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

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.

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 (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)

storage/spider/mysql-test/spider/bugfix/r/mdev_26013.result

@@ -0,0 +1,42 @@
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+
+MDEV-26013 distinct not work properly in some cases for spider tables
+
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+`a`int,
+`b`int,
+PRIMARY KEY (`a`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+insert into `tbl_a` VALUES (1,999), (2,999);
+connection master_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+`a`int,
+`b`int,
+PRIMARY KEY (`a`)
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS(`a`) (
+PARTITION `pt1` DEFAULT COMMENT = 'srv "s_2_1"'
+);
+connection master_1;
+SELECT distinct b FROM tbl_a WHERE b=999;
+b
+999
+connection master_1;
+DROP DATABASE IF EXISTS auto_test_remote;
+connection child2_1;
+DROP DATABASE IF EXISTS auto_test_remote;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_26013.cnf

@@ -0,0 +1,3 @@
+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_26013.test

@@ -0,0 +1,51 @@
+--disable_warnings
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+--enable_warnings
+
+--echo
+--echo MDEV-26013 distinct not work properly in some cases for spider tables
+--echo
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+
+eval CREATE TABLE tbl_a (
+    `a`int,
+    `b`int,
+    PRIMARY KEY (`a`)
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+
+insert into `tbl_a` VALUES (1,999), (2,999);
+
+--connection master_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+
+eval CREATE TABLE tbl_a (
+    `a`int,
+    `b`int,
+    PRIMARY KEY (`a`)
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS(`a`) (
+    PARTITION `pt1` DEFAULT COMMENT = 'srv "s_2_1"'
+);
+
+--connection master_1
+SELECT distinct b FROM tbl_a WHERE b=999;
+
+--connection master_1
+DROP DATABASE IF EXISTS auto_test_remote;
+--connection child2_1
+DROP DATABASE IF EXISTS auto_test_remote;
+
+--disable_warnings
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log
+--enable_warnings