Currently arrayCompact doesn't work with tuples like other array functions arraySort or arrayFilter.
Please consider adding such functionality to arrayCompact.
We need something like this... SELECT arrayCompact( (x,y) -> y, arr1, arr2) AS v

Current behavior:
`SELECT
    [('a', 1), ('a', 2), ('c', 3), ('a', 2)] AS arr,
    arrayCompact(x -> (x.1), arr) AS v,
    [('a', 1), ('c', 3), ('a', 2)] AS expected_v
`
![изображение](https://user-images.githubusercontent.com/33029350/93028896-5fca5e00-f617-11ea-9d55-e0a5d3c36697.png)


#include <DataTypes/DataTypesNumber.h>
#include <DataTypes/DataTypesDecimal.h>
#include <Columns/ColumnsNumber.h>
#include <Columns/ColumnDecimal.h>
#include <Common/HashTable/HashTable.h>
#include <Functions/array/FunctionArrayMapped.h>
#include <Functions/FunctionFactory.h>
namespace DB
{
namespace ErrorCodes
{
}
struct ArrayCompactImpl
{
    static bool needBoolean() { return false; }