Impact of non-clustered indexes on INSERT and UPDATE operations

22 Sep 2023

Non-clustered indexes are critical for improving the performance of query operations in a database. They provide a fast way to locate data by creating a separate data structure that points to the actual data in a table. While non-clustered indexes greatly enhance query performance, they can also have an impact on INSERT and UPDATE operations.

INSERT Operations

When inserting new data into a table that has non-clustered indexes, the presence of these indexes can slow down the INSERT operation. This is because each non-clustered index must be updated to reflect the new data. The more indexes a table has, the longer it will take to perform an INSERT operation. However, the impact can vary depending on the size of the table and the number of indexes.

To minimize the impact of non-clustered indexes on INSERT operations, you can consider the following strategies:

Disable Non-Clustered Indexes: Temporarily disabling non-clustered indexes before performing the INSERT operation can significantly improve the performance. Once the data has been inserted, the indexes can be re-enabled.

-- Disable indexes
ALTER INDEX ALL ON your_table DISABLE;
   
-- Insert data
INSERT INTO your_table (column1, column2) VALUES (value1, value2);
   
-- Enable indexes
ALTER INDEX ALL ON your_table REBUILD;

Batch Inserts: Instead of inserting one record at a time, you can optimize performance by using bulk INSERT operations or batch inserts. This reduces the overhead of updating indexes for each individual record.
```
INSERT INTO your_table (column1, column2)
VALUES (value1, value2),
       (value3, value4),
       (value5, value6);
```

UPDATE Operations

Non-clustered indexes can also impact the performance of UPDATE operations. When updating data in a table, non-clustered indexes need to be updated to maintain consistency. If a column being updated is part of one or more non-clustered indexes, the indexes will need to be updated accordingly.

Similar to INSERT operations, there are a few strategies to minimize the impact on UPDATE operations:

Disabling Non-Clustered Indexes: Temporarily disabling non-clustered indexes can improve the UPDATE performance. This can be done using the same method as mentioned earlier for INSERT operations.
Bulk Updates: Just like batch inserts, performing bulk updates rather than individual updates can improve the performance. Updating multiple rows in a single SQL statement reduces the need for updating indexes multiple times.
```
UPDATE your_table
SET column1 = value1,
    column2 = value2
WHERE condition;
```

In conclusion, while non-clustered indexes greatly enhance query performance, they can have a direct impact on INSERT and UPDATE operations. By employing strategies like disabling indexes or performing bulk operations, you can optimize the performance of these operations. It’s important to weigh the benefits of improved query performance against the potential impact on data modification operations when deciding whether to use non-clustered indexes.