[#24069] DocDB: Merge function update to take vector of indexes as input

Summary:
This diff continues work started at D39478.
It makes Merge function able to take vector of indexes as input

Jira:DB-12962

Test Plan: Jenkins

Reviewers: sergei, arybochkin

Reviewed By: arybochkin

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D40718

src/yb/vector_index/index_merge-test.cc

@@ -84,7 +84,7 @@ class IndexMergeTest : public YBTest {
     auto data_b = CreateAndFillIndex(index_factory, half_size, half_size);
 
     VectorIndexIfPtr<FloatVector, float> merged_index =
-      ASSERT_RESULT(Merge(index_factory, data_a.index, data_b.index));
+      ASSERT_RESULT(Merge(index_factory, {data_a.index, data_b.index}));
 
     // Check that the merged index contains all entries.
     auto result_a = ASSERT_RESULT(merged_index->Search(input_vectors_[0], 1));
@@ -112,8 +112,8 @@ class IndexMergeTest : public YBTest {
     // Generate indexes for the input set.
     auto data_a = CreateAndFillIndex(index_factory, 0, input_vectors_.size() / 2);
 
-    // Merge empty_index into data_a.
-    auto merged_index = ASSERT_RESULT(Merge(index_factory, data_a.index, empty_index));
+    // Merge empty_index with data_a.
+    auto merged_index = ASSERT_RESULT(Merge(index_factory, {data_a.index, empty_index}));
 
     // Check that the merged index contains only the entries from data_a.
     auto all_results = ASSERT_RESULT(merged_index->Search(

src/yb/vector_index/vectorann_util.h

@@ -120,23 +120,21 @@ std::vector<VertexWithDistance<DistanceResult>> BruteForcePreciseNearestNeighbor
 template <IndexableVectorType Vector, ValidDistanceResultType DistanceResult>
 Result<VectorIndexIfPtr<Vector, DistanceResult>> Merge(
     VectorIndexFactory<Vector, DistanceResult> index_factory,
-    VectorIndexIfPtr<Vector, DistanceResult> index_a,
-    VectorIndexIfPtr<Vector, DistanceResult> index_b) {
+    const std::vector<VectorIndexIfPtr<Vector, DistanceResult>>& indexes) {
   VectorIndexIfPtr<Vector, DistanceResult> merged_index = index_factory();
 
-  size_t max_vectors_a = index_a->MaxVectors();
-  size_t max_vectors_b = index_b->MaxVectors();
+  size_t total_max_vectors = 0;
+  for (const auto& index : indexes) {
+    total_max_vectors += index->MaxVectors();
+  }
 
   RETURN_NOT_OK(merged_index->Reserve(
-      max_vectors_a + max_vectors_b, std::thread::hardware_concurrency(),
-      std::thread::hardware_concurrency()));
-
-  for (const auto& [vertex_id, vector] : *index_a) {
-    RETURN_NOT_OK(merged_index->Insert(vertex_id, vector));
-  }
+      total_max_vectors, std::thread::hardware_concurrency(), std::thread::hardware_concurrency()));
 
-  for (const auto& [vertex_id, vector] : *index_b) {
-    RETURN_NOT_OK(merged_index->Insert(vertex_id, vector));
+  for (const auto& index : indexes) {
+    for (const auto& [vertex_id, vector] : *index) {
+      RETURN_NOT_OK(merged_index->Insert(vertex_id, vector));
+    }
   }
 
   return merged_index;