diff --git a/dbms/src/Storages/MergeTree/MergeTreeData.h b/dbms/src/Storages/MergeTree/MergeTreeData.h
index 189c1b4618d..a01740e62a8 100644
--- a/dbms/src/Storages/MergeTree/MergeTreeData.h
+++ b/dbms/src/Storages/MergeTree/MergeTreeData.h
@@ -46,10 +46,10 @@ namespace ErrorCodes
 /// The date column is specified. For each part min and max dates are remembered.
 /// Essentially it is an index too.
 ///
-/// Data is partitioned by month. Parts belonging to different months are not merged - for the ease of
-/// administration (data sync and backup).
+/// Data is partitioned by the value of the partitioning expression.
+/// Parts belonging to different partitions are not merged - for the ease of administration (data sync and backup).
 ///
-/// File structure:
+/// File structure of old-style month-partitioned tables (format_version = 0):
 /// Part directory - / min-date _ max-date _ min-id _ max-id _ level /
 /// Inside the part directory:
 /// checksums.txt - contains the list of all files along with their sizes and checksums.
@@ -58,6 +58,13 @@ namespace ErrorCodes
 /// [Column].bin - contains compressed column data.
 /// [Column].mrk - marks, pointing to seek positions allowing to skip n * k rows.
 ///
+/// File structure of tables with custom partitioning (format_version >= 1):
+/// Part directory - / partiiton-id _ min-id _ max-id _ level /
+/// Inside the part directory:
+/// The same files as for month-partitioned tables, plus
+/// partition.dat - contains the value of the partitioning expression
+/// minmax_[Column].idx - MinMax indexes (see MergeTreeDataPart::MinMaxIndex class) for the columns required by the partitioning expression.
+///
 /// Several modes are implemented. Modes determine additional actions during merge:
 /// - Ordinary - don't do anything special
 /// - Collapsing - collapse pairs of rows with the opposite values of sign_columns for the same values
diff --git a/dbms/src/Storages/MergeTree/MergeTreeDataFormatVersion.h b/dbms/src/Storages/MergeTree/MergeTreeDataFormatVersion.h
index 74f26fcdf45..e08bfd44656 100644
--- a/dbms/src/Storages/MergeTree/MergeTreeDataFormatVersion.h
+++ b/dbms/src/Storages/MergeTree/MergeTreeDataFormatVersion.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <Core/Types.h>
 #include <common/strong_typedef.h>
 
 namespace DB
diff --git a/dbms/src/Storages/MergeTree/MergeTreePartition.cpp b/dbms/src/Storages/MergeTree/MergeTreePartition.cpp
index 4bf7bb352de..e33896a3cdf 100644
--- a/dbms/src/Storages/MergeTree/MergeTreePartition.cpp
+++ b/dbms/src/Storages/MergeTree/MergeTreePartition.cpp
@@ -19,17 +19,18 @@ static ReadBufferFromFile openForReading(const String & path)
     return ReadBufferFromFile(path, std::min(static_cast<Poco::File::FileSize>(DBMS_DEFAULT_BUFFER_SIZE), Poco::File(path).getSize()));
 }
 
+/// NOTE: This ID is used to create part names which are then persisted in ZK and as directory names on the file system.
+/// So if you want to change this method, be sure to guarantee compatibility with existing table data.
 String MergeTreePartition::getID(const MergeTreeData & storage) const
 {
     if (value.size() != storage.partition_expr_columns.size())
         throw Exception("Invalid partition key size: " + toString(value.size()), ErrorCodes::LOGICAL_ERROR);
 
     if (value.empty())
-        return "all";
+        return "all"; /// It is tempting to use an empty string here. But that would break directory structure in ZK.
 
-    /// In case all partition fields are represented by integral types, try to produce a human-readable partition id.
+    /// In case all partition fields are represented by integral types, try to produce a human-readable ID.
     /// Otherwise use a hex-encoded hash.
-
     bool are_all_integral = true;
     for (const Field & field : value)
     {
@@ -51,9 +52,12 @@ String MergeTreePartition::getID(const MergeTreeData & storage) const
                 result += '-';
 
             if (typeid_cast<const DataTypeDate *>(storage.partition_expr_column_types[i].get()))
-                result += toString(DateLUT::instance().toNumYYYYMMDD(DayNum_t(value[i].get<UInt64>())));
+                result += toString(DateLUT::instance().toNumYYYYMMDD(DayNum_t(value[i].safeGet<UInt64>())));
             else
                 result += applyVisitor(to_string_visitor, value[i]);
+
+            /// It is tempting to output DateTime as YYYYMMDDhhmmss, but that would make partition ID
+            /// timezone-dependent.
         }
 
         return result;
diff --git a/dbms/src/Storages/MergeTree/MergeTreePartition.h b/dbms/src/Storages/MergeTree/MergeTreePartition.h
index 65263520e83..d32b5f4401a 100644
--- a/dbms/src/Storages/MergeTree/MergeTreePartition.h
+++ b/dbms/src/Storages/MergeTree/MergeTreePartition.h
@@ -10,6 +10,7 @@ namespace DB
 class MergeTreeData;
 struct MergeTreeDataPartChecksums;
 
+/// This class represents a partition value of a single part and encapsulates its loading/storing logic.
 struct MergeTreePartition
 {
     Row value;