CREATE INDEX IF NOT EXISTS and ALTER TABLE ADD COLUMN IF NOT EXISTS take
their relation lock before the server evaluates the existence test, so a
boot on an already-migrated database still joins the lock queue. Relation
locks are granted in queue order, so every writer queues behind it.
The shared runner now asks pg_catalog whether the index or column is
already there and skips the statement when a row comes back, and 55P03
is no longer retried by default: with the pre-check ahead of it, a lock
timeout means the object is genuinely missing and each retry re-enters
the queue. Push keeps the old retry behind an explicit option.
Jinwoo-H
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