SQL Database Version Control and RENAMEs

You’ve just watched Josh Berkus’ presentation “Ten Ways to Wreck Your Database” and bearing in mind point #2 “ranDom_naming(s),” your team has decided to get your act together and adopt a naming convention (although this may not be advantageous to your job security, per josh :-)).

PostgreSQL makes it fairly easy to rename schemas, tables and columns via an appropriate ALTER object RENAME statement. Aside: Other SQL implementations provide similar capabilities to various degrees of completeness. Some SQL version control tools support such changes. For example, Liquibase supports renaming tables, columns and views.

On the other hand, an ALTER statement –like a biological mutation– destroys information, which creates a problem for difference engines such as apgdiff, Andromeda and Pyrseas. It may be obvious to a human that the film.rating column and category table in the production database correspond to the renamed film.mpaa_rating column and categories table, respectively, in the development database. However, there is nothing in the latter’s system catalogs that tie the new names to the previous ones. As a result, difference engines will usually DROP the former objects and CREATE others with the new names.

The Pyrseas solution to this problem is to add an oldname field to the YAML spec. For example, if you have the following abbreviated spec (generated by dbtoyaml from the development database):

schema public:
  table film:
    - mpaa_rating:
        type: character(5)

By editing the spec and making the following change to the mpaa_rating column:

    - mpaa_rating:
        oldname: rating
        type: character(5)

Then yamltodb will, when run against a database that still uses the previous colum name, generate the following SQL script:

ALTER TABLE film RENAME COLUMN rating TO mpaa_rating;

Similarly, you can use oldname on a table to generate ALTER TABLE name RENAME TO new_name and on a schema to generate ALTER SCHEMA name RENAME TO new_name.

This is not a foolproof solution because the rename is a one-time action. You could commit the changed spec to your VCS, but if you ran yamltodb against a database after the RENAME has been applied, it will give an error because it cannot find the oldname object. Aside: I’m considering changing that to simply a warning. Another option would be to allow yamltodb to read more than one YAML spec or one that lists some objects more than once, so the extra information could be added when needed. For these types of changes, a complementary tool such as depesz Versioning can be used to introduce the RENAMEs into production.

Pyrseas 0.1.0 includes the RENAME capability for schemas and tables and I’ve recently committed the changes needed for RENAMEing columns.

In other project news, I’ve also added support for FOREIGN KEY ON UPDATE and ON DELETE actions, support for COMMENT statements on schemas, tables and columns, and corrected a problem with indexes being created in the wrong schema. With these changes, the autodoc regression database schemas, tables, columns, primary keys, foreign keys, indexes and comments on these objects can all be properly recreated using dbtoyaml and yamltodb, with the exception of the inheritance tables.

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