A Merge JOIN compares two inputs that are ordered by the JOIN keys and moves through them to find matching values. It can be highly efficient when both inputs are already appropriately sorted.
Understanding Merge JOIN helps explain database execution plans and performance behavior for large sorted datasets.
Join two large datasets that are already ordered by CustomerID.
SELECT
c.CustomerID,
c.CustomerName,
o.OrderID
FROM Customers c
INNER JOIN Orders o
ON c.CustomerID = o.CustomerID
ORDER BY c.CustomerID;Common in query optimization, data warehouses, and large analytical workloads.
- Merge JOIN works with ordered inputs.
- It compares values in sequence.
- Sorting may be required if inputs are not already ordered.
- It can perform well on large datasets.
- The optimizer chooses the appropriate strategy.
What is a Merge JOIN?
A Merge JOIN matches two ordered inputs by moving through their JOIN keys in sequence.
When can Merge JOIN be efficient?
It can be efficient when both inputs are already sorted or can be efficiently provided in JOIN-key order.