ruby 14 lines · 1 tab

Database indexes for query optimization

Alex Kumar Jan 2026

1 tab

class AddIndexesToPosts < ActiveRecord::Migration[6.1]
  def change
    add_index :posts, :author_id
    add_index :posts, :published_at
    add_index :posts, [:author_id, :published_at]
    add_index :posts, :created_at, order: { created_at: :desc }

    # Partial index for active published posts
    add_index :posts, :id, where: 'published_at IS NOT NULL AND deleted_at IS NULL', name: 'index_posts_published_active'

    # GIN index for array column tag searches
    add_index :posts, :tags, using: :gin
  end
end

1 file · ruby Explain with highlit

Proper indexing is the difference between millisecond and multi-second query response times. I add indexes to foreign keys automatically since Rails doesn't do this by default, and I create composite indexes for common query patterns that filter on multiple columns. The EXPLAIN output guides index decisions—when I see sequential scans on large tables, that's a signal to add an index. Partial indexes are particularly useful for queries that filter on a specific condition frequently, like WHERE deleted_at IS NULL. I also use index: { unique: true } in migrations to enforce data integrity constraints at the database level. Monitoring slow query logs in production reveals which indexes deliver the most value.

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