How does MySQL execute JOIN operations internally (nested loop, hash join, etc.)?

• The most common and fastest approach when the join condition column is indexed.

• MySQL probes the inner table index for each row of the outer table.

• Used when the inner table lacks a suitable index.

• MySQL loads chunks (blocks) of rows from the outer table into memory.

• Scans the entire inner table for matches.

• Can be expensive for large joins.

• MySQL supports hash joins for equality-based INNER JOINs.

• MySQL builds an in-memory hash table of the smaller table.

• Then probes it for each row in the larger table.

• Much faster than nested loops on large unindexed tables.

• Not used for non-equality joins (>, <, BETWEEN).

• Requires enabling via optimizer switch.

• MySQL gathers multiple lookup keys and performs batched index lookups.

• Reduces random disk I/O by accessing index pages in sorted order.

• Beneficial for large joins and slow disks.

• MySQL may place outer rows into a join buffer (controlled by join_buffer_size).

• Avoids repeatedly scanning outer table rows.

• Used for non-indexed joins or complex ON conditions.

• MySQL may use temp tables for ORDER BY, GROUP BY, DISTINCT, and subqueries.

• Temp tables can be in-memory or on disk.

• Slows JOIN performance when large intermediate results are needed.