From 3656aa0606bc77f1a5a7bbaddbce99f97dcbe78b Mon Sep 17 00:00:00 2001
From: Alexey Milovidov <milovidov@yandex-team.ru>
Date: Tue, 13 Feb 2018 00:06:13 +0300
Subject: [PATCH] Improved performance of function arrayElement with constant
 array when one of element is an empty array #1889

---
 dbms/src/Functions/FunctionsArray.cpp                  | 10 ++++++++--
 .../0_stateless/00570_empty_array_is_const.reference   |  3 +++
 .../queries/0_stateless/00570_empty_array_is_const.sql |  3 +++
 3 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 dbms/tests/queries/0_stateless/00570_empty_array_is_const.reference
 create mode 100644 dbms/tests/queries/0_stateless/00570_empty_array_is_const.sql

diff --git a/dbms/src/Functions/FunctionsArray.cpp b/dbms/src/Functions/FunctionsArray.cpp
index 1cdf17a500c..3db0e32e794 100644
--- a/dbms/src/Functions/FunctionsArray.cpp
+++ b/dbms/src/Functions/FunctionsArray.cpp
@@ -61,6 +61,13 @@ void FunctionArray::executeImpl(Block & block, const ColumnNumbers & arguments,
 {
     size_t num_elements = arguments.size();
 
+    if (num_elements == 0)
+    {
+        /// We should return constant empty array.
+        block.getByPosition(result).column = block.getByPosition(result).type->createColumnConstWithDefaultValue(block.rows());
+        return;
+    }
+
     const DataTypePtr & return_type = block.getByPosition(result).type;
     const DataTypePtr & elem_type = static_cast<const DataTypeArray &>(*return_type).getNestedType();
 
@@ -91,8 +98,7 @@ void FunctionArray::executeImpl(Block & block, const ColumnNumbers & arguments,
         columns[i] = columns_holder[i].get();
     }
 
-    /** Create and fill the result array.
-        */
+    /// Create and fill the result array.
 
     auto out = ColumnArray::create(elem_type->createColumn());
     IColumn & out_data = out->getData();
diff --git a/dbms/tests/queries/0_stateless/00570_empty_array_is_const.reference b/dbms/tests/queries/0_stateless/00570_empty_array_is_const.reference
new file mode 100644
index 00000000000..1eedb9f964c
--- /dev/null
+++ b/dbms/tests/queries/0_stateless/00570_empty_array_is_const.reference
@@ -0,0 +1,3 @@
+Array(Nothing), Const(size = 1, Array(size = 1, UInt64(size = 1), Nothing(size = 0)))
+Array(Array(Array(Nothing))), Const(size = 1, Array(size = 1, UInt64(size = 1), Array(size = 1, UInt64(size = 1), Array(size = 1, UInt64(size = 1), Nothing(size = 0)))))
+Array(Array(UInt8)), Const(size = 1, Array(size = 1, UInt64(size = 1), Array(size = 2, UInt64(size = 2), UInt8(size = 1))))
diff --git a/dbms/tests/queries/0_stateless/00570_empty_array_is_const.sql b/dbms/tests/queries/0_stateless/00570_empty_array_is_const.sql
new file mode 100644
index 00000000000..62d56a7745d
--- /dev/null
+++ b/dbms/tests/queries/0_stateless/00570_empty_array_is_const.sql
@@ -0,0 +1,3 @@
+SELECT dumpColumnStructure([]);
+SELECT dumpColumnStructure([[[]]]);
+SELECT DISTINCT dumpColumnStructure([[], [1]]) FROM numbers(2);