add comments and doc

docs/en/operations/system-tables/tables.md

@@ -57,6 +57,8 @@ Columns:
     - If the table stores data on disk, returns used space on disk (i.e. compressed).
     - If the table stores data in memory, returns approximated number of used bytes in memory.
 
+- `total_bytes_uncompressed` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) - Total number of uncompressed bytes, if it is possible to quickly determine exact number of bytes from checksums for the table on storage, otherwise `NULL` (does not includes any underlying storage).
+
 - `lifetime_rows` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) - Total number of rows INSERTed since server start (only for `Buffer` tables).
 
 - `lifetime_bytes` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) - Total number of bytes INSERTed since server start (only for `Buffer` tables).

src/Storages/IStorage.h

@@ -694,6 +694,8 @@ public:
     ///
     /// Used for:
     /// - For total_bytes_uncompressed column in system.tables
+    ///
+    /// Does not takes underlying Storage (if any) into account
     virtual std::optional<UInt64> totalBytesUncompressed(const Settings &) const { return {}; } 
 
     /// Number of rows INSERTed since server start.

src/Storages/MergeTree/MergeTreeDataPartChecksum.cpp

@@ -1,6 +1,5 @@
 #include "MergeTreeDataPartChecksum.h"
 #include <Common/SipHash.h>
-#include "base/types.h"
 #include <base/hex.h>
 #include <IO/ReadHelpers.h>
 #include <IO/WriteHelpers.h>

src/Storages/StorageReplicatedMergeTree.h

@@ -2,7 +2,6 @@
 
 #include <base/UUID.h>
 #include <atomic>
-#include <optional>
 #include <pcg_random.hpp>
 #include <Storages/IStorage.h>
 #include <Storages/MergeTree/AsyncBlockIDsCache.h>