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MDEV-26013 distinct not work properly in some cases for spider tables

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The bug is caused by the following reasons:

* spider_group_by_handler::init_scan() generates a query for a data node.
* The function adds DISTINCT if and only if
  spider_group_by_handler::query::distinct is TRUE.
* spider_group_by_handler::query::distinct is set to the value of
  JOIN::select_distinct in JOIN::make_aggr_tables_info().
* In the test case, DISTINCT is not added because JOIN::select_distinct
  is FALSE at the call of JOIN::make_aggr_tables_info().

Why JOIN::select_distinct is set to FALSE? That is because the function
JOIN::optimize_stage2() convert DISTINCT into GROUP BY and then optimizes
away GROUP BY.
This commit is contained in:
Nayuta Yanagisawa
2021-07-13 07:53:19 +00:00
parent 7ffa801cf2
commit aafb888657
4 changed files with 107 additions and 3 deletions
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14
sql/sql_select.cc
View File

@ -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 (group_list). 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 || group_optimized_away
to Query::distinct.
*/
Query query= {&all_fields, select_distinct || group_optimized_away, tables_list,
conds, group_list, order ? order : group_list, having};
group_by_handler *gbh= ht->create_group_by(thd, &query);
if (gbh)