The document discusses migrating a database table's column from storing percentage values in the range of 0 to 100 to storing decimal values in the range of 0 to 1, without downtime or changing the database schema. It proposes doing this by adding a new column to store the decimal values, populating it from the existing percentage column, then dropping the original column and modifying the new one to be the main value column. This allows applications to continue reading and writing as normal during the migration.

1. Database compatibility(i.e. Application talking to a DB) Select statement App. is reader, DB is writer Insert App. is writer, DB is reader Company Confidential - Wealthfront Inc.

2. DB: Rules of Engagement Adding a table: ✔ Removing a table: ✔ Table is unused Removing a column: ✔ Column is unused Adding a column: ✔ NullableOR Not-null with default value Renaming a column: ✖ Company Confidential - Wealthfront Inc.

3. DB: Dogfighting Table users with two columns id and value Value is a percentage, stored as [0, 100] Migrate to [0, 1] representation Clustered servers talking to the DB No downtime Company Confidential - Wealthfront Inc.

4. DB: Dogfighting Company Confidential - Wealthfront Inc. Reads id & value Writes [0, 100] in value create table users ( id int not null, value int not null);

5. DB: Dogfighting Company Confidential - Wealthfront Inc. alter table users modify value int, add value_dec decimal;

6. DB: Dogfighting Company Confidential - Wealthfront Inc. Reads id & value Writes [0, 100] in value Writes [0, 1] in value_dec

7. DB: Dogfighting Company Confidential - Wealthfront Inc. update users set value_dec = value / 100 where value_dec is null;

8. DB: Dogfighting Company Confidential - Wealthfront Inc. Reads id & value_dec Writes [0, 1] in value_dec

9. DB: Dogfighting Company Confidential - Wealthfront Inc. alter table users drop column value, modify value_dec decimal not null;

10. DB: Top Gun Table users with two columns id and value Value is a percentage, stored as [0, 100] Migrate to [0, 1] representation Clustered servers talking to the DB No downtime No DDL change (value is decimal) Company Confidential - Wealthfront Inc. NEW