MDEV-33679 Spider group by handler: skip on multiple equalities

The spider group by handler is created in
JOIN::make_aggr_tables_info(), by which time calls to
substitute_for_best_equal_field() should have already removed all the
multiple equalities (i.e. Item_equal, with MULT_EQUAL_FUNC func_type).
Therefore, if there is still such items, it is deemed as an optimizer
bug and should be skipped.

storage/spider/mysql-test/spider/bugfix/r/mdev_33679.result

+#
+# MDEV-33679 spider returns parsing failure on valid left join select by translating the on expression to ()
+#
+for master_1
+for child2
+for child3
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE `t1` (`c` INT(10) UNSIGNED NOT NULL, `b` VARCHAR(255) NOT NULL , PRIMARY KEY (`c`) USING BTREE ) ENGINE=MYISAM;
+CREATE TABLE `t2` (`a` INT(10) UNSIGNED NOT NULL, `c` INT(10) UNSIGNED NOT NULL ) ENGINE=MYISAM;
+SET spider_same_server_link= on;
+CREATE TABLE `t1_spider` (`c` INT(10) UNSIGNED NOT NULL, `b` VARCHAR(255) NOT NULL , PRIMARY KEY (`c`) USING BTREE ) COMMENT='wrapper "mysql",srv "srv", table "t1"' ENGINE=SPIDER;
+CREATE TABLE `t2_spider` (`a` INT(10) UNSIGNED NOT NULL, `c` INT(10) UNSIGNED NOT NULL
+, PRIMARY KEY (`a`) USING BTREE
+) COMMENT='wrapper "mysql",srv "srv",table "t2"' ENGINE=SPIDER;
+INSERT INTO t1_spider VALUES(1,'oooo');
+INSERT INTO t2_spider VALUES(1,1);
+SELECT t2_spider.a,t1_spider.c FRoM t2_spider LEFT join t1_spider ON (t2_spider.c = t1_spider.c) WHERE t2_spider.a = 1;
+a	c
+1	1
+Warnings:
+Warning	1815	Internal error: Spider group by handler: Encountered multiple equalities, likely an optimizer bug
+drop table t1, t2, t1_spider, t2_spider;
+drop server srv;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_33679.test

+--echo #
+--echo # MDEV-33679 spider returns parsing failure on valid left join select by translating the on expression to ()
+--echo #
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+CREATE TABLE `t1` (`c` INT(10) UNSIGNED NOT NULL, `b` VARCHAR(255) NOT NULL , PRIMARY KEY (`c`) USING BTREE ) ENGINE=MYISAM;
+CREATE TABLE `t2` (`a` INT(10) UNSIGNED NOT NULL, `c` INT(10) UNSIGNED NOT NULL ) ENGINE=MYISAM;
+SET spider_same_server_link= on;
+CREATE TABLE `t1_spider` (`c` INT(10) UNSIGNED NOT NULL, `b` VARCHAR(255) NOT NULL , PRIMARY KEY (`c`) USING BTREE ) COMMENT='wrapper "mysql",srv "srv", table "t1"' ENGINE=SPIDER;
+CREATE TABLE `t2_spider` (`a` INT(10) UNSIGNED NOT NULL, `c` INT(10) UNSIGNED NOT NULL
+, PRIMARY KEY (`a`) USING BTREE
+) COMMENT='wrapper "mysql",srv "srv",table "t2"' ENGINE=SPIDER;
+INSERT INTO t1_spider VALUES(1,'oooo');
+INSERT INTO t2_spider VALUES(1,1);
+SELECT t2_spider.a,t1_spider.c FRoM t2_spider LEFT join t1_spider ON (t2_spider.c = t1_spider.c) WHERE t2_spider.a = 1;
+
+drop table t1, t2, t1_spider, t2_spider;
+drop server srv;
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log

storage/spider/spd_db_mysql.cc

@@ -5566,6 +5566,17 @@ int spider_db_mbase_util::check_item_func(
       if (spider_db_check_ft_idx(item_func, spider) == MAX_KEY)
         DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM);
       break;
+    case Item_func::MULT_EQUAL_FUNC:
+      /* If there is still Item_equal by the time of
+      JOIN::make_aggr_tables_info() where the spider group by handler
+      is created, it indicates a bug in the optimizer, because there
+      shouldn't be any. */
+      push_warning_printf(
+        spider->trx->thd, SPIDER_WARN_LEVEL_WARN, ER_INTERNAL_ERROR,
+        ER_THD(spider->trx->thd, ER_INTERNAL_ERROR),
+        "Spider group by handler: Encountered multiple equalities, likely "
+        "an optimizer bug");
+      DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM);
     default:
       break;
   }
@@ -6492,6 +6503,17 @@ int spider_db_mbase_util::print_item_func(
       last_str = SPIDER_SQL_CLOSE_PAREN_STR;
       last_str_length = SPIDER_SQL_CLOSE_PAREN_LEN;
       break;
+    case Item_func::MULT_EQUAL_FUNC:
+      /* If there is still Item_equal by the time of
+      JOIN::make_aggr_tables_info() where the spider group by handler
+      is created, it indicates a bug in the optimizer, because there
+      shouldn't be any. */
+      push_warning_printf(
+        spider->trx->thd, SPIDER_WARN_LEVEL_WARN, ER_INTERNAL_ERROR,
+        ER_THD(spider->trx->thd, ER_INTERNAL_ERROR),
+        "Spider group by handler: Encountered multiple equalities, likely "
+        "an optimizer bug");
+      DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM);
     default:
       THD *thd = spider->trx->thd;
       SPIDER_SHARE *share = spider->share;