Update index_sorting.md

doc/src/index_sorting.md

@@ -1,9 +1,10 @@
 
 - [Index Sorting](#index-sorting)
-    + [Motivation](#motivation)
+    + [Why Sorting](#why-sorting)
         * [Compression](#compression)
         * [Top-N Optimization](#top-n-optimization)
         * [Pruning](#pruning)
+        * [Other](#other)
     + [Usage](#usage)
 
 # Index Sorting
@@ -32,8 +33,12 @@ Let's say we want all documents and want to apply the filter `>= 2010-08-11`. Wh
 
 Note: Tantivy 0.16 does not do this optimization yet.
 
+###### Other?
+
+In principle there are many algorithms possible that exploit the monotonically increasing nature. (aggregations maybe?)
+
 ## Usage
-The index sorting can be configured setting `sort_by_field` on `IndexSettings` and passing it to a `IndexBuilder`. As of tantvy 0.16 only fast fields are allowed to be used.
+The index sorting can be configured setting [`sort_by_field`](https://github.com/tantivy-search/tantivy/blob/000d76b11a139a84b16b9b95060a1c93e8b9851c/src/core/index_meta.rs#L238) on `IndexSettings` and passing it to a `IndexBuilder`. As of tantvy 0.16 only fast fields are allowed to be used.
 
 ```
 let settings = IndexSettings {
@@ -48,3 +53,9 @@ index_builder = index_builder.settings(settings);
 let index = index_builder.create_in_ram().unwrap();
 ```
 
+## Implementation details
+
+Sorting an index is applied in the serialization step. In general there are two serialization steps: [Finishing a single segment](https://github.com/tantivy-search/tantivy/blob/000d76b11a139a84b16b9b95060a1c93e8b9851c/src/indexer/segment_writer.rs#L338) and [merging multiple segments](https://github.com/tantivy-search/tantivy/blob/000d76b11a139a84b16b9b95060a1c93e8b9851c/src/indexer/merger.rs#L1073).
+
+In both cases we generate a docid mapping reflecting the sort. This mapping is used when serializing the different components (doc store, fastfields, posting list, normfield, facets).
+