perf: add parquetReadMultiColumn benchmark, update README with full comparison

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

README.md

@@ -283,11 +283,12 @@ JMH throughput (ops/s = full-file scans per second). Higher is better.
 Source: 47.6 MB Parquet → 12.0 MB Vortex (4× compression). Both sides scalar decode
 (Hardwood disables SIMD on JDK 25; Vortex Java uses FFM scalar reads throughout).
 
-| Benchmark | ops/s | vs Parquet |
-|---|---|---|
-| `parquetRead` — Hardwood, single col (`trip_distance`) | 66.96 ± 1.99 | baseline |
-| `vortexRead` — single col (`trip_distance`) | 201.61 ± 1.63 | **3.0×** |
-| `vortexReadMultiColumn` — two cols (`fare_amount` + `PULocationID`) | 140.31 ± 5.42 | **2.1×** |
+| Benchmark | ops/s | ms/scan | vs Parquet |
+|---|---|---|---|
+| `parquetRead` — Hardwood, 1 col (`trip_distance`) | 66.96 ± 1.99 | 14.9 ms | baseline |
+| `vortexRead` — 1 col (`trip_distance`) | 201.61 ± 1.63 | 4.96 ms | **3.0×** |
+| `parquetReadMultiColumn` — 2 cols (`fare_amount`, `PULocationID`) | 53.07 ± 1.25 | 18.8 ms | baseline |
+| `vortexReadMultiColumn` — 2 cols (`fare_amount`, `PULocationID`) | 140.31 ± 5.42 | 7.13 ms | **2.6×** |
 
 ### Why fewer layers = faster
 

performance/src/main/java/io/github/dfa1/vortex/performance/ParquetVsVortexReadBenchmark.java

@@ -111,11 +111,11 @@ public void tearDown() throws IOException {
         }
     }
 
-    /// Hardwood: scan all rows, sum trip_distance.
+    /// Hardwood: scan all rows, sum trip_distance (single column).
     @Benchmark
     public double parquetRead() throws IOException {
         double sum = 0.0;
-        ColumnProjection projection = ColumnProjection.columns("trip_distance", "fare_amount", "PULocationID");
+        ColumnProjection projection = ColumnProjection.columns("trip_distance");
         try (ParquetFileReader reader = ParquetFileReader.open(InputFile.of(parquetFile));
              RowReader rows = reader.buildRowReader().projection(projection).build()) {
             while (rows.hasNext()) {
@@ -126,6 +126,23 @@ public double parquetRead() throws IOException {
         return sum;
     }
 
+    /// Hardwood: scan all rows, sum fare_amount + PULocationID (two columns).
+    @Benchmark
+    public double parquetReadMultiColumn() throws IOException {
+        double fareSum = 0.0;
+        long idSum = 0L;
+        ColumnProjection projection = ColumnProjection.columns("fare_amount", "PULocationID");
+        try (ParquetFileReader reader = ParquetFileReader.open(InputFile.of(parquetFile));
+             RowReader rows = reader.buildRowReader().projection(projection).build()) {
+            while (rows.hasNext()) {
+                rows.next();
+                fareSum += rows.getDouble("fare_amount");
+                idSum += rows.getInt("PULocationID");
+            }
+        }
+        return fareSum + idSum;
+    }
+
     /// Vortex: scan all rows, sum trip_distance via fold.
     @Benchmark
     public double vortexRead() throws IOException {