add tests and comments

src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp

@@ -373,6 +373,8 @@ bool MergeTreeDataMergerMutator::selectAllPartsToMergeWithinPartition(
         return false;
     }
 
+    /// If final, optimize_skip_merged_partitions is true and we have only one part in partition with level > 0
+    /// than we don't select it to merge
     if (final && data.getSettings()->optimize_skip_merged_partitions && parts.size() == 1 && parts[0]->info.level > 0)
     {
         *is_single_merged_part = true;
@@ -637,6 +639,8 @@ MergeTreeData::MutableDataPartPtr MergeTreeDataMergerMutator::mergePartsToTempor
 {
     static const String TMP_PREFIX = "tmp_merge_";
 
+
+
     if (merges_blocker.isCancelled())
         throw Exception("Cancelled merging parts", ErrorCodes::ABORTED);
 

src/Storages/MergeTree/MergeTreeDataMergerMutator.h

@@ -88,7 +88,9 @@ public:
         String * out_disable_reason = nullptr);
 
     /** Select all the parts in the specified partition for merge, if possible.
-      * final - choose to merge even a single part - that is, allow to merge one part "with itself".
+      * final - choose to merge even a single part - that is, allow to merge one part "with itself",
+      * but if setting optimize_skip_merged_partitions is true (it's true as default) than single part with level > 0
+      * won't be merged with itself.
       */
     bool selectAllPartsToMergeWithinPartition(
         FutureMergedMutatedPart & future_part,

src/Storages/StorageMergeTree.cpp

@@ -654,6 +654,9 @@ bool StorageMergeTree::merge(
         };
 
         bool selected = false;
+
+        /// This flag is true when there is only one part in partition, it's level > 0
+        /// and setting optimize_skip_merged_partitions is true
         bool is_single_merged_part = false;
 
         if (partition_id.empty())
@@ -710,6 +713,7 @@ bool StorageMergeTree::merge(
 
         if (!selected)
         {
+            /// If is_single_merged_part is true we treat this part as already merged
             if (final && is_single_merged_part)
             {
                 return true;

src/Storages/StorageReplicatedMergeTree.cpp

@@ -3761,12 +3761,16 @@ bool StorageReplicatedMergeTree::optimize(
                     ReplicatedMergeTreeMergePredicate can_merge = queue.getMergePredicate(zookeeper);
 
                     FutureMergedMutatedPart future_merged_part;
+
+                    /// This flag is true when there is only one part in partition, it's level > 0
+                    /// and setting optimize_skip_merged_partitions is true
                     bool is_single_merged_part = false;
                     bool selected = merger_mutator.selectAllPartsToMergeWithinPartition(
                         future_merged_part, disk_space, can_merge, partition_id, true, &is_single_merged_part, nullptr);
 
                     if (!selected)
                     {
+                        /// If is_single_merged_part is true we treat this part as already merged
                         if (is_single_merged_part)
                             return true;
                         break;
@@ -3803,6 +3807,9 @@ bool StorageReplicatedMergeTree::optimize(
                 FutureMergedMutatedPart future_merged_part;
                 String disable_reason;
                 bool selected = false;
+
+                /// This flag is true when there is only one part in partition, it's level > 0
+                /// and setting optimize_skip_merged_partitions is true
                 bool is_single_merged_part = false;
                 if (!partition)
                 {
@@ -3819,6 +3826,7 @@ bool StorageReplicatedMergeTree::optimize(
 
                 if (!selected)
                 {
+                    /// If is_single_merged_part is true we treat this part as already merged
                     if (final && is_single_merged_part)
                         return true;
                     std::stringstream message;

tests/queries/0_stateless/01533_optimize_skip_merged_partitions.reference

@@ -0,0 +1,2 @@
+optimize_final	200001	1	1
+optimize_final	202001	1	1

tests/queries/0_stateless/01533_optimize_skip_merged_partitions.sql

@@ -0,0 +1,18 @@
+DROP TABLE IF EXISTS optimize_final;
+
+CREATE TABLE optimize_final(t DateTime, x Int32) ENGINE = MergeTree() PARTITION BY toYYYYMM(t) ORDER BY x;
+
+INSERT INTO optimize_final SELECT toDate('2000-01-01'), number FROM numbers(5);
+INSERT INTO optimize_final SELECT toDate('2000-01-01'), number + 5 FROM numbers(5);
+
+OPTIMIZE TABLE optimize_final FINAL;
+
+INSERT INTO optimize_final SELECT toDate('2020-01-01'), number FROM numbers(5);
+INSERT INTO optimize_final SELECT toDate('2020-01-01'), number + 5 FROM numbers(5);
+
+OPTIMIZE TABLE optimize_final FINAL;
+
+SELECT table, partition, active, level from system.parts where table = 'optimize_final' and active = 1;
+
+DROP TABLE optimize_final;
+