"""videos: a random key per video, so a seeded shuffle walks an index instead of sorting the scope

Revision ID: 0118_videos_shuffle_key
Revises: 0117_videos_length_sort_indexes
Create Date: 2026-11-07

The feed's Shuffle sort and the Home surprise ordered the whole feed scope by md5(id || seed) on every
call: ~350 ms on prod for a scope of ~213k videos, and a Shuffle page deeper in the list cost more
(767 ms on page 7 on localdev, against 4 ms for Newest). Each video now carries a random number drawn
once (`Video.shuffle_key`); a seed picks a point on that ring and the draw reads the index from there
(`feedquery.shuffle_walk`).

One ADD COLUMN with a volatile default rewrites the table and fills every row with its own value. On
localdev (278k videos) that took 28 s, against 84 s for an UPDATE of a nullable column, so the rewrite
it is: the table is locked for that time, and a deploy has the app down while it migrates anyway.
The index is (shuffle_key, id): the walk's keyset cursor compares the pair.
"""

from alembic import op

revision: str = "0118_videos_shuffle_key"
down_revision: str | None = "0117_videos_length_sort_indexes"
branch_labels: str | None = None
depends_on: str | None = None


def upgrade() -> None:
    op.execute(
        "ALTER TABLE videos ADD COLUMN IF NOT EXISTS shuffle_key double precision "
        "NOT NULL DEFAULT random()"
    )
    with op.get_context().autocommit_block():
        op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_shuffle_key")
        op.execute("CREATE INDEX CONCURRENTLY ix_videos_shuffle_key ON videos (shuffle_key, id)")


def downgrade() -> None:
    with op.get_context().autocommit_block():
        op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_shuffle_key")
    op.execute("ALTER TABLE videos DROP COLUMN IF EXISTS shuffle_key")