improve comments

Author: alamb

datafusion-examples/examples/parquet_index.rs

@@ -113,6 +113,7 @@ async fn main() -> Result<()> {
         .await?
         .show()
         .await?;
+    println!("Files pruned: {}", provider.index().last_num_pruned());
 
     // Run a query that uses the index to prune files.
     //
@@ -221,8 +222,12 @@ impl TableProvider for IndexTableProvider {
             // that always evaluates to true we can pass to the index
             .unwrap_or_else(|| datafusion_physical_expr::expressions::lit(true));
 
-        // Use the index to find the files that might have data that matches the predicate.
+        // Use the index to find the files that might have data that matches the
+        // predicate. Any file that can not have data that matches the predicate
+        // will not be returned.
         let files = self.index.get_files(predicate.clone())?;
+
+        // Transform to the format needed to pass to ParquetExec
         // Create one file group per file (default to scanning them all in parallel)
         let file_groups = files
             .into_iter()
@@ -534,8 +539,8 @@ impl ParquetMetadataIndexBuilder {
         assert_eq!(value_column_mins.null_count(), 0);
         assert_eq!(value_column_maxes.null_count(), 0);
 
-        // compute the total row count, and overall min and max of the "value"
-        // column in this file
+        // The statistics above are one for row group so we need to compute the
+        // overall file row count, and min and max .
         let row_count = row_counts
             .iter()
             .flatten() // skip nulls (should be none)