change function and the up & down functions in Ecto migrations:

Ecto migrations, the change , the up and down functions

change Function:

Automatic Reversibility: Used when Ecto can automatically infer how to reverse the migration.

Simple Operations: Suitable for straightforward schema changes like creating/dropping tables,
adding/removing columns, and modifying indexes.


up and down Functions:

Explicit Reversibility: Required when Ecto cannot automatically reverse the migration steps.
Complex Operations: Ideal for complex migrations involving data transformation, executing raw SQL, or irreversible commands.


Use change: For reversible migrations where Ecto can handle rollback automatically.
Use up and down: When migrations involve operations that need explicit instructions for both applying and rolling back.
Key Differences:

Automatic vs. Manual: change relies on Ecto's automatic reversal capabilities, while up and down require manual definitions.
Complexity Handling: change is for simple, reversible changes; up and down are for complex or non-reversible changes.

Best Practices:     

Assess Reversibility: Use change when all operations are reversible; otherwise, opt for up and down.
Maintain Clarity: Use explicit functions (up, down) for clarity in complex migrations.