Revert "Parallelize knn query rewrite across slices rather than segments (#12325)" (#12385)

This reverts commit 10bebde269.

Based on a recent discussion in
https://github.com/apache/lucene/pull/12183#discussion_r1235739084 we
agreed it makes more sense to parallelize knn query vector rewrite
across leaves rather than leaf slices.

lucene/core/src/java/org/apache/lucene/search/AbstractKnnVectorQuery.java

@@ -19,12 +19,12 @@ package org.apache.lucene.search;
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executor;
 import java.util.concurrent.FutureTask;
 import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.index.FieldInfo;
@@ -81,12 +81,11 @@ abstract class AbstractKnnVectorQuery extends Query {
       filterWeight = null;
     }
 
-    SliceExecutor sliceExecutor = indexSearcher.getSliceExecutor();
-    // in case of parallel execution, the leaf results are not ordered by leaf context's ordinal
+    Executor executor = indexSearcher.getExecutor();
     TopDocs[] perLeafResults =
-        (sliceExecutor == null)
+        (executor == null)
             ? sequentialSearch(reader.leaves(), filterWeight)
-            : parallelSearch(indexSearcher.getSlices(), filterWeight, sliceExecutor);
+            : parallelSearch(reader.leaves(), filterWeight, executor);
 
     // Merge sort the results
     TopDocs topK = TopDocs.merge(k, perLeafResults);
@@ -110,40 +109,27 @@ abstract class AbstractKnnVectorQuery extends Query {
   }
 
   private TopDocs[] parallelSearch(
-      IndexSearcher.LeafSlice[] slices, Weight filterWeight, SliceExecutor sliceExecutor) {
-
-    List<FutureTask<TopDocs[]>> tasks = new ArrayList<>(slices.length);
-    int segmentsCount = 0;
-    for (IndexSearcher.LeafSlice slice : slices) {
-      segmentsCount += slice.leaves.length;
-      tasks.add(
-          new FutureTask<>(
-              () -> {
-                TopDocs[] results = new TopDocs[slice.leaves.length];
-                int i = 0;
-                for (LeafReaderContext context : slice.leaves) {
-                  results[i++] = searchLeaf(context, filterWeight);
-                }
-                return results;
-              }));
-    }
+      List<LeafReaderContext> leafReaderContexts, Weight filterWeight, Executor executor) {
+    List<FutureTask<TopDocs>> tasks =
+        leafReaderContexts.stream()
+            .map(ctx -> new FutureTask<>(() -> searchLeaf(ctx, filterWeight)))
+            .toList();
 
+    SliceExecutor sliceExecutor = new SliceExecutor(executor);
     sliceExecutor.invokeAll(tasks);
 
-    TopDocs[] topDocs = new TopDocs[segmentsCount];
-    int i = 0;
-    for (FutureTask<TopDocs[]> task : tasks) {
-      try {
-        for (TopDocs docs : task.get()) {
-          topDocs[i++] = docs;
-        }
-      } catch (ExecutionException e) {
-        throw new RuntimeException(e.getCause());
-      } catch (InterruptedException e) {
-        throw new ThreadInterruptedException(e);
-      }
-    }
-    return topDocs;
+    return tasks.stream()
+        .map(
+            task -> {
+              try {
+                return task.get();
+              } catch (ExecutionException e) {
+                throw new RuntimeException(e.getCause());
+              } catch (InterruptedException e) {
+                throw new ThreadInterruptedException(e);
+              }
+            })
+        .toArray(TopDocs[]::new);
   }
 
   private TopDocs searchLeaf(LeafReaderContext ctx, Weight filterWeight) throws IOException {

lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java

@@ -962,10 +962,6 @@ public class IndexSearcher {
     return executor;
   }
 
-  SliceExecutor getSliceExecutor() {
-    return sliceExecutor;
-  }
-
   /**
    * Thrown when an attempt is made to add more than {@link #getMaxClauseCount()} clauses. This
    * typically happens if a PrefixQuery, FuzzyQuery, WildcardQuery, or TermRangeQuery is expanded to