More comments.

dbms/src/Functions/IFunction.cpp

@@ -42,6 +42,9 @@ namespace ErrorCodes
 }
 
 
+/// Cache for functions result if it was executed on low cardinality column.
+/// It's LRUCache which stores function result executed on dictionary and index mapping.
+/// It's expected that cache_size is a number of reading streams (so, will store single cached value per thread).
 class PreparedFunctionLowCardinalityResultCache
 {
 public:

dbms/src/Interpreters/ExpressionActions.h

@@ -89,8 +89,11 @@ public:
     ColumnPtr added_column;
 
     /// For APPLY_FUNCTION and LEFT ARRAY JOIN.
+    /// FunctionBuilder is used before action was added to ExpressionActions (when we don't know types of arguments).
     FunctionBuilderPtr function_builder;
+    /// Can be used after action was added to ExpressionActions if we want to get function signature or properties like monotonicity.
     FunctionBasePtr function_base;
+    /// Prepared function which is used in function execution.
     PreparedFunctionPtr function;
     Names argument_names;
     bool is_function_compiled = false;