14 join algorithms

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Join Algorithms

1. Nested Loop Join

The Nested Loop Join algorithm evaluates each tuple from the outer side by searching for matching tuples on the inner side.

Key Points:

• Naive Strategy: Scans all tuples on the inner side for each outer-side tuple, making it inefficient.
• Optimized Usage:
  • Leverage an operator on the inner side to enhance lookup performance.
  • Examples:
    • Using a data node indexed by the lookup column.
    • Applying a materialized operation that reorders data for efficient access.

Example:

The illustration below demonstrates two query trees.

• Left Tree: The movie_cast data node is on the outer side, while the person data node, indexed by person_id, is on the inner side. This setup is efficient since the index facilitates fast lookups for each tuple.

• Right Tree: The sides are reversed, with movie_cast as the inner side. This is inefficient because:
  • The movie_cast node is indexed by both movie_id and person_id, but person_id is the secondary key.
  • Each lookup requires scanning the entire index for matches, resulting in high overhead.

Solutions for Inefficiency:

1. Switch the Sides: Configure the person data node as the inner side to leverage its index (as in the left tree).
2. Add a Materialized Operation: Connect movie_cast to a materialized operation that offers efficient lookups (as in the next example).
3. Use a Hash Join: Replace the Nested Loop Join with a Hash Join for improved performance (as in the next example).

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2. Hash Join

The Hash Join algorithm builds a hash table from the inner side using the join column(s) as the key. The outer side tuples are then matched against this hash table.

Key Points:

• Efficiency: Performs better than Nested Loop Join for larger datasets.
• Memory Usage: Requires memory to store the hash table.
• Equivalent Process:
  • Combines elements of a Nested Loop Join, Hash operator, and Projection operator.

Example:

The figure below compares two equivalent approaches to Hash Join:

• Left Tree: Directly builds a hash table from the inner side and uses it to match tuples from the outer side, avoiding repeated scans.

• Right Tree: Projects relevant columns from the inner side, builds a hash table using these columns, and uses a Nested Loop Join for lookups. The Hash operator is a materialized operation that dynamically aligns its keys with the parent node (the join operator). Both trees achieve the same result, but the left tree is a simplified representation.

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3. Merge Join

The Merge Join algorithm processes both sides of the join simultaneously in a single, forward-only scan, identifying matches as they are encountered.

Key Points:

• Prerequisite: Both sides must be sorted by the join column(s).
  • A Sort operator can be added to ensure the correct order.
• Pitfalls: Unsuitable for unsorted data, as matches may be missed.
• Efficiency:
  • When pre-sorted:
    • It is the most efficient join algorithm.
    • Requires no additional memory.
    • Processes each side exactly once.

Example:

The image below compares two query trees using Merge Join:

• Left Tree: Joins movie and movie_cast nodes, both already sorted by the join column (movie_id). This is an optimal setup for Merge Join.

• Right Tree: Joins movie_cast and person nodes, but requires a Sort operator for the inner side (person) to align tuples by the join column (person_id). When sorting is needed, alternative join strategies may offer better performance.

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By understanding the nuances of these join algorithms, their optimal scenarios, and trade-offs, you can design efficient query plans to maximize relational query performance.

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