In this lesson, you will learn how to resolve "hotspot" contention in B-Tree indexes caused by high-velocity, monotonically increasing data.
The Problem: The "Right-Side" Bottleneck
When you define a primary key using a sequence or an auto-incrementing integer, every new row you insert gets a value higher than the last. Because a B-Tree index maintains its data in a sorted order, every new entry must be inserted into the "right-most" leaf node.
In a high-concurrency system, this creates a performance bottleneck. All concurrent insert transactions compete for a single lock on the same leaf node. The tree isn't balanced in terms of traffic; it is "right-heavy." While the rest of the index remains idle, the final node becomes a massive contention point, limiting your throughput.
The Solution: Reversing the Key
A Reverse Key Index solves this by physically storing the index entries with their bytes reversed. For example, if you are inserting IDs 1001, 1002, and 1003, the database will store them as 1001, 2001, and 3001 (simplified).
By reversing the bytes, sequential numbers are distributed across the leaf nodes rather than clustering at the very end of the tree.
-- Example: Creating a reverse key index on a primary key column
CREATE INDEX idx_orders_id_reverse
ON Orders (order_id) REVERSE;
Impact on Performance
When you use a reverse key index, the database spreads the inserts across the entire range of the B-Tree. This transforms a single-node contention problem into a parallelized operation. Multiple leaf nodes are updated simultaneously, reducing lock waits and allowing the hardware to work at its full capacity.
Note of Caution: Because the bytes are reversed, the index is no longer efficient for range scans. If you execute a query like WHERE order_id BETWEEN 1000 AND 2000, the database cannot perform an efficient range scan because the logical order of the values does not match their physical storage order. Reverse key indexes are strictly for point-lookup performance and alleviating insert pressure.
Key Takeaway: Reverse Key Indexes spread sequential inserts across the index tree to eliminate hotspot contention at the cost of losing range-scan efficiency.