Sort Merge Joins

What is a Sort-Merge-Join

A sort-merge join is one of the join methods used by the Oracle optimizer to combine rows from two tables based on a join condition. It is particularly efficient when both inputs to the join are sorted on the join key, either inherently or through sorting during query execution. 

How Does a Sort-Merge-Join Work

The process involves two main steps

1. Sort Phase:

   • Oracle sorts both tables (or data streams) on the join key if they are not already sorted.

2. Merge Phase:

   • Oracle iterates through the sorted data from both tables, merging rows that satisfy the join condition.

When Does Oracle Choose a Sort-Merge-Join

Oracle chooses a sort-merge-join in scenarios such as, although not limited to, the following

• Lack of Indexes: When neither of the tables in the join has a suitable index for a nested loop join.

• Large Data Sets: The optimizer determines that sorting both inputs and merging is more efficient than hash joins or nested loop joins.

• Sorted Input: One or both inputs are already sorted on the join key (e.g., due to an ORDER BY, index range scan, or sorted subquery output), reducing the cost of sorting.

Advantages of a Sort-Merge-Join

The following are some of the advantages of sort-merge-join

• Efficient for Large Datasets: Handles large volumes of data better than nested loop joins in certain cases.

• Streaming Capability: Can process data in a streaming fashion, reducing memory overhead compared to hash joins.

• Works Well for Sorted Inputs: When one or both inputs are pre-sorted, the sort phase is skipped or minimized.

Disadvantages of a Sort-Merge-Join

The following are some of the disadvantages of sort-merge-join

• Sorting Overhead: If inputs are unsorted, the sorting phase can be expensive, especially for very large tables.

• Equality Condition Limitation: Not suitable for non-equality joins.

• Higher Disk I/O: If sorting does not fit into memory, temporary disk I/O might increase.

Comparisons with Other Joins

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Published 26th November 2024