Impact of ordered vs. random index insertion in InnoDB

The impact of ordered versus random index insertion in InnoDB depends on several factors, including the size and complexity of the table, the frequency of inserts, and the workload of the database.

Ordered index insertion is more efficient than random index insertion for InnoDB tables because InnoDB uses a clustered index that physically orders the data based on the primary key. If the primary key values are inserted in order, the new rows are inserted into the correct physical location, which minimizes the amount of page splits and disk I/O. This reduces fragmentation and improves the overall performance of the table.

Random index insertion, on the other hand, can cause more page splits and disk I/O. This is because InnoDB may need to move existing rows to make room for the new row if it is inserted between two existing rows. This can cause fragmentation and decrease the overall performance of the table.

However, if the workload of the database involves frequent updates and deletes, random index insertion may be preferable because it can reduce the likelihood of hot spots, where many inserts are concentrated in a small area of the index. This can improve concurrency and reduce contention for table-level locks.

In general, the best approach to index insertion in InnoDB is to choose a primary key that is meaningful for the data, and to avoid inserting rows in a completely random order unless it is necessary for the workload of the database.

Visual impact of ordered vs. random index insertion in InnoDB

The visual impact of ordered versus random index insertion in InnoDB can be seen in the physical layout of the table on disk.

With ordered index insertion, the data is physically ordered on disk based on the primary key, so the new rows are inserted into the correct physical location. This results in a compact table with minimal fragmentation and a clear logical order to the data.

In contrast, with random index insertion, the data is not inserted into a predictable physical location, and InnoDB may need to move existing rows to make room for new rows. This can result in a fragmented table with less predictable physical order, making it more difficult to optimize the performance of the table.

To illustrate this visually, consider the following example where we have an InnoDB table with a primary key on a column named “id”. The first diagram shows the physical layout of the table after inserting rows with ordered values for “id”, while the second diagram shows the physical layout of the table after inserting rows with randomly generated values for “id”.

Ordered index insertion:

Random index insertion: 

As you can see, the table with ordered index insertion has a more compact and predictable physical layout, while the table with random index insertion has a less predictable physical layout and more fragmentation.

Conclusion

In InnoDB, ordered index insertion is more efficient than random insertion, reducing fragmentation and improving table performance. While random insertion can reduce hot spots in certain scenarios, it often leads to increased fragmentation and disk I/O. Choosing a meaningful primary key and avoiding random insertion is recommended for optimal InnoDB performance.

About Shiv Iyer 444 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.