Add comment for used_flags in hash join, fix build

src/Common/ColumnsHashingImpl.h

@@ -87,6 +87,9 @@ public:
     bool isInserted() const { return inserted; }
 };
 
+/// FindResult optionally may contain pointer to value and offset in hashtable buffer.
+/// Only bool found is required.
+/// So we will have 4 different specializations for FindResultImpl
 class FindResultImplBase
 {
     bool found;
@@ -241,6 +244,8 @@ protected:
     {
         if constexpr (Cache::consecutive_keys_optimization)
         {
+            /// It's possible to support such combination, but code will became more complex.
+            /// Now there's not place where we need this options enabled together
             static_assert(!FindResult::has_offset, "`consecutive_keys_optimization` and `has_offset` are conflicting options");
             if (cache.check(key))
             {

src/Common/HashTable/FixedHashTable.h

@@ -476,6 +476,10 @@ public:
 
     size_t getBufferSizeInCells() const { return NUM_CELLS; }
 
+    /// Return offset for result in internal buffer.
+    /// Result can have value up to `getBufferSizeInCells() + 1`
+    /// because offset for zero value considered to be 0
+    /// and for other values it will be `offset in buffer + 1`
     size_t offsetInternal(ConstLookupResult ptr) const
     {
         if (ptr->isZero(*this))

src/Common/HashTable/HashTable.h

@@ -1214,6 +1214,10 @@ public:
         return grower.bufSize();
     }
 
+    /// Return offset for result in internal buffer.
+    /// Result can have value up to `getBufferSizeInCells() + 1`
+    /// because offset for zero value considered to be 0
+    /// and for other values it will be `offset in buffer + 1`
     size_t offsetInternal(ConstLookupResult ptr) const
     {
         if (ptr->isZero(*this))

src/Interpreters/HashJoin.cpp

@@ -57,6 +57,7 @@ struct NotProcessedCrossJoin : public ExtraBlock
 
 namespace JoinStuff
 {
+    /// Version of `getUsed` with dynamic dispatch
     bool JoinUsedFlags::getUsedSafe(size_t i) const
     {
         if (flags.empty())
@@ -576,6 +577,7 @@ namespace
             APPLY_FOR_JOIN_VARIANTS(M)
         #undef M
         }
+        __builtin_unreachable();
     }
 }
 

src/Interpreters/HashJoin.h

@@ -350,6 +350,9 @@ private:
 
     /// Right table data. StorageJoin shares it between many Join objects.
     std::shared_ptr<RightTableData> data;
+    /// Flags that indicate that particular row already used in join.
+    /// Flag is stored for every record in hash map.
+    /// Number of this flags equals to hashtable buffer size (plus one for zero value).
     mutable JoinStuff::JoinUsedFlags used_flags;
     Sizes key_sizes;