Optimize prepared scalar execution

DuckDB.NET.Benchmarks/PreparedCommandBenchmark.cs

@@ -46,6 +46,15 @@ public int ExecutePrepared()
         return (int)preparedCommand.ExecuteScalar()!;
     }
 
+    // Preserves the former ExecuteScalar path as a direct same-process comparison.
+    [Benchmark]
+    public int ExecutePreparedViaReader()
+    {
+        preparedParameter.Value = nextValue++;
+        using var reader = preparedCommand.ExecuteReader();
+        return reader.Read() ? (int)reader.GetValue(0) : default;
+    }
+
     private DuckDBCommand CreateCommand(out DuckDBParameter changingParameter)
     {
         var command = connection.CreateCommand();

DuckDB.NET.Data/DuckDBCommand.cs

@@ -5,6 +5,7 @@
 using Apache.Arrow;
 using Apache.Arrow.Ipc;
 using DuckDB.NET.Data.Arrow;
+using DuckDB.NET.Data.DataChunk.Reader;
 using PreparedStatementBase = DuckDB.NET.Data.PreparedStatement.PreparedStatement;
 using ReusablePreparedStatement = DuckDB.NET.Data.PreparedStatement.ReusablePreparedStatement;
 
@@ -121,6 +122,11 @@ public override int ExecuteNonQuery()
     {
         EnsureConnectionOpen();
 
+        if (preparedStatement is { } reusableStatement)
+        {
+            return ExecutePreparedScalar(reusableStatement, connection!.NativeConnection);
+        }
+
         using var reader = ExecuteReader();
         return reader.Read() ? reader.GetValue(0) : null;
     }
@@ -292,6 +298,66 @@ private int ExecutePreparedNonQuery(
         }
     }
 
+    private object? ExecutePreparedScalar(
+        ReusablePreparedStatement reusableStatement,
+        DuckDBNativeConnection nativeConnection)
+    {
+        BeginPreparedExecution();
+
+        try
+        {
+            var result = reusableStatement.Execute(parameters, UseStreamingMode, nativeConnection);
+
+            try
+            {
+                if (NativeMethods.Query.DuckDBResultReturnType(result) != DuckDBResultType.QueryResult ||
+                    NativeMethods.Query.DuckDBColumnCount(ref result) == 0)
+                {
+                    return null;
+                }
+
+                return ReadFirstValue(ref result);
+            }
+            finally
+            {
+                result.Close();
+            }
+        }
+        finally
+        {
+            CompletePreparedExecution();
+        }
+    }
+
+    private static object? ReadFirstValue(ref DuckDBResult result)
+    {
+        var streamingResult = NativeMethods.Types.DuckDBResultIsStreaming(result) > 0;
+        long chunkIndex = 0;
+
+        while (true)
+        {
+            using var chunk = streamingResult
+                ? NativeMethods.StreamingResult.DuckDBStreamFetchChunk(result)
+                : NativeMethods.Types.DuckDBResultGetChunk(result, chunkIndex++);
+
+            if (chunk is null || chunk.IsInvalid)
+            {
+                return null;
+            }
+
+            if (NativeMethods.DataChunks.DuckDBDataChunkGetSize(chunk) == 0)
+            {
+                continue;
+            }
+
+            var vector = NativeMethods.DataChunks.DuckDBDataChunkGetVector(chunk, 0);
+            using var logicalType = NativeMethods.Query.DuckDBColumnLogicalType(ref result, 0);
+            using var reader = VectorDataReaderFactory.CreateReader(vector, logicalType);
+
+            return reader.IsValid(0) ? reader.GetValue(0) : DBNull.Value;
+        }
+    }
+
     private void BeginPreparedExecution()
     {
         activeExecutions++;

DuckDB.NET.Test/DuckDBCommandTests.cs

@@ -35,6 +35,81 @@ public void PreparedCommandCanBeExecutedRepeatedlyWithNewParameterValues()
         command.ExecuteScalar().Should().Be(22);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void PreparedExecuteScalarHandlesValuesNullsAndEmptyResults(bool useStreamingMode)
+    {
+        using var command = Connection.CreateCommand();
+        command.CommandText = "SELECT $value::INTEGER";
+        command.Parameters.Add(new DuckDBParameter("value", 42));
+        command.UseStreamingMode = useStreamingMode;
+        command.Prepare();
+
+        command.ExecuteScalar().Should().Be(42);
+
+        command.Parameters["value"].Value = DBNull.Value;
+        command.ExecuteScalar().Should().Be(DBNull.Value);
+
+        command.CommandText = "SELECT 42 WHERE FALSE";
+        command.Parameters.Clear();
+        command.Prepare();
+        command.ExecuteScalar().Should().BeNull();
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void PreparedExecuteScalarHandlesRepresentativeReaderTypes(bool useStreamingMode)
+    {
+        using var command = Connection.CreateCommand();
+        command.UseStreamingMode = useStreamingMode;
+
+        command.CommandText = "SELECT 'duckdb'::VARCHAR";
+        command.Prepare();
+        command.ExecuteScalar().Should().Be("duckdb");
+
+        command.CommandText = "SELECT DATE '2026-07-21'";
+        command.Prepare();
+        command.ExecuteScalar().Should().Be(new DateOnly(2026, 7, 21));
+
+        command.CommandText = "SELECT [1, 2, 3]::INTEGER[]";
+        command.Prepare();
+        command.ExecuteScalar().Should().BeEquivalentTo(new[] { 1, 2, 3 });
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void PreparedExecuteScalarCanBeReusedAfterMaterializationFailure(bool useStreamingMode)
+    {
+        using var command = Connection.CreateCommand();
+        command.CommandText = "SELECT CASE WHEN $infinite THEN DATE 'infinity' ELSE DATE '2026-07-21' END";
+        command.Parameters.Add(new DuckDBParameter("infinite", true));
+        command.UseStreamingMode = useStreamingMode;
+        command.Prepare();
+
+        command.Invoking(value => value.ExecuteScalar())
+            .Should().Throw<InvalidOperationException>()
+            .WithMessage("Cannot convert infinite date value*");
+
+        command.Parameters["infinite"].Value = false;
+        command.ExecuteScalar().Should().Be(new DateOnly(2026, 7, 21));
+    }
+
+    [Fact]
+    public void PreparedExecuteScalarReturnsNullForNonQueryStatement()
+    {
+        using var connection = new DuckDBConnection("DataSource=:memory:");
+        connection.Open();
+
+        using var command = connection.CreateCommand();
+        command.CommandText = "CREATE TABLE prepared_scalar(value INTEGER)";
+        command.Prepare();
+
+        command.ExecuteScalar().Should().BeNull();
+    }
+
     [Fact]
     public void PreparedCommandClearsBindingsBeforeReuse()
     {