What does a Hash Join operation mean in terms of a query? Are there alternatives?

Hash Join is a table join algorithm used in databases to efficiently perform JOIN operations, especially when suitable indexes are not available.

Working principle:

1. A hash table is built for the smaller table based on the join key.
2. For each row in the larger table, the hash key is computed, and a quick search for matching rows in the hash table is performed.

This significantly speeds up the join compared to nested loops.

Alternatives:

• Nested Loop Join — iterates over all pairs of rows, suitable for small tables or when indexes are present.
• Sort-Merge Join — sorts both tables by the key and merges them sequentially, effective with sorted data.

The choice depends on data volume, index availability, and statistics.