-- 021_session_blocker_and_closed_at.up.sql
-- Track why a session ended partial/failed and when it actually closed.
-- See plans/2026-07-20-session-review-ten-sessions.md P1.5.
--
-- `blocker` is a short structured reason: "approval_timeout",
-- "classifier_overreach", "user_abandoned", "tool_error", "model_refusal",
-- etc. Set by complete_task when outcome is partial/failed, derived from the
-- last assistant message's text. Empty for success outcomes.
--
-- `closed_at` is when the session reached its terminal state. Distinct from
-- `last_active_at`, which is touched on any access (including the operator
-- just opening the transcript) — `closed_at` is set ONCE at completion.
-- Without it, "session duration" can only be computed as
-- `last_active - created`, which lies for reopened sessions (a51e2086
-- reported 4-day duration because the operator reopened it to close it).

ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS blocker   TEXT NOT NULL DEFAULT '';
ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS closed_at TIMESTAMPTZ;

-- Backfill closed_at for already-terminal sessions so the new column isn't
-- NULL forever on existing rows. Use last_active_at as the best proxy — it's
-- the most recent touch, which is the closest we have to "when it ended"
-- for historical sessions. New sessions set closed_at explicitly on
-- complete_task.
UPDATE agent_sessions
SET closed_at = last_active_at
WHERE closed_at IS NULL
  AND status IN ('done', 'failed');

-- Index for "show me partial sessions in the last N days" — the common
-- audit query. Covers the blocker column too so the planner can answer
-- "blocker breakdown over the last week" with an index-only scan.
CREATE INDEX IF NOT EXISTS idx_agent_sessions_closed
  ON agent_sessions (closed_at DESC)
  WHERE status IN ('done', 'failed');