GetDecimal(ulong offset)
             default: throw new DuckDBException($"Invalid type {DuckDBType} ({(int)DuckDBType}) for column {ColumnName}");
         }
     }
+
 }

DuckDB.NET.Test/DuckDBDataReaderTests.cs

@@ -323,4 +323,35 @@ public void ReadNonQueryAsResult()
 
         reader.Invoking(r => r.Close()).Should().NotThrow();
     }
+
+    [Fact]
+    public void ReadDecimalSchema() 
+    {
+        Command.CommandText = "CREATE TABLE decimaltbl(foo decimal(10,2));";
+        Command.ExecuteNonQuery();
+
+        Command.CommandText = "INSERT INTO decimaltbl VALUES (3.45), (9.35), (7.24);";
+        Command.ExecuteNonQuery();
+
+        Command.CommandText = "SELECT foo FROM decimaltbl";
+        using var reader = Command.ExecuteReader();
+
+        var schemaTable = reader.GetSchemaTable();
+        schemaTable.Rows[0]["NumericScale"].Should().Be(2);
+        schemaTable.Rows[0]["NumericPrecision"].Should().Be(10);
+    }
+
+    [Fact]
+    public void ReadDecimalSchemaWithoutTableRow() 
+    {
+        Command.CommandText = "CREATE TABLE decimaltbl(foo decimal(10,2));";
+        Command.ExecuteNonQuery();
+
+        Command.CommandText = "SELECT foo FROM decimaltbl";
+        using var reader = Command.ExecuteReader();
+
+        var schemaTable = reader.GetSchemaTable();
+        schemaTable.Rows[0]["NumericScale"].Should().Be(0);
+        schemaTable.Rows[0]["NumericPrecision"].Should().Be(0);
+    }
 }