chore: add some comments.

Author: youngsofun

src/query/storages/delta/src/table.rs

@@ -73,6 +73,17 @@ pub struct DeltaTableMeta {
     partition_columns: Vec<String>,
 }
 
+/// In a delta table, partition columns are not stored in parquet file.
+/// so it needs a few efforts to make pushdown work:
+/// - context:
+///   - Table store partition column names in meta.engine_options.
+///   - Each partition carries all partition column values in the same order.
+///   - With this order, we can get need info with a PartitionIndex.
+/// - pushdown:
+///   - projections (mask): partition columns are excluded when read parquet file and inserted at last.
+///   - filter pass to parquet reader: all partition columns are appended to the filter input columns.
+///   - pruner: ColumnRef of partition columns in filter expr are replace with const scalars.
+/// Type of partition columns can only be simple primitive types.
 impl DeltaTable {
     #[async_backtrace::framed]
     pub fn try_create(info: TableInfo) -> Result<Box<dyn Table>> {

src/query/storages/parquet/src/parquet_rs/parquet_reader/reader/full_reader.rs

@@ -68,6 +68,7 @@ pub struct ParquetRSFullReader {
 }
 
 impl ParquetRSFullReader {
+    // partition_fields is only used for delta table engine.
     pub async fn prepare_data_stream(
         &self,
         loc: &str,

src/query/storages/parquet/src/parquet_rs/pruning.rs

@@ -114,6 +114,8 @@ impl ParquetRSPruner {
     /// Return the selected row groups' indices in the meta and omit filter flags.
     ///
     /// If `stats` is not [None], we use this statistics to prune but not collect again.
+    ///
+    /// `partition_values` is used only for Delta table engine.
     pub fn prune_row_groups(
         &self,
         meta: &ParquetMetaData,
@@ -178,6 +180,7 @@ impl ParquetRSPruner {
     /// Prune pages of a parquet file.
     ///
     /// Return a vector of [`RowSelection`] to represent rows to read.
+    /// `partition_values` is used only for Delta table engine.
     pub fn prune_pages(
         &self,
         meta: &ParquetMetaData,