maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Nayuta!

On Jul 21, Nayuta Yanagisawa wrote:
> revision-id: 5e16a49f5d9 (mariadb-10.4.20-32-g5e16a49f5d9)
> parent(s): 78735dcaf75
> author: Nayuta Yanagisawa
> committer: Nayuta Yanagisawa
> timestamp: 2021-07-21 09:58:34 +0000
> message:
> 
> MDEV-26013 distinct not work properly in some cases for spider tables
> 
> diff --git a/sql/sql_select.cc b/sql/sql_select.cc
> index d8116b20cbe..cfd04d93662 100644
> --- a/sql/sql_select.cc
> +++ b/sql/sql_select.cc
> @@ -3274,9 +3274,17 @@ bool JOIN::make_aggr_tables_info()
>  
>      if (ht && ht->create_group_by)
>      {
> -      /* Check if the storage engine can intercept the query */
> -      Query query= {&all_fields, select_distinct, tables_list, conds,
> -                    group_list, order ? order : group_list, having};
> +      /*
> +        Check if the storage engine can intercept the query.
> +
> +        JOIN::optimize_stage2() might convert DISTINCT into GROUP BY and then
> +        optimize away GROUP BY. In such a case, we need to notify a storage engine
> +        supporting a group by handler of the existence of the original DISTINCT.
> +        Thus, we set select_distinct || (no_order && group_optimized_away) to
> +        Query::distinct.
> +      */
> +      Query query= {&all_fields, select_distinct || (no_order && group_optimized_away),
> +                    tables_list, conds, group_list, order ? order : group_list, having};

If DISTINCT was coverted to a GROUP BY, why would the engine need to
know whether there was DISTINCT or not originally? There is no DISTINCT
on the execution plan now, that should be sufficient, shouldn't it?

Why does the query fail with GROUP BY?

>        group_by_handler *gbh= ht->create_group_by(thd, &query);
>  
>        if (gbh)

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