Prune columns from select distinct

[max-hoffman]

The plans below indicate that we are deserializing ProductName unnecessarily for the SELECT DISTINCT query, while we appropriately prune the GROUP BY. For tables with many TEXT columns this is a performance hindrance.

db/main*> CREATE TABLE Products(
                                     ProductID Char(38) PRIMARY KEY, /*UUID*/
                                     Manufacturer TEXT,
                                     ProductSeries TEXT,
                                     ProductName TEXT);
db/main*> explain plan SELECT DISTINCT Manufacturer, ProductSeries FROM Products;
+------------------------------------------------------------------+
| plan                                                             |
+------------------------------------------------------------------+
| Distinct                                                         |
|  └─ Project                                                      |
|      ├─ columns: [products.Manufacturer, products.ProductSeries] |
|      └─ Table                                                    |
|          └─ name: Products                                       |
+------------------------------------------------------------------+
5 rows in set (0.00 sec)

db/main*> explain plan SELECT Manufacturer, ProductSeries FROM Products GROUP BY Manufacturer, ProductSeries;
+------------------------------------------------------------------+
| plan                                                             |
+------------------------------------------------------------------+
| GroupBy                                                          |
|  ├─ SelectedExprs(products.Manufacturer, products.ProductSeries) |
|  ├─ Grouping(products.manufacturer, products.productseries)      |
|  └─ Table                                                        |
|      ├─ name: Products                                           |
|      └─ columns: [manufacturer productseries]                    |
+------------------------------------------------------------------+