Adding new column is slow on large sql table

The joys of working with large SQL tables!

Adding a new column to a large SQL table can be a slow and resource-intensive operation. Here are some reasons why:

1. Index maintenance: When you add a new column, the database needs to update all existing indexes that reference the column. This can be a time-consuming process, especially if the table has many indexes.
2. Data rewriting: The database needs to rewrite the entire table to accommodate the new column. This involves reading and rewriting all existing data, which can take a long time for large tables.
3. Locking and concurrency: When you add a new column, the database may need to lock the table to ensure data consistency. This can lead to concurrency issues and slow down other queries that are running concurrently.
4. Statistics updates: The database needs to update its statistics about the table, including the number of rows, data distribution, and other metadata. This can take some time, especially if the table is very large.

To speed up the process of adding a new column to a large SQL table, you can try the following:

1. Use a temporary table: Create a temporary table with the new column and then swap it with the original table. This can reduce the amount of data that needs to be rewritten.
2. Use a parallel query: If your database supports parallel queries, you can use this feature to speed up the process. This can be especially helpful if you have a multi-core processor.
3. Use a smaller data type: If the new column will only contain a small amount of data, consider using a smaller data type (e.g., int instead of varchar) to reduce the amount of data that needs to be rewritten.
4. Use a partitioned table: If your table is partitioned, you can add the new column to a specific partition instead of the entire table. This can reduce the amount of data that needs to be rewritten.
5. Use a database-specific feature: Some databases, such as Oracle, have features like "online" or "parallel" DDL operations that can help speed up the process of adding a new column.

Here are some specific tips for popular databases:

• MySQL: Use the ALTER TABLE statement with the FAST option to speed up the process. For example: ALTER TABLE mytable ADD COLUMN new_column INT FAST;
• PostgreSQL: Use the ALTER TABLE statement with the REINDEX option to speed up the process. For example: ALTER TABLE mytable ADD COLUMN new_column INT REINDEX;
• Oracle: Use the ALTER TABLE statement with the PARALLEL option to speed up the process. For example: ALTER TABLE mytable ADD COLUMN new_column INT PARALLEL;

Remember to always test your queries and database configuration before making changes to a large table.