Indexing Long URLs in MySQL

In MySQL, storing and efficiently querying URLs can be challenging, especially when dealing with long URLs that exceed the maximum index key prefix length. In this technical blog, we will explore the problem faced when indexing URLs as multiple byte words and discuss a solution to overcome this limitation.

The Problem:

MySQL has a default index key prefix length of 3072 bytes, which means it can only index a certain number of characters within a column. However, when dealing with URLs, the URLs can be up to 2048 characters or even longer. Additionally, URLs often consist of multi-byte characters due to internationalization and the use of non-ASCII characters. These multi-byte characters require more than one byte to represent, further complicating the indexing process.

When attempting to index a URL column that exceeds the maximum index key prefix length, MySQL will only index first 3072 bytes of URLs. This limitation hinders the ability to enforce uniqueness constraints and efficiently query the URL column.

The Solution: Hashing URLs for Indexing

To overcome the problem of indexing long URLs in MySQL, one possible solution is to use a hash function to generate a fixed-size hash value for each URL. By storing and indexing these hash values instead of the complete URLs, we can enforce uniqueness and perform efficient queries on the URL column.

Here's how the solution can be implemented:

1. Create a new column in the URL table to store the hash values.
2. Before inserting a URL into the table, apply a hash function (e.g., MD5, SHA-256) to generate a hash value for the URL.
3. Index the hash value column.
4. When performing insert operations, first compute the hash value for the URL being inserted.
5. Check if the computed hash value already exists in the hash value column. If a match is found, it indicates a duplicate URL, and the insertion can be rejected.
6. If the computed hash value is unique, insert the URL into the table along with its corresponding hash value.
7. When querying the URL column, apply the same hash function to the search URL and search for its corresponding hash value in the indexed hash value column.

You can see sample implementation here - https://github.com/ssv445/symfony-URL-importer/blob/main/src/Controller/URLImportController.php

Benefits of the Solution:

• Fixed-size index keys: Since hash values have a fixed size regardless of the length of the URL, they can be easily accommodated within the index key prefix length limitation.
• Efficient uniqueness enforcement: By indexing the hash values, duplicate URLs can be efficiently detected during insertion operations.
• Improved query performance: Querying the hash value column can be faster compared to querying the entire URL column, especially when the URL column contains long URLs