MDEV-21265: IN predicate conversion to IN subquery should be allowed for a broader set of datatype comparison

Allow materialization strategy when collations on the
inner and outer sides of an IN subquery are the same and the
character set of the inner side is a proper subset of the character
set on the outer side.
This allows conversion from utf8mb3 to utf8mb4
as the former is a subset of the later.
This is only allowed when IN predicate is converted to an IN subquery

Backported part of the patch (d6a00d9b18f) of MDEV-17905.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/sql_tvc.cc b/sql/sql_tvc.cc
index def519035d9..10a279b92ed 100644
--- a/sql/sql_tvc.cc
+++ b/sql/sql_tvc.cc
@@ -829,7 +829,8 @@ static bool cmp_row_types(Item* item1, Item* item2)
     Item *inner= item1->element_index(i);
     Item *outer= item2->element_index(i);
     if (!inner->type_handler()->subquery_type_allows_materialization(inner,
-                                                                     outer))
+                                                                     outer,
+                                                                     true))
       return true;
   }
   return false;
@@ -895,7 +896,7 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
   
   for (uint i=1; i < arg_count; i++)
   {
-    if (!args[i]->const_item() || cmp_row_types(args[0], args[i]))
+    if (!args[i]->const_item() || cmp_row_types(args[i], args[0]))
       return this;
   }
 
@@ -975,6 +976,7 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
   if (!(in_subs=
           new (thd->mem_root) Item_in_subselect(thd, args[0], sq_select)))
     goto err;
+  in_subs->converted_from_in_predicate= TRUE;
   sq= in_subs;
   if (negated)
     sq= negate_expression(thd, in_subs);