pub const CLAIM_ACTION: &str = "DELETE FROM queue WHERE id = :id RETURNING id";Expand description
Deletes the row an owner is about to apply, and reports whether it was still there.
It runs first in the applying transaction, not last: the pending rows
are read outside any transaction, so a second owner reading the same
list would otherwise apply every action twice, and add and copy
are not idempotent. Claiming the row first makes exactly-once a
property of the statement rather than a convention about who drains.