Migrations

My talk: http://qasimk.io/talks/2019/database-migrations-2#1

An excellent article: https://benchling.engineering/move-fast-and-migrate-things-how-we-automated-migrations-in-postgres-d60aba0fc3d4

Cliffnotes for Postgres

  • Adding a NOT NULL field , or one with a dynamic default value (NB: < Postgres 11) causes the entire table and its indexes to be rewritten
    • Do multi-step deploy
  • Adding a new NOT NULL with DEFAULT applies default to all existing rows
    • Add default after column is created applies it to new rows only
  • Removing a NOT NULL field can cause the ORM to read/write a non-existant field
    • Migrate to NULL before removing code
  • Creating an index locks the entire table, preventing writes
    • Use CREATE INDEX CONCURRENTLY
  • Creating constraints (inc. NOT NULL) locks the entire table.
    • Use 2-step creation: ... NOT VALID and VALIDATE CONSTRAINT - NOT VALID uses ACCESS EXCLUSIVE, but VALIDATE only uses SHARE UPDATE EXCLUSIVE and a full table scan.
  • Most ALTER TABLE operations (including ADD COLUMN, DROP COLUMN) require an ACCESS EXCLUSIVE lock which waits for ALL other queries to complete (including SELECT!)
    • Set a very low lock_timeout and statement_timeout (< 10 seconds) to allow migration to fail without affecting normal operation.
    • Reduce size of transactions/queries; beware of SQLAlchemy's automatic start of transaction even on a first select.
  • Default ORM queries select all columns to populate the full object, this is a problem because...
  • DROP COLUMN hides column, and gradually, as writes occur, replaces it with NULLs to free up space.

Automated Checks

  • Check indexes are created concurrently
  • Check constraints are created in the background
  • Check new columns are NULLABLE [[without a default value]]
  • Check removed columns are NULLABLE
  • In tests, check queries against a new deprecated_column to ensure they are not being used before removal
  • Use a renamed_to column-thing to automatically create SQL triggers to populate data bi-directionally
  • (SQLAlchemy) Check every foreign key is covered by an index

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