How Can I Efficiently Retrieve Multiple Random Results from a Database without Using `ORDER BY RAND()`?

Avoiding Performance Bottlenecks with Random Data Retrieval in MySQL

MySQL's ORDER BY RAND() is a simple way to get random database entries. However, retrieving multiple random rows using this method becomes extremely slow, especially with large datasets.

Consider this query aiming to select 18 random user profiles:

<code class="language-sql">SELECT u.id, 
       p.photo 
FROM users u, profiles p 
WHERE p.memberid = u.id 
  AND p.photo != '' 
  AND (u.ownership=1 OR u.stamp=1) 
ORDER BY RAND() 
LIMIT 18</code>

The inefficiency stems from ORDER BY RAND()'s inherent need to sort the entire result set before applying the LIMIT clause.

A More Efficient Approach

A superior strategy utilizes an indexed column to dramatically speed up random selection. Here's an optimized query:

<code class="language-sql">SELECT 
    g.*
FROM
    table g
        JOIN
    (SELECT 
        id
    FROM
        table
    WHERE
        RAND() < (SELECT 4 / COUNT(*) ) * 10
    LIMIT 4) AS subquery
ON g.id = subquery.id;</code>

Explanation of Optimizations

This revised query incorporates several key improvements:

• Indexed Column Utilization: The subquery filters rows using RAND() against a percentage of the total rows. Indexing the id column (or whichever column is used in the WHERE clause) allows for rapid row identification.
• Randomized IDs Only: Randomization is applied only to IDs, minimizing memory usage and enhancing performance.
• Consistent Limit: The LIMIT clause in both the subquery (4) and the overall query must be identical.
• Refined Subquery: The subquery incorporates the same WHERE conditions as the main query, ensuring data integrity.

This method significantly reduces query execution time, even with massive tables, providing a much more scalable solution for retrieving multiple random results.

Conclusion

These optimization techniques offer a substantial performance boost when dealing with random data selection in MySQL. By avoiding ORDER BY RAND(), you can create more efficient and robust database applications.

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