MDEV-9764: MariaDB does not limit memory used for range optimization

Part #2: make tree_or(tree1, tree2) to reuse tree1 for the result object
for simple cases. These include key IN (c1, ... cN).

The reuse was happening in old MySQL versions, but we stopped doing it
in the "fair choice between range and index_merge" patch.

sql/opt_range.cc

@@ -8566,14 +8566,30 @@ tree_or(RANGE_OPT_PARAM *param,SEL_TREE *tree1,SEL_TREE *tree2)
     imerge[0]= new SEL_IMERGE(tree1->merges.head(), 0, param);
   }
   bool no_imerge_from_ranges= FALSE;
-  if (!(result= new (param->mem_root) SEL_TREE(param->mem_root, param->keys)))
-    DBUG_RETURN(result);
 
   /* Build the range part of the tree for the formula (1) */ 
   if (sel_trees_can_be_ored(param, tree1, tree2, &ored_keys))
   {
     bool must_be_ored= sel_trees_must_be_ored(param, tree1, tree2, ored_keys);
     no_imerge_from_ranges= must_be_ored;
+
+    if (no_imerge_from_ranges && no_merges1 && no_merges2)
+    {
+      /*
+        Reuse tree1 as the result in simple cases. This reduces memory usage
+        for e.g. "key IN (c1, ..., cN)" which produces a lot of ranges.
+      */
+      result= tree1;
+    }
+    else
+    {
+      if (!(result= new (param->mem_root) SEL_TREE(param->mem_root,
+                                                   param->keys)))
+      {
+        DBUG_RETURN(result);
+      }
+    }
+
     key_map::Iterator it(ored_keys);
     int key_no;
     while ((key_no= it++) != key_map::Iterator::BITMAP_END)
@@ -8590,7 +8606,13 @@ tree_or(RANGE_OPT_PARAM *param,SEL_TREE *tree1,SEL_TREE *tree2)
     }
     result->type= tree1->type;
   }
-      
+  else
+  {
+    if (!result && !(result= new (param->mem_root) SEL_TREE(param->mem_root,
+                                                            param->keys)))
+      DBUG_RETURN(result);
+  }
+
   if (no_imerge_from_ranges && no_merges1 && no_merges2)
   {
     if (result->keys_map.is_clear_all())