嵌套循环联接和哈希联接之间的区别

1.嵌套循环连接：
这是一种物理联接算法，用于联接2个关系的情况。此联接是内部联接技术，这意味着我们看不到联接。这是所有类型的联接中最简单的。这是适用于小数据和小交易的最佳算法。对于名为R和S的2个关系，嵌套循环连接的算法如下：

For each record x of R read in, do
Use the index on B for S
Get all the matching records (having B=x.A)
End

2.哈希加入：
哈希联接也是一种物理联接算法，在内部联接两个表的情况下使用。联接是一种内部联接技术，这意味着我们看不到联接。联接选择由查询优化器自动完成。哈希联接使用两个步骤(构建和探测)执行。在两个关系分别为R和S的情况下，哈希联接的算法如下：

Hash records of R, one by one, using A values
(Use the same M buckets and same hash function h)
Hash matching pair of records into the same bucket
End

嵌套循环联接和哈希联接之间的区别：

S.No.	Nested Loop Join	Hash Join
1.	It is processed by forming an outer loop within an inner loop after which the inner loop is individually processed for the fewer entries that it has.	It is specifically used in case of joining of larger tables.
2.	The nested join has the least performance in case of large tables.	It has best performance in case of large and sorted and non-indexed inputs.
3.	There are two phases in this, outer and inner loop processing.	The two phases in this are build and probe.
4.	Steps involved include identifying an outer driving table and assigning the inner table to it, and processing the rows of inner table for every outer table row.	The steps involved are building a Hash table on a small table. It is used to probe the hash value of the Hash table is applicable for each element in the second row.
5.	Index range scan is done here.	Full-table scan of the smaller table is done in case of hash join.
6.	This uses lesser RAM resources.	It uses more RAM resources.
7.	It is the most common type of join.	It is not as common as the nested loop join.
8.	Least number of comparisons are required in case of nested loop join.	It needs more comparisons than the nested loop join thereby using more RAM resources.
9.	It is the fastest join algorithm due to least number of comparisons.	It is not as fast due to more number of comparisons.
10.	It is better than all other types of join for small transactions and small data.	It is not as good as nested loop join in case of smaller data.
11.	It is of three types, namely, nested loop join, indexed nested loop join and Temporary indexed nested loop join.	Its types are classic hash join, Grace hash join, hybrid hash join, hash anti join, hash semi-join, recursive hash join and hash bailout.
12.	It is not automatically selected.	This join is automatically selected in case there is no specific reason to adopt other types of join algorithms. It is also known as the go-to guy of all the join operators.