-- ═══════════════════════════════════════════════════════════════
-- 007: DB Indexes Optimization (audit 2026-05-22)
--
-- Adds 18 missing indexes for hot-path queries identified by
-- comprehensive audit. Skips users.email — UNIQUE constraint
-- already provides equality-lookup index automatically.
--
-- Expected impact: 30-60% speedup for admin overview / analytics,
-- 10-20% for per-user notification + session timelines, and
-- prevents full-table scans on CASCADE DELETE of classroom_sessions.
-- ═══════════════════════════════════════════════════════════════

-- ── users: filter/sort by created_at, last_login, role ──────────
CREATE INDEX IF NOT EXISTS idx_users_created_at  ON users(created_at);
CREATE INDEX IF NOT EXISTS idx_users_last_login  ON users(last_login);
CREATE INDEX IF NOT EXISTS idx_users_role        ON users(role);
CREATE INDEX IF NOT EXISTS idx_users_banned_role ON users(is_banned, role);

-- ── test_sessions: date range filters + JOIN on subject_id ──────
CREATE INDEX IF NOT EXISTS idx_test_sessions_started_at         ON test_sessions(started_at);
CREATE INDEX IF NOT EXISTS idx_test_sessions_finished_at        ON test_sessions(finished_at);
CREATE INDEX IF NOT EXISTS idx_test_sessions_subject_id         ON test_sessions(subject_id);
CREATE INDEX IF NOT EXISTS idx_test_sessions_status_finished    ON test_sessions(status, finished_at DESC);

-- ── notifications: per-user timeline (was indexed on is_read, not created_at) ──
CREATE INDEX IF NOT EXISTS idx_notifications_user_created ON notifications(user_id, created_at DESC);

-- ── admin_audit_log: target LIKE 'user:%' + action filters ──────
CREATE INDEX IF NOT EXISTS idx_audit_log_target      ON admin_audit_log(target);
CREATE INDEX IF NOT EXISTS idx_audit_log_action      ON admin_audit_log(action);
CREATE INDEX IF NOT EXISTS idx_audit_log_action_date ON admin_audit_log(action, created_at DESC);

-- ── classroom_*: CASCADE DELETE safety (FKs without indexes) ────
CREATE INDEX IF NOT EXISTS idx_classroom_muted_session             ON classroom_muted(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_attendance_session        ON classroom_attendance(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_invites_session           ON classroom_invites(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_notes_session             ON classroom_notes(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_draw_permissions_session  ON classroom_draw_permissions(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_hands_session             ON classroom_hands(session_id);
CREATE INDEX IF NOT EXISTS idx_classroom_chat_session_date         ON classroom_chat(session_id, created_at);

-- ── submissions: per-student timeline ────────────────────────────
CREATE INDEX IF NOT EXISTS idx_submissions_student_date ON submissions(student_id, submitted_at DESC);

-- ── questions.difficulty: exam builder filter ────────────────────
CREATE INDEX IF NOT EXISTS idx_questions_difficulty ON questions(difficulty);