What Is External Sorting in Data Structure?


Larry Thompson

What Is External Sorting in Data Structure?

In the world of data structures, sorting is a fundamental operation that arranges data in a specific order. It helps in efficient searching, indexing, and processing of information. While sorting algorithms like Bubble Sort, Insertion Sort, and Quick Sort are well-known for handling small datasets, they may struggle when dealing with large amounts of data that cannot fit into the main memory.

This is where External Sorting comes into play. External Sorting is a technique used to sort large datasets that do not fit entirely into the main memory (RAM) but instead reside on external storage devices such as hard disks or solid-state drives (SSDs).

Why Do We Need External Sorting?

Before we dive into the specifics of external sorting, let’s understand why it is necessary.

In most modern computing systems, the main memory has limited capacity compared to secondary storage devices like hard disks. As a result, it becomes challenging to perform operations on large datasets that cannot be accommodated entirely in RAM.

Sorting algorithms typically require frequent random access to elements during their execution. Random access means accessing any element directly without having to iterate through all previous elements. In the case of large datasets stored externally, accessing elements randomly becomes time-consuming due to disk read/write operations.

Hence, external sorting provides an efficient way to sort large datasets by minimizing disk I/O operations and utilizing both internal and external memory effectively.

How Does External Sorting Work?

The process of external sorting involves several steps:

  1. Splitting: The initial step is to divide the input dataset into smaller subsets called runs. Each run can fit comfortably into the available internal memory.

    The size of each run depends on the available memory and the size of the input dataset.

  2. Sorting: Once the dataset is divided into runs, each run is sorted using an efficient sorting algorithm such as Merge Sort or Quick Sort. Sorting smaller subsets in memory is faster compared to sorting the entire dataset stored externally.
  3. Merging: After sorting each run, the runs are merged together to create a single sorted output file. The merge process involves comparing and combining elements from different runs until all elements are merged into a single sorted sequence.

The splitting, sorting, and merging steps are repeated iteratively until the entire dataset is completely sorted.

Advantages of External Sorting

External Sorting offers several advantages:

  • Efficient Memory Utilization: External Sorting optimizes memory usage by dividing the dataset into smaller runs that can fit into available internal memory resources.
  • Reduced Disk I/O Operations: By minimizing random disk access and performing sequential disk operations during merging, external sorting reduces disk I/O overheads, which can be time-consuming.
  • Scalability: External Sorting enables efficient sorting of large datasets that cannot fit entirely in RAM. It allows processing datasets that exceed the system’s main memory capacity.


In summary, external sorting is a crucial technique for efficiently sorting large datasets that cannot fit into main memory. By dividing the data into smaller subsets, performing in-memory sorting on them, and then merging them back together, external sorting minimizes disk I/O operations and maximizes memory utilization. This technique ensures efficient processing of vast amounts of data by leveraging both internal and external memory resources.

So, the next time you encounter a sorting task involving massive datasets, consider implementing external sorting to optimize performance and achieve better results.

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