## Summary
Drops the `TaskRun_status_runtimeEnvironmentId_createdAt_id_idx` index
from the `TaskRun` table. After #3742 gated the legacy
`WAITING_FOR_DEPLOY` drain to V1-engine workers only, this index sees
zero scans on both writer and reader replicas. Removing it cuts index
maintenance on every `TaskRun` INSERT/UPDATE.
## Why
The index existed to support `WHERE status = X AND runtimeEnvironmentId
= Y` queries from `ExecuteTasksWaitingForDeployService`, which is
V1-only and no longer triggered on V2 deployments. A code grep across
`apps/webapp` and `internal-packages/run-engine` confirmed no V2
production query uses this access pattern — every other `status:` filter
on `TaskRun` is paired with `id`/`friendlyId`/`parentSpanId` and uses a
different index.
Dropping it also unlocks HOT updates on the dequeue path. The dequeue
`UPDATE` modifies `status` (`QUEUED` -> `DEQUEUED`), and `status` is the
leading column of this index — its presence blocked HOT eligibility for
every `TaskRun` UPDATE. With the index gone, dequeue UPDATEs can become
HOT, reducing WAL bytes and removing the B-tree page contention on this
index's right-edge leaves.
Uses `DROP INDEX CONCURRENTLY` to avoid blocking writes during the drop.
## Sequencing
Should only ship once #3742 has soaked long enough to confirm the index
is genuinely cold (24h+ of zero scans on `pg_stat_user_indexes`).
Eric Allam
9211032733