perf(#4842): pre-aggregate latest-message ordering for missing-index cron scans

[rodboev]

Thinking Path

• The earlier #4842 slices already removed the per-row cron sidecar I/O path and the streaming cache churn, but the cron-only sidebar rescue can still hit multi-second cold rebuilds when state.db has no usable idx_messages_session.
• The common indexed path needs to stay on the existing correlated ordering, because that is still the faster plan on a healthy migrated store.
• This follow-up stays independent of #4952, keeps indexed scans unchanged, and only switches the cron-only capped pass to a pre-aggregated latest-message relation when the best-effort index prime cannot restore the normal path.

What Changed

• api/agent_sessions.py: keep the existing indexed ordering path, keep the defensive idx_messages_session prime, and add a cron-only fallback that joins a pre-aggregated latest_messages CTE only when the messages index is still unavailable after that prime.
• tests/test_issue2628_cli_sessions_perf.py: update the source-shape guard to the missing-index gate, keep the resumed-old-session coverage, and add a deterministic progress-budget proof that compares the old correlated ordering against the new fallback on a read-only missing-index database.

Why It Matters

This fixes the cold cron fallback that still goes quadratic on degraded or read-only stores, without regressing the normal indexed sidebar path. Cron-heavy installs keep the same rows and ordering, but the missing-index branch no longer pays one latest-message lookup per session row.

Verification

• pytest tests/test_issue2628_cli_sessions_perf.py tests/test_issue3887_index_prime.py tests/test_issue3172_cron_session_limit.py tests/test_issue3930_source_filter_pushdown.py tests/test_issue3585_cron_session_overflow.py -v --timeout=60

Full-suite CI context, not a required local check unless requested: pytest tests/ -v --timeout=60.

Upstream

Follow-up for #4842.

Attribution: Nathan's request for help and Rod Boev's profiling plus proposed SQL shape defined this slice.

Model Used

GPT 5.5 via Codex CLI

[greptile-apps]

Greptile Summary

This PR adds a faster cron-only fallback for degraded session scans. The main changes are:

• Detects whether idx_messages_session is present before choosing the query shape.
• Keeps the indexed correlated ordering path for normal migrated stores.
• Uses a pre-aggregated latest-message CTE for cron-only missing-index scans.
• Expands performance tests for missing-index cron ordering.

Confidence Score: 5/5

The changed flow looks mergeable after a small deterministic-ordering cleanup.

• The normal indexed path remains unchanged.
• The missing-index cron fallback can pick arbitrary tied sessions at the candidate limit boundary.
• No blocking security or data-loss issue was found in the changed code.

api/agent_sessions.py

Important Files Changed

Filename	Overview
api/agent_sessions.py	Adds the index-presence gate and cron-only latest-message CTE fallback; the fallback candidate window still needs deterministic tie handling.
tests/test_issue2628_cli_sessions_perf.py	Adds missing-index database helpers and progress-budget coverage for the cron fallback path.

_{Reviews (1): Last reviewed commit: "fix(sidebar): restrict cron pre-aggregat..." | Re-trigger Greptile}

[nesquena-hermes]

Shipped in v0.51.672 (Release YB, just deployed) — thanks @rodboev! The cron sidebar rescue scan on a state.db missing idx_messages_session now uses a single pre-aggregated latest-message ordering instead of the correlated per-row subquery; the indexed path is unchanged. Gate: Codex SAFE — verified result-equivalence with a direct SQL probe (no-message / null-timestamp / tied-latest-message all match old output), pre-agg fires only on missing-index, no interaction with #4952. Suite 10654. Verified on prod.