1 建表如下
CREATE TABLE t92 ( 
 a1 int(10) unsigned NOT NULL , 
 b1 int(10) DEFAULT NULL, 
 UNIQUE KEY (a1) 
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;CREATE TABLE t93 ( 
 a2 int(10) unsigned NOT NULL, 
 b2 int(10) DEFAULT NULL, 
 UNIQUE KEY (a2) 
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO t92 (a1, b1) VALUES (1, 11); 
 INSERT INTO t92 (a1, b1) VALUES (2, 12); 
 INSERT INTO t92 (a1, b1) VALUES (3, 13); 
 INSERT INTO t92 (a1, b1) VALUES (4, 14); 
 INSERT INTO t92 (a1, b1) VALUES (5, 15); 
 INSERT INTO t92 (a1, b1) VALUES (6, 16); 
 INSERT INTO t92 (a1, b1) VALUES (7, 17); 
 INSERT INTO t92 (a1, b1) VALUES (8, 18); 
 INSERT INTO t92 (a1, b1) VALUES (9, 19); 
 INSERT INTO t92 (a1, b1) VALUES (10, 20);INSERT INTO t93 (a2, b2) VALUES (1, 21); 
 INSERT INTO t93 (a2, b2) VALUES (2, 22); 
 INSERT INTO t93 (a2, b2) VALUES (3, 23); 
 INSERT INTO t93 (a2, b2) VALUES (4, 24); 
 INSERT INTO t93 (a2, b2) VALUES (5, 25); 
 INSERT INTO t93 (a2, b2) VALUES (6, 26); 
 INSERT INTO t93 (a2, b2) VALUES (7, 27); 
 INSERT INTO t93 (a2, b2) VALUES (8, 28); 
 INSERT INTO t93 (a2, b2) VALUES (9, 29); 
 INSERT INTO t93 (a2, b2) VALUES (10, 30);2 查询执行计划如下
mysql> EXPLAIN EXTENDED (SELECT a1 FROM t92 WHERE a1>=1 ORDER BY a1) UNION ALL (SELECT a2 FROM t93 WHERE a2>=1 ORDER BY a2); 
 +——+————–+————+——-+—————+——+———+——+——+———-+————————–+ 
 | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | 
 +——+————–+————+——-+—————+——+———+——+——+———-+————————–+ 
 | 1 | PRIMARY | t92 | index | a1 | a1 | 4 | NULL | 10 | 100 | Using where; Using index | 
 | 2 | UNION | t93 | index | a2 | a2 | 4 | NULL | 10 | 100 | Using where; Using index | 
 | NULL | UNION RESULT |