mysql-透视非规范化的行-将多行转换为单行-导致缓慢的LEFT JOIN
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我正在尝试创建一个表,在该表中,字符串是在由三个字符串组成的集群中排列的,并返回一个数据在一条记录中的查询.
该表的定义是：
CREATE TABLE Xerox.FIM2 (
KPIN CHAR(18) NOT NULL COMMENT 'Part Number',
KSEQ DECIMAL(2, 0) NOT NULL COMMENT 'Sequence Number',
SDS1 CHAR(40) NOT NULL COMMENT 'Supplemental Description 1',
SDS2 CHAR(30) NOT NULL COMMENT 'Supplemental Description 2',
SDS3 CHAR(30) NOT NULL COMMENT 'Supplemental Description 3',
PRIMARY KEY (KPIN, KSEQ),
INDEX IDX_FIM2_KSEQ (KSEQ),
INDEX IDX_FIM2_SDS1 (SDS1),
INDEX IDX_FIM2_SDS2 (SDS2),
INDEX IDX_FIM2_SDS3 (SDS3),
INDEX UK_FIM2_KPIN (KPIN)
)
ENGINE = INNODB
AVG_ROW_LENGTH = 180
CHARACTER SET latin1
COLLATE latin1_swedish_ci
COMMENT = 'Supplemental Part Descriptions';
给定的零件号可能没有记录,也可能没有8条记录.这八个记录包含字符串数据(描述,每个记录三个),我尝试将其转换为：
KPIN KSEQ SDS1 SDS2 SDS3
R0205 1 COLD ROLLED A1008/A CS TYPE B MATTE FINISH OIL / EXPOSED MIN YIELD 30,000 RB 45-60
R0205 2 THICKNESS .032 TOL +.006/-.000 WIDTH 48.000 TOL +.188/-.000 LENGTH 84.875 TOL +/-.020
R0205 3 SQUARENESS TOL +/-.062 MAX LIFT WEIGHT 10,000 LBS <null>
像这样的数据：
KPIN KSEQ DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9 DES10 DES11 DES12
R0205 1 COLD ROLLED A1008/A CS TYPE B MATTE FINISH OIL / EXPOSED MIN YIELD 30,000 RB 45-60 THICKNESS .032 TOL +.006/-.000 WIDTH 48.000 TOL +.188/-.000 LENGTH 84.875 TOL +/-.020 THICKNESS .032 TOL +.006/-.000 WIDTH 48.000 TOL +.188/-.000 LENGTH 84.875 TOL +/-.020 SQUARENESS TOL +/-.062 MAX LIFT WEIGHT 10,000 LBS <null><null><null><null>...
我的SQL命令是这样的：
SELECT FIM1.KPIN, FIM1.DES, p.DES1, p.DES2, p.DES3, p.DES4, p.DES5, p.DES6,
p.DES7, p.DES8, p.DES9, p.DES10, p.DES11, p.DES12,
p.DES13, p.DES14, p.DES15, p.DES16, p.DES17, p.DES18,
p.DES19, p.DES20, p.DES21, p.DES22, p.DES23, p.DES24
FROM FIM1
JOIN
(SELECT KPIN, KSEQ,
MAX(CASE WHEN (KSEQ = 1) THEN SDS1 END) AS DES1,
MAX(CASE WHEN (KSEQ = 1) THEN SDS2 END) AS DES2,
MAX(CASE WHEN (KSEQ = 1) THEN SDS3 END) AS DES3,
MAX(CASE WHEN (KSEQ = 2) THEN SDS1 END) AS DES4,
MAX(CASE WHEN (KSEQ = 2) THEN SDS2 END) AS DES5,
MAX(CASE WHEN (KSEQ = 2) THEN SDS3 END) AS DES6,
MAX(CASE WHEN (KSEQ = 3) THEN SDS1 END) AS DES7,
MAX(CASE WHEN (KSEQ = 3) THEN SDS2 END) AS DES8,
MAX(CASE WHEN (KSEQ = 3) THEN SDS3 END) AS DES9,
MAX(CASE WHEN (KSEQ = 4) THEN SDS1 END) AS DES10,
MAX(CASE WHEN (KSEQ = 4) THEN SDS2 END) AS DES11,
MAX(CASE WHEN (KSEQ = 4) THEN SDS3 END) AS DES12,
MAX(CASE WHEN (KSEQ = 5) THEN SDS1 END) AS DES13,
MAX(CASE WHEN (KSEQ = 5) THEN SDS2 END) AS DES14,
MAX(CASE WHEN (KSEQ = 5) THEN SDS3 END) AS DES15,
MAX(CASE WHEN (KSEQ = 6) THEN SDS1 END) AS DES16,
MAX(CASE WHEN (KSEQ = 6) THEN SDS2 END) AS DES17,
MAX(CASE WHEN (KSEQ = 6) THEN SDS3 END) AS DES18,
MAX(CASE WHEN (KSEQ = 7) THEN SDS1 END) AS DES19,
MAX(CASE WHEN (KSEQ = 7) THEN SDS2 END) AS DES20,
MAX(CASE WHEN (KSEQ = 7) THEN SDS3 END) AS DES21,
MAX(CASE WHEN (KSEQ = 8) THEN SDS1 END) AS DES22,
MAX(CASE WHEN (KSEQ = 8) THEN SDS2 END) AS DES23,
MAX(CASE WHEN (KSEQ = 8) THEN SDS3 END) AS DES24
FROM FIM2
GROUP BY KPIN
ORDER BY KPIN, KSEQ) as p
ON FIM1.KPIN=p.KPIN ORDER BY FIM1.KPIN
表FIM1(主要零件编号列表)中有118,552条记录.
表FIM2中有66,303条记录(所有这些描述)
表FIM1中的记录62,163中的FIM2中至少有一个描述记录.
如上所述的查询在大约3秒钟内返回62,163条记录.如果将其更改为LEFT JOIN而不是JOIN,我希望获得所有118,552条记录,其中包含很多空值.该操作需要几分钟和几分钟才能完成.我想了解为什么这两个操作之间会有如此巨大的差异.似乎LEFT JOIN迫使引擎一次又一次地遍历FIM2表,但是为什么对于简单的JOIN不必这样做呢？我意识到我正在为每个项目创建一个类似临时记录的东西,但是简单的连接仍然可以对其进行优化.
此行合并技术基于此处详细介绍的概念：
How to pivot? How to convert multiple rows into one row with multiple columns?
我的查询计划：
(对于JOIN)：
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL (null) (null) (null) (null) 64512 Using temporary; Using filesort
1 PRIMARY FIM1 eq_ref PRIMARY,UK_FIM1_KPIN PRIMARY 18 p.KPIN 1
2 DERIVED FIM2 index (null) PRIMARY 19 (null) 64085 Using temporary; Using filesort
对于左联接：
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY FIM1 index (null) IDX_FIM1_DES 30 (null) 124199 Using index; Using temporary; Using filesort
1 PRIMARY <derived2> ALL (null) (null) (null) (null) 64512
2 DERIVED FIM2 index (null) PRIMARY 19 (null) 64085 Using temporary; Using filesort
解决方法:
啊,标准化数据集的乐趣.这不是您要的内容,但是您可以像这样重做此FIM2表,因此每行只有一项而不是三项吗？如果这样做,您的生活会变得更加轻松.
KPIN KSEQ KFLD ATTRIBUTE
R0205 1 1 COLD ROLLED A1008/A CS TYPE B
R0205 1 2 MATTE FINISH OIL / EXPOSED
R0205 1 3 MIN YIELD 30,000 RB 45-60
R0205 2 1 THICKNESS .032 TOL +.006/-.000
R0205 2 2 WIDTH 48.000 TOL +.188/-.000
R0205 2 3 LENGTH 84.875 TOL +/-.020
R0205 3 1 SQUARENESS TOL +/-.062
R0205 3 2 MAX LIFT WEIGHT 10,000 LBS
但是,那不是您要的.您想改进报告程序,以便它可以在合理的时间内左联接这两个表.
分解查询很重要.首先,您尝试左联接FIM1和FIM2表,然后将结果集旋转以将内容扩展到一行.
在功能上正确的方法是,先进行枢轴旋转,然后进行连接.让我们尝试另一种方法,首先加入,然后进行透视.那应该减少满足查询所需的运算次数.
SELECT ff.KPIN, ff.DES,
a.SDS1 AS DES1,
a.SDS2 AS DES2,
a.SDS3 AS DES3,
b.SDS1 AS DES4,
b.SDS2 AS DES5,
b.SDS3 AS DES6,
c.SDS1 AS DES7,
c.SDS2 AS DES8, ...
h.SDS1 AS DES23,
h.SDS2 AS DES24
FROM FIM1 AS ff
LEFT JOIN FIM2 AS a ON ff.KPIN = a.KPIN AND a.KSEQ=1
LEFT JOIN FIM2 AS b ON ff.KPIN = b.KPIN AND b.KSEQ=2
LEFT JOIN FIM2 AS c ON ff.KPIN = c.KPIN AND c.KSEQ=3
LEFT JOIN FIM2 AS d ON ff.KPIN = d.KPIN AND d.KSEQ=4
LEFT JOIN FIM2 AS e ON ff.KPIN = e.KPIN AND e.KSEQ=5
LEFT JOIN FIM2 AS f ON ff.KPIN = f.KPIN AND f.KSEQ=6
LEFT JOIN FIM2 AS g ON ff.KPIN = g.KPIN AND g.KSEQ=7
LEFT JOIN FIM2 AS h ON ff.KPIN = h.KPIN AND h.KSEQ=8
如果使用复合索引(KSEQ,KPIN)对FIM2进行索引,则可能会很快.当然,它通过八个LEFT JOIN操作看起来确实很毛茸茸,但是您的MAX()… GROUP BY查询也很毛茸茸.您还应该尝试使用索引(KSEQ,KPIN)来查看哪个更快.
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