diff --git a/src/Storages/Statistic/Estimator.h b/src/Storages/Statistic/Estimator.h
index a52351f4879..53ea46cbfd5 100644
--- a/src/Storages/Statistic/Estimator.h
+++ b/src/Storages/Statistic/Estimator.h
@@ -7,6 +7,7 @@ namespace DB
 
 class RPNBuilderTreeNode;
 
+/// It estimates the selectivity of a condition.
 class ConditionEstimator
 {
 private:
@@ -20,11 +21,13 @@ private:
 
     UInt64 total_count = 0;
 
+    /// Minimum estimator for values in a part. It can contains multiple types of statistics.
+    /// But right now we only have tdigest;
     struct PartColumnEstimator
     {
         UInt64 part_count = 0;
 
-        std::shared_ptr<TDigestStatistic> t_digest;
+        std::shared_ptr<TDigestStatistic> tdigest;
 
         void merge(StatisticPtr statistic)
         {
@@ -34,25 +37,27 @@ private:
 
             if (typeid_cast<TDigestStatistic *>(statistic.get()))
             {
-                t_digest = std::static_pointer_cast<TDigestStatistic>(statistic);
+                tdigest = std::static_pointer_cast<TDigestStatistic>(statistic);
             }
         }
 
         Float64 estimateLess(Float64 val) const
         {
-            if (t_digest != nullptr)
-                return t_digest -> estimateLess(val);
+            if (tdigest != nullptr)
+                return tdigest -> estimateLess(val);
             return part_count * default_normal_cond_factor;
         }
 
         Float64 estimateGreator(Float64 val) const
         {
-            if (t_digest != nullptr)
-                return part_count - t_digest -> estimateLess(val);
+            if (tdigest != nullptr)
+                return part_count - tdigest -> estimateLess(val);
             return part_count * default_normal_cond_factor;
         }
     };
 
+    /// An estimator for a column consists of several PartColumnEstimator.
+    /// We simply get selectivity for every part estimator and combine the result.
     struct ColumnEstimator
     {
         std::map<std::string, PartColumnEstimator> estimators;
diff --git a/src/Storages/Statistic/Statistic.h b/src/Storages/Statistic/Statistic.h
index 0bb416b9feb..f474fffafa5 100644
--- a/src/Storages/Statistic/Statistic.h
+++ b/src/Storages/Statistic/Statistic.h
@@ -23,7 +23,7 @@ class IStatistic;
 using StatisticPtr = std::shared_ptr<IStatistic>;
 using Statistics = std::vector<StatisticPtr>;
 
-/// Statistic for a column
+/// Statistic contains the distribution of values in a column.
 /// right now we support
 /// - tdigest
 class IStatistic
@@ -60,6 +60,7 @@ protected:
 
 };
 
+/// TDigestStatistic is a kind of histogram.
 class TDigestStatistic : public IStatistic
 {
     QuantileTDigest<Float64> data;
diff --git a/tests/queries/0_stateless/00002_log_and_exception_messages_formatting.sql b/tests/queries/0_stateless/00002_log_and_exception_messages_formatting.sql
index aa7603f8c35..38626fc2928 100644
--- a/tests/queries/0_stateless/00002_log_and_exception_messages_formatting.sql
+++ b/tests/queries/0_stateless/00002_log_and_exception_messages_formatting.sql
@@ -45,7 +45,7 @@ create temporary table known_short_messages (s String) as select * from (select
 'Unknown identifier: ''{}''', 'User name is empty', 'Expected function, got: {}',
 'Attempt to read after eof', 'String size is too big ({}), maximum: {}',
 'Processed: {}%', 'Creating {}: {}', 'Table {}.{} doesn''t exist', 'Invalid cache key hex: {}',
-'User has been dropped', 'Illegal type {} of argument of function {}. Should be DateTime or DateTime64'
+'User has been dropped', 'Illegal type {} of argument of function {}. Should be DateTime or DateTime64',
 'Unknown statistic column: {}'
 ] as arr) array join arr;