add setting aggregate_functions_null_for_empty

src/Core/Settings.h

@@ -400,7 +400,8 @@ class IColumn;
     M(Bool, optimize_trivial_insert_select, true, "Optimize trivial 'INSERT INTO table SELECT ... FROM TABLES' query", 0) \
     M(Bool, allow_experimental_database_atomic, true, "Obsolete setting, does nothing. Will be removed after 2021-02-12", 0) \
     M(Bool, allow_non_metadata_alters, true, "Allow to execute alters which affects not only tables metadata, but also data on disk", 0) \
-    M(Bool, enable_global_with_statement, false, "Propagate WITH statements to UNION queries and all subqueries", 0)
+    M(Bool, enable_global_with_statement, false, "Propagate WITH statements to UNION queries and all subqueries", 0) \
+    M(Bool, aggregate_functions_null_for_empty, false, "Rewrite all aggregate functions in a query, adding -OrNull suffix to them", 0)
 
 // End of COMMON_SETTINGS
 // Please add settings related to formats into the FORMAT_FACTORY_SETTINGS below.

src/Interpreters/TreeRewriter.cpp

@@ -31,6 +31,7 @@
 #include <IO/WriteHelpers.h>
 #include <Storages/IStorage.h>
 
+#include <AggregateFunctions/AggregateFunctionFactory.h>
 
 namespace DB
 {
@@ -110,6 +111,25 @@ struct CustomizeFunctionsSuffixData
 char ifDistinct[] = "ifdistinct";
 using CustomizeIfDistinctVisitor = InDepthNodeVisitor<OneTypeMatcher<CustomizeFunctionsSuffixData<ifDistinct>>, true>;
 
+/// Used to rewrite all aggregate functions to add -OrNull suffix to them if setting `aggregate_functions_null_for_empty` is set.
+struct CustomizeAggregateFunctionsSuffixData
+{
+    using TypeToVisit = ASTFunction;
+
+    const String & customized_func_suffix;
+
+    void visit(ASTFunction & func, ASTPtr &)
+    {
+        if (AggregateFunctionFactory::instance().isAggregateFunctionName(func.name)
+            && !endsWith(func.name, customized_func_suffix))
+        {
+            func.name = func.name + customized_func_suffix;
+        }
+    }
+};
+
+using CustomizeAggregateFunctionsOrNullVisitor = InDepthNodeVisitor<OneTypeMatcher<CustomizeAggregateFunctionsSuffixData>, true>;
+
 /// Translate qualified names such as db.table.column, table.column, table_alias.column to names' normal form.
 /// Expand asterisks and qualified asterisks with column names.
 /// There would be columns in normal form & column aliases after translation. Column & column alias would be normalized in QueryNormalizer.
@@ -692,6 +712,13 @@ void TreeRewriter::normalize(ASTPtr & query, Aliases & aliases, const Settings &
         CustomizeGlobalNotInVisitor(data_global_not_null_in).visit(query);
     }
 
+    // Rewrite all aggregate functions to add -OrNull suffix to them
+    if (settings.aggregate_functions_null_for_empty)
+    {
+        CustomizeAggregateFunctionsOrNullVisitor::Data data_or_null{"OrNull"};
+        CustomizeAggregateFunctionsOrNullVisitor(data_or_null).visit(query);
+    }
+
     /// Creates a dictionary `aliases`: alias -> ASTPtr
     QueryAliasesVisitor(aliases).visit(query);
 

tests/queries/0_stateless/01528_setting_aggregate_functions_null_for_empty.reference

@@ -0,0 +1,8 @@
+0
+\N
+\N
+\N
+45
+45
+45
+45

tests/queries/0_stateless/01528_setting_aggregate_functions_null_for_empty.sql

@@ -0,0 +1,28 @@
+DROP TABLE IF EXISTS defaults;
+
+CREATE TABLE defaults
+(
+	n Int8
+)ENGINE = Memory();
+
+SELECT sum(n) FROM defaults;
+SELECT sumOrNull(n) FROM defaults;
+
+SET aggregate_functions_null_for_empty=1;
+
+SELECT sum(n) FROM defaults;
+SELECT sumOrNull(n) FROM defaults;
+
+INSERT INTO defaults SELECT * FROM numbers(10);
+
+SET aggregate_functions_null_for_empty=0;
+
+SELECT sum(n) FROM defaults;
+SELECT sumOrNull(n) FROM defaults;
+
+SET aggregate_functions_null_for_empty=1;
+
+SELECT sum(n) FROM defaults;
+SELECT sumOrNull(n) FROM defaults;
+
+DROP TABLE defaults;