Question

请问您能帮我解决这个疑问吗？

SELECT p.patid, MAX(c1.eventdate) as eventdate 
from patient as p 
left join op_adv_effects._clinical as c1 on p.patid = c1.patid 
where c1.eventdate < p.case_index 
group by p.patid

以下是2个表的SHOW CREATE TABLE的输出：

patient CREATE TABLE `patient` (
  `patid` int(10) unsigned NOT NULL,
  `case_index` date NOT NULL,
  PRIMARY KEY (`patid`,`case_index`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_cs

_clinical   CREATE TABLE `_clinical` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `patid` int(10) unsigned NOT NULL,
  `eventdate` date NOT NULL,
  `medcode` mediumint(8) unsigned DEFAULT NULL,
  `adid` mediumint(8) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_clin_eventdate_medcode` (`patid`,`eventdate`,`medcode`),
  KEY `idx_clin_eventdate_adid` (`patid`,`eventdate`,`adid`)
) ENGINE=InnoDB AUTO_INCREMENT=62407536 DEFAULT CHARSET=latin1 COLLATE=latin1_general_cs

＆＃34;解释＆＃34;返回以下内容：

*************************** 1. row ********************
           id: 1
  select_type: SIMPLE
        table: p
         type: index
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 182939
        Extra: Using index
*************************** 2. row ********************
           id: 1
  select_type: SIMPLE
        table: c1
         type: ref
possible_keys: idx_clin_eventdate_medcode,idx_clin_eventdate_adid
          key: idx_clin_eventdate_medcode
      key_len: 4
          ref: gprd_opadveff_extra_elisa.p.patid
         rows: 171
        Extra: Using where; Using index

为什么不使用idx_clin_eventdate_medcode的前两个字段，即（patid，eventdate），而只使用patid（参见ref列）？

如果我将where条件更改为相等，则可以正常工作：

SELECT p.patid, MAX(c1.eventdate) as eventdate 
from patient as p 
left join op_adv_effects._clinical as c1 on p.patid = c1.patid 
where c1.eventdate = p.case_index 
group by p.patid

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: p
         type: index
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 182939
        Extra: Using index
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: c1
         type: ref
possible_keys: idx_clin_eventdate_medcode,idx_clin_eventdate_adid
          key: idx_clin_eventdate_medcode
      key_len: 7
          ref: gprd_opadveff_extra_elisa.p.patid,gprd_opadveff_extra_elisa.p.cas
e_index
         rows: 1
        Extra: Using index

某些建议变体的结果相同：

explain SELECT  patid,
(SELECT  eventdate
FROM  op_adv_effects._clinical
WHERE  patid = p.patid
AND eventdate < p.case_index
ORDER BY  eventdate DESC
LIMIT  1 ) AS eventdate
FROM  patient AS p;

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: p
         type: index
possible_keys: NULL
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 182939
        Extra: Using index
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: _clinical
         type: ref
possible_keys: idx_clin_eventdate_medcode,idx_clin_eventdate_adid
          key: idx_clin_eventdate_medcode
      key_len: 4
          ref: gprd_opadveff_extra_elisa.p.patid
         rows: 171
        Extra: Using where; Using index; Using filesort


explain SELECT  patid, 
( SELECT  MAX(eventdate)
FROM  op_adv_effects._clinical
WHERE  patid = p.patid
AND  eventdate < p.case_index) AS eventdate
FROM  patient AS p;

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: p
         type: index
possible_keys: NULL
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 182939
        Extra: Using index
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: _clinical
         type: ref
possible_keys: idx_clin_eventdate_medcode,idx_clin_eventdate_adid
          key: idx_clin_eventdate_medcode
      key_len: 4
          ref: gprd_opadveff_extra_elisa.p.patid
         rows: 171
        Extra: Using where; Using index

查询是更复杂的查询的一部分，如下所述。但是，这只是几个复杂查询的一个示例，它们应该在eventdate上使用索引的缺失部分。因此，它非常重要。

CREATE TABLE bmi_lp
(PRIMARY KEY (patid))
ENGINE=INNODB DEFAULT CHARSET=latin1 COLLATE=latin1_general_cs
SELECT tmp.patid, a2.data3 as bmi_lp, tmp.eventdate as bmi_lp_date 
from ( 
SELECT p.patid, MAX(c.eventdate) as eventdate 
from patient as p 
left join op_adv_effects._clinical as c1 on p.patid = c1.patid 
left join op_adv_effects._additional as a1 on c1.patid = a1.patid 
where c1.adid <> 0 and c1.adid = a1.adid 
and a1.enttype = 13 
and a1.data3 is not null 
and c1.eventdate < p.case_index 
group by p.patid 
order by p.patid) as tmp 
left join op_adv_effects._clinical   as c2 on tmp.patid = c2.patid 
left join op_adv_effects._additional as a2 on c2.patid = a2.patid 
where tmp.eventdate = c2.eventdate and c2.adid = a2.adid

Answer 1

由于WHERE你正在做INNER JOIN。你打算这么做吗？

无论如何，由于<目前无法使用索引，如果您有一个使用不同顺序的索引，它可以正常工作。

例如，在PostgreSQL中你可以这样做：

CREATE INDEX idx_clin_eventdate_medcode ON _clinical (patid ASC, eventdate DESC);

在MySQL中，DESC和ASC运算符很不幸（至少每个MySQL版本都达到5.7）。因此，除非您可以反转查询（使用>而不是<），否则MySQL无法有效地使用该索引。

请注意，不使用索引甚至可能更快，这取决于具体情况。因为它只经过171行我不会太担心。

Answer 2

尝试一下：

SELECT  patid, 
      ( SELECT  MAX(eventdate)
            FROM  op_adv_effects._clinical
            WHERE  patid = p.patid
              AND  eventdate < p.case_index 
      ) AS eventdate
    FROM  patient AS p;

（需要GROUP BY。）

这是使用LIMIT 1代替MAX的变体：

SELECT  patid, 
      ( SELECT  eventdate
            FROM  op_adv_effects._clinical
            WHERE  patid = p.patid
              AND  eventdate < p.case_index
            ORDER BY  eventdate DESC
            LIMIT  1 
      ) AS eventdate
    FROM  patient AS p;

输出中有多少行？

为什么这个MySQL查询没有使用完整的索引？

2 个答案: