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The Migration That Locks The Table
Description
Your Rails migration adds a column with a default value to a table with 12 million rows. Postgres 11+ handles this instantly — no table rewrite needed. And indeed, the ALTER TABLE ADD COLUMN completes in 12ms. But the migration still holds a table lock for 4+ minutes, killing production.
Something else in the migration is the problem. Find the slow operation.
What is the exact Rails migration method call that causes the 4-minute lock?
The column was instant. The index was not.
Input Data
```ruby
class AddStatusToOrders < ActiveRecord::Migration[7.1]
def change
add_column :orders, :status, :string, default: 'pending', null: false
add_index :orders, :status
end
end
```
```
# PostgreSQL log:
LOG: ALTER TABLE "orders" ADD "status" varchar DEFAULT 'pending' NOT NULL
duration: 12.345 ms <-- instant ✓
LOG: CREATE INDEX "index_orders_on_status" ON "orders" ("status")
duration: 245891.432 ms <-- 4+ MINUTES, holds AccessExclusive lock
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