cache store readers to utilize lru cache (4x performance)

cache store readers, to utilize lru cache (4x faster performance, due to less decompress calls on the block)

src/indexer/merger.rs

@@ -915,9 +915,14 @@ impl IndexMerger {
         store_writer: &mut StoreWriter,
         doc_id_mapping: &Option<Vec<(DocId, SegmentReaderWithOrdinal)>>,
     ) -> crate::Result<()> {
+        let store_readers: Vec<_> = self
+            .readers
+            .iter()
+            .map(|reader| reader.get_store_reader())
+            .collect::<Result<_, _>>()?;
         if let Some(doc_id_mapping) = doc_id_mapping {
             for (old_doc_id, reader_with_ordinal) in doc_id_mapping {
-                let store_reader = reader_with_ordinal.reader.get_store_reader()?;
+                let store_reader = &store_readers[reader_with_ordinal.ordinal as usize];
                 let raw_doc = store_reader.get_raw(*old_doc_id)?;
                 store_writer.store_bytes(raw_doc.get_bytes())?;
             }

src/indexer/merger_sorted_index_test.rs

@@ -62,6 +62,7 @@ mod tests {
         let mut schema_builder = schema::Schema::builder();
         let int_options = IntOptions::default()
             .set_fast(Cardinality::SingleValue)
+            .set_stored()
             .set_indexed();
         let int_field = schema_builder.add_u64_field("intval", int_options);
 
@@ -201,6 +202,8 @@ mod tests {
                 doc.get_first(my_text_field).unwrap().text(),
                 Some("blubber")
             );
+            let doc = searcher.doc(DocAddress::new(0, 0)).unwrap();
+            assert_eq!(doc.get_first(int_field).unwrap().u64_value(), Some(1000));
         }
     }
 

src/indexer/segment_updater.rs

@@ -521,8 +521,11 @@ impl SegmentUpdater {
             }
         });
 
-        Ok(merging_future_recv
-            .unwrap_or_else(|e| Err(crate::TantivyError::SystemError("Merge failed:".to_string() + &e.to_string()))))
+        Ok(merging_future_recv.unwrap_or_else(|e| {
+            Err(crate::TantivyError::SystemError(
+                "Merge failed:".to_string() + &e.to_string(),
+            ))
+        }))
     }
 
     async fn consider_merge_options(&self) {

src/store/reader.rs

@@ -88,6 +88,7 @@ impl StoreReader {
     /// Calling `.get(doc)` is relatively costly as it requires
     /// decompressing a compressed block. The store utilizes a LRU cache,
     /// so accessing docs from the same compressed block should be faster.
+    /// For that reason a store reader should be kept and reused.
     ///
     /// It should not be called to score documents
     /// for instance.
@@ -103,6 +104,7 @@ impl StoreReader {
     /// Calling `.get(doc)` is relatively costly as it requires
     /// decompressing a compressed block. The store utilizes a LRU cache,
     /// so accessing docs from the same compressed block should be faster.
+    /// For that reason a store reader should be kept and reused.
     ///
     pub fn get_raw(&self, doc_id: DocId) -> crate::Result<RawDocument> {
         let checkpoint = self.block_checkpoint(doc_id).ok_or_else(|| {