MDEV-25116 Spider: IF(COUNT( trigger SQL Error (1054)_ Unknown column '' in field list

The original query "SELECT IF(COUNT(a.`id`)>=0,'Y','N') FROM t" is
transformed to "SELECT COUNT(a.`id`), IF(ref >= 0, 'Y', 'N') FROM t",
where ref is Item_ref to "COUNT(a.`id`)", by split_sum_func().

Spider walks the item list twice, invoking spider_db_print_item_type().
The first invocation is in spider_create_group_by_handler() with
str == NULL. The second one is in spider_group_by_handler::init_scan()
with str != NULL.

spider_db_print_item_type() prints nothing at the first invocation,
and it prints item at the second invocation. However, at the second
invocation, the above mentioned ref to "COUNT(a.`id`)" points to
a field in a temporary table where the result will be stored. Thus,
to look behind the item_ref, Spider need to generate the query earlier.

A possible fix would be to generate a query to send in
spider_create_group_by_handler(). However, the fix requires a
considerable amount of changes of the Spider's GROUP BY handler.
I'd like to avoid that.

So, I fix the problem by not to use the GROUP BY handler when a
query contains Item_ref whose table_name, name, and alias_name_used
are not set.

storage/spider/mysql-test/spider/bugfix/r/mdev_25116.result

+#
+# MDEV-25116 Spider: IF(COUNT( trigger SQL Error (1054)_ Unknown column '' in field list
+#
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (id INT);
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+id INT
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a", srv "s_2_1"';
+connection master_1;
+SELECT IF(COUNT(id > 0),'Y','N') FROM tbl_a;
+IF(COUNT(id > 0),'Y','N')
+N
+connection master_1;
+DROP DATABASE IF EXISTS auto_test_local;
+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_25116.cnf

+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_25116.test

+--echo #
+--echo # MDEV-25116 Spider: IF(COUNT( trigger SQL Error (1054)_ Unknown column '' in field list
+--echo #
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+
+CREATE TABLE tbl_a (id INT);
+
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+
+eval CREATE TABLE tbl_a (
+    id INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a", srv "s_2_1"';
+
+--connection master_1
+SELECT IF(COUNT(id > 0),'Y','N') FROM tbl_a;
+
+--connection master_1
+DROP DATABASE IF EXISTS auto_test_local;
+--connection child2_1
+DROP DATABASE IF EXISTS auto_test_remote;
+
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log

storage/spider/spd_db_conn.cc

@@ -9298,8 +9298,7 @@ int spider_db_open_item_ref(
       }
       DBUG_RETURN(0);
     }
-    DBUG_RETURN(spider_db_print_item_type(*(item_ref->ref), NULL, spider, str,
-      alias, alias_length, dbton_id, use_fields, fields));
+    DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM); // MDEV-25116
   }
   DBUG_RETURN(spider_db_open_item_ident((Item_ident *) item_ref, spider, str,
     alias, alias_length, dbton_id, use_fields, fields));