Bug#34554865: Hypergraph: Duplicate FLOAT values with SELECT DISTINCT

When SELECT DISTINCT is performed as a "sort with duplicate removal"
and the select list contains FLOAT values, duplicates may be returned
because Filesort compares the values as DOUBLE and sees differences in
insignificant digits.

Fixed by truncating the double values to float to remove the
insignificant digits before the sort key is created.

Change-Id: Iad9c1f2b3fdb69001ef9b68773e45f5c1dde490e

Author: kahatlen

mysql-test/t/distinct.test

@@ -984,8 +984,8 @@ INSERT INTO t1 VALUES
  (-2007568257), (-2007568260), (-2007570000), (-2007567234), (-2007567230),
  (2007568257), (2007568260), (2007570000), (2007567234), (2007567230);
 
---skip_if_hypergraph  # Bug#34119506
 SELECT SQL_SMALL_RESULT DISTINCT CAST(v AS FLOAT) FROM t1;
+--skip_if_hypergraph  # Incorrect results recorded. Bug#34554755.
 SELECT SQL_BIG_RESULT DISTINCT CAST(v AS FLOAT) FROM t1;
 
 DROP TABLE t1;

sql/filesort.cc

@@ -1356,6 +1356,12 @@ size_t make_sortkey_from_item(Item *item, Item_result result_type,
     case REAL_RESULT: {
       assert(!is_varlen);
       double value = item->val_real();
+      if (item->data_type() == MYSQL_TYPE_FLOAT) {
+        // Get rid of extra precision. Otherwise, duplicate removal may make the
+        // wrong decision and treat equal values as distinct values due to
+        // differences in the insignificant digits.
+        value = static_cast<float>(value);
+      }
       if (item->null_value) {
         assert(item->is_nullable());
         *null_indicator = 0;