simple detect functional index

I want to create the following index
```ruby
add_index :products, '(price_usual-price_retail)'
```
and have this error
```
PG::Error: ERROR:  column "(price_usual-price_retail)" does not exist
```

I think, detected a functional index should be easier
```ruby

def quoted_columns_for_index(column_names, options = {})
	column_names.map do |name|
	  if name.to_s.include?('(')
	    name
	  else
	    quote_column_name(name)
	  end
	end
end

```