優化前：
SELECT a.* FROM t1 a,
(SELECT obj_id,MAX(PRE_DETAIL_INST_ID) PRE_DETAIL_INST_ID FROM t1 WHERE DATE_FORMAT(crt_date,'%Y-%m-%d %H') < DATE_FORMAT(NOW(),'%Y-%m-%d %H') AND
DATE_FORMAT(crt_date,'%Y-%m-%d %H') >= DATE_FORMAT(DATE_ADD(NOW(),INTERVAL -1 HOUR),'%Y-%m-%d %H') GROUP BY obj_id) b
WHERE a.pre_detail_inst_id = b.pre_detail_inst_id;
索引字段：
+---------------------+------------+---------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------------------+------------+---------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1| 0 | PRIMARY | 1 | PRE_DETAIL_INST_ID | A | 14897 | NULL | NULL | | BTREE | | |
| t1| 1 | PRE_INST_OBJ_ID_xxx | 1 | OBJ_ID | A | 14897 | NULL | NULL | YES | BTREE | | |
| t1| 1 | PRE_INST_PRE_INST_ID_xxx | 1 | PRE_INST_ID | A | 14897 | NULL | NULL | YES | BTREE | | |
| t1| 1 | pre_inst_pre_rule_id_xxx | 1 | PRE_RULE_ID | A | 1354 | NULL | NULL | YES | BTREE | | |
| t1| 1 | idx_bil_cust_order_id_xxx | 1 | CUST_ORDER_ID | A | 1489 | NULL | NULL | YES | BTREE | | |
| t1| 1 | idx_crt_date | 1 | CRT_DATE | A | 1354 | NULL | NULL | YES | BTREE | | |
+---------------------+------------+---------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
執行計划如下：
+----+-------------+---------------------+--------+---------------------+---------------------+---------+----------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+--------+---------------------+---------------------+---------+----------------------+-------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 14897 | Using where |
| 1 | PRIMARY | a | eq_ref | PRIMARY | PRIMARY | 8 | b.PRE_DETAIL_INST_ID | 1 | NULL |
| 2 | DERIVED | tb_bil_pre_inst_xxx | index | PRE_INST_OBJ_ID_xxx | PRE_INST_OBJ_ID_xxx | 9 | NULL | 14897 | Using where |
+----+-------------+---------------------+--------+---------------------+---------------------+---------+----------------------+-------+-------------+
對謂詞crt_date加date_format函數無法使用索引。從而導致使用全表掃描。由於是innodb引擎，且無法使用索引導致行鎖升級為表鎖，在高並發環境下，導致大量的等待。
數據量： 2500000
執行時間：00:00:16:274
優化后：
SELECT a.* FROM
(SELECT * FROM t1 WHERE crt_date >= CONCAT(DATE_FORMAT(DATE_SUB(NOW(),INTERVAL 1 HOUR),'%Y-%m-%d %H'),':00:00')
AND crt_date < CONCAT(DATE_FORMAT(NOW(),'%Y-%m-%d %H'),':00:00')) a,
(SELECT obj_id,MAX(pre_detail_inst_id) pre_detail_inst_id FROM t1 WHERE crt_date >= CONCAT(DATE_FORMAT(DATE_SUB(NOW(),INTERVAL 1 HOUR),'%Y-%m-%d %H'),':00:00')
AND crt_date < CONCAT(DATE_FORMAT(NOW(),'%Y-%m-%d %H'),':00:00') GROUP BY obj_id) b
WHERE a.pre_detail_inst_id = b.pre_detail_inst_id;
執行計划如下：
+----+-------------+---------------------+-------+--------------------------------------+------------------+---------+----------------------+------+--------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+-------+--------------------------------------+------------------+---------+----------------------+------+--------------------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
| 1 | PRIMARY | <derived3> | ref | <auto_key0> | <auto_key0> | 9 | a.PRE_DETAIL_INST_ID | 2 | NULL |
| 3 | DERIVED | t1| range | PRE_INST_OBJ_ID_xxx ,idx_crt_date_xxx | idx_crt_date_xxx | 6 | NULL | 1 | Using index condition; Using temporary; Using filesort |
| 2 | DERIVED | t1| range | idx_crt_date_xxx | idx_crt_date_xxx | 6 | NULL | 1 | Using index condition |
+----+-------------+---------------------+-------+--------------------------------------+------------------+---------+----------------------+------+--------------------------------------------------------+
改進crt_date，mysql使用索引范圍查找，利用行鎖，規避了表鎖和高並發下的表鎖等待問題。
數據量： 2500000
執行時間：00:00:00:188