What are the relational database join algorithms?

Relational database join algorithms are methods used to combine rows from two or more tables based on a related column between them. The choice of algorithm affects query performance, especially for large datasets. Here are the main types:

1. Nested Loop Join

   • How it works: For each row in the first table (outer table), scan the second table (inner table) to find matching rows.
   • Best for: Small tables or when one table is significantly smaller than the other.
   • Example: If Table A has 10 rows and Table B has 1,000 rows, the algorithm checks each of the 10 rows against all 1,000 rows in Table B.
   • Use in Tencent Cloud: TencentDB for MySQL optimizes such joins for small-scale queries.

2. Hash Join

   • How it works: Build a hash table from one table (usually the smaller one), then probe the hash table with rows from the other table to find matches.
   • Best for: Equi-joins (joins using =) where one table can fit in memory.
   • Example: Joining Orders and Customers on CustomerID, where Customers is hashed for fast lookup.
   • Use in Tencent Cloud: Tencent Cloud’s distributed databases use hash joins for efficient in-memory operations.

3. Merge Join (Sort-Merge Join)

   • How it works: Both tables are sorted on the join key, then merged by scanning both in order to find matches.
   • Best for: Large tables that are already sorted or can be sorted efficiently.
   • Example: Joining two large tables Sales and Products on ProductID after sorting both by ProductID.
   • Use in Tencent Cloud: TencentDB for PostgreSQL employs merge joins for sorted datasets.

4. Index Nested Loop Join

   • How it works: Uses an index on the inner table to avoid full scans, improving efficiency.
   • Best for: When the inner table has an index on the join column.
   • Example: Joining Employees and Departments where Departments has an index on DepartmentID.
   • Use in Tencent Cloud: Tencent Cloud’s managed databases leverage indexes to optimize such joins.

5. Block Nested Loop Join

   • How it works: A variation of nested loop where blocks (chunks) of rows are loaded into memory at once, reducing I/O overhead.
   • Best for: Larger datasets when memory is limited.
   • Example: Processing batches of rows from Table A and Table B in chunks.
   • Use in Tencent Cloud: Tencent Cloud’s databases optimize block processing for memory efficiency.

Choosing the right algorithm depends on table sizes, indexes, and available memory. Tencent Cloud’s database services automatically select optimal join strategies for performance.