Hive 的隐式连接总是内部连接吗? [英] Are Hive's implicit joins always inner joins?

问题描述

Hive 的 join 文档鼓励使用隐式连接，即

The join documentation for Hive encourages the use of implicit joins, i.e.

SELECT * 
FROM table1 t1, table2 t2, table3 t3 
WHERE t1.id = t2.id AND t2.id = t3.id AND t1.zipcode = '02535';

这相当于

SELECT t1.*, t2.*, t3.*
FROM table1 t1
INNER JOIN table2 t2 ON
  t1.id = t2.id 
INNER JOIN table3 t3 ON
  t2.id = t3.id
WHERE t1.zipcode = '02535'

，或者上面会返回额外的记录?

, or will the above return additional records?

推荐答案

并非总是如此.您的查询是等效的.但是如果没有 WHERE t1.id = t2.id AND t2.id = t3.id，它将是 CROSS JOIN.

Not always. Your queries are equivalent. But without WHERE t1.id = t2.id AND t2.id = t3.id it will be CROSS JOIN.

更新:

这是一个有趣的问题，我决定添加一些演示.让我们创建两个表:

This is interesting question and I decided to add some demo. Let's create two tables:

A(c1 int, c2 string) 和 B(c1 int, c2 string).

加载数据:

insert into table A
 select 1, 'row one' union all
 select 2, 'row two';

insert into table B
 select 1, 'row one' union all
 select 3, 'row three';

检查数据:

hive> select * from A;
OK
1       row one
2       row two
Time taken: 1.29 seconds, Fetched: 2 row(s)
hive> select * from B;
OK
1       row one
3       row three
Time taken: 0.091 seconds, Fetched: 2 row(s)

检查交叉连接(没有where转换为交叉的隐式连接):

Check cross join (implicit join without where transformed to cross):

hive> select a.c1, a.c2, b.c1, b.c2 from a,b;
Warning: Map Join MAPJOIN[14][bigTable=a] in task 'Stage-3:MAPRED' is a cross product
Warning: Map Join MAPJOIN[22][bigTable=b] in task 'Stage-4:MAPRED' is a cross product
Warning: Shuffle Join JOIN[4][tables = [a, b]] in Stage 'Stage-1:MAPRED' is a cross product

OK
1       row one 1       row one
2       row two 1       row one
1       row one 3       row three
2       row two 3       row three
Time taken: 54.804 seconds, Fetched: 4 row(s)

检查内连接(使用 where 作为 INNER 的隐式连接):

Check inner join (implicit join with where works as INNER):

hive> select a.c1, a.c2, b.c1, b.c2 from a,b where a.c1=b.c1;
OK
1       row one 1       row one
Time taken: 38.413 seconds, Fetched: 1 row(s)

尝试通过将 OR b.c1 is null 添加到 where 来执行左连接:

Try to perform left join by adding OR b.c1 is null to the where:

hive> select a.c1, a.c2, b.c1, b.c2 from a,b where (a.c1=b.c1) OR (b.c1 is null);
OK
1       row one 1       row one
Time taken: 57.317 seconds, Fetched: 1 row(s)

如您所见，我们再次获得了内连接.或b.c1为空被忽略

As you can see we got inner join again. or b.c1 is null is ignored

现在 left join 没有 where 和 ON 子句(转换为 CROSS):

Now left join without where and ON clause (transformed to CROSS):

 select a.c1, a.c2, b.c1, b.c2 from a left join b;
OK
1       row one 1       row one
1       row one 3       row three
2       row two 1       row one
2       row two 3       row three
Time taken: 37.104 seconds, Fetched: 4 row(s)

正如你所看到的，我们再次交锋.

尝试使用 where 子句和不使用 ON 的左连接(用作 INNER):

Try left join with where clause and without ON (works as INNER):

select a.c1, a.c2, b.c1, b.c2 from a left join b where a.c1=b.c1;
OK
1       row one 1       row one
Time taken: 40.617 seconds, Fetched: 1 row(s)

我们得到了 INNER 连接

尝试使用 where 子句进行左连接，而不使用 ON+ 尝试允许空值:

Try left join with where clause and without ON+ try to allow nulls:

 select a.c1, a.c2, b.c1, b.c2 from a left join b where a.c1=b.c1 or b.c1 is null;
OK
1       row one 1       row one
Time taken: 53.873 seconds, Fetched: 1 row(s)

再次获得内在.或 b.c1 is null 被忽略.

Again got INNER. or b.c1 is null is ignored.

左连接与 on 子句:

Left join with on clause:

hive> select a.c1, a.c2, b.c1, b.c2 from a left join b on a.c1=b.c1;
OK
1       row one 1       row one
2       row two NULL    NULL
Time taken: 48.626 seconds, Fetched: 2 row(s)

是的，这是真正的左连接.

Left join with on + where(得到 INNER):

Left join with on + where (got INNER):

hive> select a.c1, a.c2, b.c1, b.c2 from a left join b on a.c1=b.c1 where a.c1=b.c1;
OK
1       row one 1       row one
Time taken: 49.54 seconds, Fetched: 1 row(s)

我们得到 INNER 是因为 WHERE 不允许 NULL.

左连接 where + 允许空值:

hive> select a.c1, a.c2, b.c1, b.c2 from a left join b on a.c1=b.c1 where a.c1=b.c1 or b.c1 is null;
OK
1       row one 1       row one
2       row two NULL    NULL
Time taken: 55.951 seconds, Fetched: 2 row(s)

是的，是左连接.

结论:

1. 隐式连接作为 INNNER (with where) 或 CROSS 如果没有 WHERE条款.
2. 如果没有 ON 和 WHERE，左连接可以作为 CROSS 工作，如果 WHERE 子句不允许空值，也可以作为 INNER 工作为了权利表.
3. 最好使用 ANSI 语法，因为它是不言自明的，并且很容易理解您期望它的工作方式.作为 INNER 或 CROSS 工作的隐式连接或左连接很难理解并且很容易出错.

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