diff --git a/datafusion/datasource-parquet/src/opener.rs b/datafusion/datasource-parquet/src/opener.rs
index 83bdf79c8fcc0..570f9b4412840 100644
--- a/datafusion/datasource-parquet/src/opener.rs
+++ b/datafusion/datasource-parquet/src/opener.rs
@@ -180,6 +180,9 @@ impl PreparedAccessPlan {
 
 impl FileOpener for ParquetOpener {
     fn open(&self, partitioned_file: PartitionedFile) -> Result<FileOpenFuture> {
+        // -----------------------------------
+        // Step: prepare configurations, etc.
+        // -----------------------------------
         let file_range = partitioned_file.range.clone();
         let extensions = partitioned_file.extensions.clone();
         let file_location = partitioned_file.object_meta.location.clone();
@@ -280,6 +283,10 @@ impl FileOpener for ParquetOpener {
                 .get_file_decryption_properties(&file_location)
                 .await?;
 
+            // ---------------------------------------------
+            // Step: try to prune the current file partition
+            // ---------------------------------------------
+
             // Prune this file using the file level statistics and partition values.
             // Since dynamic filters may have been updated since planning it is possible that we are able
             // to prune files now that we couldn't prune at planning time.
@@ -328,6 +335,10 @@ impl FileOpener for ParquetOpener {
 
             file_metrics.files_ranges_pruned_statistics.add_matched(1);
 
+            // --------------------------------------------------------
+            // Step: fetch Parquet metadata (and optionally page index)
+            // --------------------------------------------------------
+
             // Don't load the page index yet. Since it is not stored inline in
             // the footer, loading the page index if it is not needed will do
             // unnecessary I/O. We decide later if it is needed to evaluate the
@@ -428,14 +439,21 @@ impl FileOpener for ParquetOpener {
 
             metadata_timer.stop();
 
+            // ---------------------------------------------------------
+            // Step: construct builder for the final RecordBatch stream
+            // ---------------------------------------------------------
+
             let mut builder = ParquetRecordBatchStreamBuilder::new_with_metadata(
                 async_file_reader,
                 reader_metadata,
             );
 
-            let indices = projection.column_indices();
-
-            let mask = ProjectionMask::roots(builder.parquet_schema(), indices);
+            // ---------------------------------------------------------------------
+            // Step: optionally add row filter to the builder
+            //
+            // Row filter is used for late materialization in parquet decoding, see
+            // `row_filter` for details.
+            // ---------------------------------------------------------------------
 
             // Filter pushdown: evaluate predicates during scan
             if let Some(predicate) = pushdown_filters.then_some(predicate).flatten() {
@@ -464,6 +482,10 @@ impl FileOpener for ParquetOpener {
                     builder.with_row_selection_policy(RowSelectionPolicy::Selectors);
             }
 
+            // ------------------------------------------------------------
+            // Step: prune row groups by range, predicate and bloom filter
+            // ------------------------------------------------------------
+
             // Determine which row groups to actually read. The idea is to skip
             // as many row groups as possible based on the metadata and query
             let file_metadata = Arc::clone(builder.metadata());
@@ -525,9 +547,13 @@ impl FileOpener for ParquetOpener {
 
             let mut access_plan = row_groups.build();
 
+            // --------------------------------------------------------
+            // Step: prune pages from the kept row groups
+            //
             // page index pruning: if all data on individual pages can
             // be ruled using page metadata, rows from other columns
             // with that range can be skipped as well
+            // --------------------------------------------------------
             if enable_page_index
                 && !access_plan.is_empty()
                 && let Some(p) = page_pruning_predicate
@@ -545,7 +571,10 @@ impl FileOpener for ParquetOpener {
             let mut prepared_plan =
                 PreparedAccessPlan::from_access_plan(access_plan, rg_metadata)?;
 
-            // If reverse scanning is enabled, reverse the prepared plan
+            // ----------------------------------------------------------
+            // Step: potentially reverse the access plan for performance.
+            // See `ParquetSource::try_reverse_output` for the rationale.
+            // ----------------------------------------------------------
             if reverse_row_groups {
                 prepared_plan = prepared_plan.reverse(file_metadata.as_ref())?;
             }
@@ -564,6 +593,9 @@ impl FileOpener for ParquetOpener {
             // metrics from the arrow reader itself
             let arrow_reader_metrics = ArrowReaderMetrics::enabled();
 
+            let indices = projection.column_indices();
+            let mask = ProjectionMask::roots(builder.parquet_schema(), indices);
+
             let stream = builder
                 .with_projection(mask)
                 .with_batch_size(batch_size)
@@ -621,6 +653,9 @@ impl FileOpener for ParquetOpener {
                 })
             });
 
+            // ----------------------------------------------------------------------
+            // Step: wrap the stream so a dynamic filter can stop the file scan early
+            // ----------------------------------------------------------------------
             if let Some(file_pruner) = file_pruner {
                 Ok(EarlyStoppingStream::new(
                     stream,