SQL连接如何工作? [英] How does SQL join work?
问题描述
我试图了解联接在内部如何工作.以下两个查询的运行方式之间会有什么区别?
I am trying to understand how does joins work internally. What will be the difference between the way in which the following two queries would run?
For example
(A)
Select *
FROM TABLE1
FULL JOIN TABLE2 ON TABLE1.ID = TABLE2.ID
FULL JOIN TABLE3 ON TABLE1.ID = TABLE3.ID
And
(B)
Select *
FROM TABLE1
FULL JOIN TABLE2 ON TABLE1.ID = TABLE2.ID
FULL JOIN TABLE3 ON TABLE2.ID = TABLE3.ID
我在这里谈论甲骨文. 考虑表2和表3中的某些记录，但表1中没有的记录，查询A将为该记录提供两行，而B将仅提供一行.
I am talking about oracle here. Consider some records present in table 2 and table 3 but not in table 1, query A would give two rows for that record but B would give only one row.
推荐答案
您的DBMS优化器将确定如何最好地执行查询.通常，这是通过基于成本的优化"来完成的，其中考虑了多种不同的查询计划，并选择了最有效的一种. 如果，您的两个查询在逻辑上是相同的，则无论编写哪种方式，该优化器很可能最终都会使用相同的查询计划.实际上，由于SQL的这种细微差别，产生不同的查询计划的这些天来，优化器是一个糟糕的选择.
Your DBMS's optimiser will determine how best to perform the query. Usually this is done by "cost based optimisation", where a number of different query plans are considered and the most efficient one selected. If your two queries are logically identical, it is most likely that the optimiser will end up using the same query plan whichever way you write it. In fact, it would be a poor optimiser these days that produced different query plans based on such minor differences in the SQL.
但是，完全外部联接是另一回事(至少在Oracle中是如此)，因为列的联接方式会影响结果.也就是说，这两个查询不可互换.
However, full outer joins are a different matter (in Oracle at least), since the way the columns are joined influences the result. i.e. the 2 queries are not interchangeable.
您可以在SQL Plus中使用AUTOTRACE来查看不同的计划:
You can use AUTOTRACE in SQL Plus to see the different plans:
SQL> select *
2 from t1
3 full join t2 on t2.id = t1.id
4 full join t3 on t3.id = t2.id;
ID ID ID
---------- ---------- ----------
1 1
1 row selected.
Execution Plan
----------------------------------------------------------
---------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
---------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 117 | 29 (11)|
| 1 | VIEW | | 3 | 117 | 29 (11)|
| 2 | UNION-ALL | | | | |
|* 3 | HASH JOIN OUTER | | 2 | 142 | 15 (14)|
| 4 | VIEW | | 2 | 90 | 11 (10)|
| 5 | UNION-ALL | | | | |
|* 6 | HASH JOIN OUTER | | 1 | 91 | 6 (17)|
| 7 | TABLE ACCESS FULL| T1 | 1 | 52 | 2 (0)|
| 8 | TABLE ACCESS FULL| T2 | 1 | 39 | 3 (0)|
|* 9 | HASH JOIN ANTI | | 1 | 26 | 6 (17)|
| 10 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
| 11 | TABLE ACCESS FULL| T1 | 1 | 13 | 2 (0)|
| 12 | TABLE ACCESS FULL | T3 | 1 | 26 | 3 (0)|
|* 13 | HASH JOIN ANTI | | 1 | 26 | 15 (14)|
| 14 | TABLE ACCESS FULL | T3 | 1 | 13 | 3 (0)|
| 15 | VIEW | | 2 | 26 | 11 (10)|
| 16 | UNION-ALL | | | | |
|* 17 | HASH JOIN OUTER | | 1 | 39 | 6 (17)|
| 18 | TABLE ACCESS FULL| T1 | 1 | 26 | 2 (0)|
| 19 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
|* 20 | HASH JOIN ANTI | | 1 | 26 | 6 (17)|
| 21 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
| 22 | TABLE ACCESS FULL| T1 | 1 | 13 | 2 (0)|
---------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T3"."ID"(+)="T2"."ID")
6 - access("T2"."ID"(+)="T1"."ID")
9 - access("T2"."ID"="T1"."ID")
13 - access("T3"."ID"="T2"."ID")
17 - access("T2"."ID"(+)="T1"."ID")
20 - access("T2"."ID"="T1"."ID")
SQL> select *
2 from t1
3 full join t2 on t2.id = t1.id
4 full join t3 on t3.id = t1.id;
ID ID ID
---------- ---------- ----------
1
1
2 rows selected.
Execution Plan
----------------------------------------------------------
---------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
---------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 117 | 29 (11)|
| 1 | VIEW | | 3 | 117 | 29 (11)|
| 2 | UNION-ALL | | | | |
|* 3 | HASH JOIN OUTER | | 2 | 142 | 15 (14)|
| 4 | VIEW | | 2 | 90 | 11 (10)|
| 5 | UNION-ALL | | | | |
|* 6 | HASH JOIN OUTER | | 1 | 91 | 6 (17)|
| 7 | TABLE ACCESS FULL| T1 | 1 | 52 | 2 (0)|
| 8 | TABLE ACCESS FULL| T2 | 1 | 39 | 3 (0)|
|* 9 | HASH JOIN ANTI | | 1 | 26 | 6 (17)|
| 10 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
| 11 | TABLE ACCESS FULL| T1 | 1 | 13 | 2 (0)|
| 12 | TABLE ACCESS FULL | T3 | 1 | 26 | 3 (0)|
|* 13 | HASH JOIN ANTI | | 1 | 26 | 15 (14)|
| 14 | TABLE ACCESS FULL | T3 | 1 | 13 | 3 (0)|
| 15 | VIEW | | 2 | 26 | 11 (10)|
| 16 | UNION-ALL | | | | |
|* 17 | HASH JOIN OUTER | | 1 | 39 | 6 (17)|
| 18 | TABLE ACCESS FULL| T1 | 1 | 26 | 2 (0)|
| 19 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
|* 20 | HASH JOIN ANTI | | 1 | 26 | 6 (17)|
| 21 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)|
| 22 | TABLE ACCESS FULL| T1 | 1 | 13 | 2 (0)|
---------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T3"."ID"(+)="T1"."ID")
6 - access("T2"."ID"(+)="T1"."ID")
9 - access("T2"."ID"="T1"."ID")
13 - access("T3"."ID"="T1"."ID")
17 - access("T2"."ID"(+)="T1"."ID")
20 - access("T2"."ID"="T1"."ID")
实际上，除了谓词信息外，查询计划是相同的
In fact, the query plans are identical except for the Predicate information
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