Recently used.

I have seen it before, but it is useless. recently used.
Hash table and Hash Function
For a dynamic search table, 1) The table length is unknown; 2) when designing a search table, you only know the scope of the keyword, but not the exact keyword. Therefore, we usually need to establish a function relationship. The position of the table where f (key) is used as the key is recorded. This function f (key) is usually called a hash function. (Note: This function is not necessarily a mathematical function)
A hash function is an image that maps a set of keywords to an address set. Its settings are flexible as long as the size of this address set does not exceed the permitted range.
In reality, the hash function must be constructed and can be used properly.
Multi-table join query, left Outer Join, right Outer Join, full join