maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Sergei,

On May 04, Sergei Petrunia wrote:
> revision-id: 4ba5b444b4b (mariadb-10.2.43-97-g4ba5b444b4b)
> parent(s): 70555454b4c
> author: Sergei Petrunia
> committer: Sergei Petrunia
> timestamp: 2022-05-04 12:24:48 +0300
> message:
> 
> MDEV-28437: Assertion `!eliminated' failed in Item_subselect::exec
> 
> (This is the assert that was added in fix for MDEV-26047)
> 
> Table elimination may remove an ON expression from an outer join.
> However SELECT_LEX::update_used_tables() will still call
> 
>   item->walk(&Item::eval_not_null_tables)
> 
> for eliminated expressions. If the subquery is constant and cheap
> Item_cond_and will attempt to evaluate it, which will trigger an
> assert.
> The fix is not to call update_used_tables() or eval_not_null_tables()
> for ON expressions that were eliminated.
> 
> diff --git a/sql/sql_lex.cc b/sql/sql_lex.cc
> index 22ee8801e3a..8e718f2a942 100644
> --- a/sql/sql_lex.cc
> +++ b/sql/sql_lex.cc
> @@ -4229,7 +4229,7 @@ void SELECT_LEX::update_used_tables()
>        }
>      }
>      while ((embedding= embedding->embedding));
> -    if (tl->on_expr)
> +    if (tl->on_expr && !is_eliminated_table(join->eliminated_tables, tl))
>      {
>        tl->on_expr->update_used_tables();
>        tl->on_expr->walk(&Item::eval_not_null_tables, 0, NULL);
> @@ -4253,8 +4253,11 @@ void SELECT_LEX::update_used_tables()
>        if (embedding->on_expr && 
>            embedding->nested_join->join_list.head() == tl)
>        {

can embedding be eliminated?

if yes - does it make sense to do anything in tl's below that eliminated
embedding? Perhaps it should be (in the do..while above), like

    do
    {
      bool maybe_null;
      if ((maybe_null= MY_TEST(embedding->outer_join)))
      {
        tl->table->maybe_null= maybe_null;
        break;
      }
+     if (is_eliminated_table(join->eliminated_tables, embedding))
+       tl= embedding->embedding;
    }
    while ((embedding= embedding->embedding));

> +        if (!is_eliminated_table(join->eliminated_tables, embedding))
> +        {
>            embedding->on_expr->update_used_tables();
>            embedding->on_expr->walk(&Item::eval_not_null_tables, 0, NULL);
> +        }
>        }
>        tl= embedding;
>        embedding= tl->embedding;

Regards,
Sergei
VP of MariaDB Server Engineering
and security@xxxxxxxxxxx