Can you explain how the HASH JOIN operation works in databases?

The HASH JOIN operation is used in databases to join two tables based on a key. The working principle:

1. In the first stage, a hash table is built based on the key from the smaller table (build phase).
2. Then, for each record from the second table (probe phase), a search for the corresponding key in the hash table is performed.

This allows for quick matching without the need for sorting or sequentially scanning all records, which is especially efficient with large volumes of data.

Example:

SELECT * FROM orders o
JOIN customers c ON o.customer_id = c.id;

Within the DBMS, HASH JOIN may be used if the customers table is small — it will be built into a hash table, and then a quick search will be performed for each record in orders.