[GitHub] [lucene] mayya-sharipova commented on a change in pull request #204: LUCENE-10020 DocComparator don't skip docs of same docID

[GitBox <gi...@apache.org>]

mayya-sharipova commented on a change in pull request #204:
URL: https://github.com/apache/lucene/pull/204#discussion_r664573881



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File path: lucene/core/src/java/org/apache/lucene/search/comparators/DocComparator.java
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@@ -81,7 +87,12 @@ public Integer value(int slot) {
     public DocLeafComparator(LeafReaderContext context) {
       this.docBase = context.docBase;
       if (enableSkipping) {
-        this.minDoc = topValue + 1;
+        // For a single sort on _doc, we want to skip all docs before topValue.
+        // For multiple fields sort on [_doc, other fields], we want to include docs with the same
+        // docID.
+        // This is needed in a distributed search, where there are docs from different indices with
+        // the same docID.
+        this.minDoc = singleSort ? topValue + 1 : topValue;

Review comment:
       Great comment! +1 for simplifying the code at the expense for extra single case in `DocComparator`.  
   
   For `NumericComparator` though this is not the case, and there could be huge number of docs with the same value, so extra optimization for `singleSort` is important.
   
   > I guess it doesn't specifically address the case where _doc is the last sort, for example a sort on ["some_field", "_doc"], where we could also use topValue + 1.
   
   No, the sort optimizations in `DocComparator` are not applicable where `_doc` is the 1st sort. 




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