Pgroll is a tool for zero-downtime, reversible schema migrations in PostgreSQL, addressing issues like locking and data back-filling. It allows for safe application versioning and easy rollbacks by managing hidden columns and using views. The migration process integrates seamlessly with application upgrades, ensuring minimal disruption.

1. pgroll - Zero-downtime, reversible,
schema migrations for Postgres
Tudor Golubenco
July 16, 2024
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xata.io
Xata - Postgres Data Platform
Fully managed Postgres service
+ Branching
+ Replication to OpenSearch
+ Schema changes
+ File attachments
+ Excellent support

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https://github.com/xataio/pgroll

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(Some) Postgres schema changes are difficult
● Locking issues (e.g. on a busy table, in a fast a migration can wait at an exclusive lock and cause
downtime)
https://xata.io/blog/migrations-and-exclusive-locks
● Constraints might require data back-filling (if you add a NOT NULL + UNIQUE column, how do you fill the
data?)
● Backwards incompatible changes require a multiple step process (e.g. renames)
Motivation

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How
https://xata.io/blog/multi-version-schema-migrations

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How
● Hidden columns
are added to the
physical table
● Data is backfilled
and transformed in
background
● Views hide or show
the different
columns

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How - Application selects it’s version by setting the `search_path`

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● Rollback is easy - just drop the views and intermediary columns.
● The tool takes care of locking issues and common issues.
● The merging workflow is always the same:
○ Start the pgroll migration
○ Roll-out the application upgrade (can be blue-green)
○ Complete the pgroll migration
Benefits

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Thanks!
PS: we’re hiring at Xata