Schema Modification Operators (SMOs)

Last updated Feb 16, 2025

Schema Modification Operators (SMOs) are formal, operations that specify how to transform one database schema into another. Think of them as a standardized vocabulary or set of commands for making schema changes.

Here are some common examples of SMOs:

1. Adding or dropping a table
2. Adding, renaming, or dropping a column
3. Merging two tables into one
4. Splitting one table into two
5. Moving a column from one table to another
6. Changing a column’s data type

The key benefits of using SMOs are:

• Predictability: Each operation has a clear, defined outcome
• Automation: They can be scripted and executed automatically
• Reversibility: Many SMOs can be reversed, allowing rollback if needed
• Documentation: They provide a clear record of schema changes
• Validation: Changes can be verified before execution

For example, instead of making ad-hoc schema changes, a team might define a series of SMOs like:

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2
3

ADD COLUMN customer.email VARCHAR(255);
RENAME COLUMN customer.phone TO customer.phone_number;
MERGE TABLE address INTO customer;

This systematic approach makes schema evolution more manageable, especially in large databases where changes need to be carefully controlled and documented.

Origin: Schema Evolution vs. Data Contracts vs. NoSQL
References: Schema Evolution, Liquibase
Created 2025-02-16