Goal

When pg_ash is installed on the server, /ash should immediately show the full history available in ash.samples, then continue streaming live data. Today it always uses the live ring buffer (600 samples max, ~10 min).

Current behavior

AshState.pg_ash_installed is detected but ViewMode::History never activates — the code falls back to live ring buffer unconditionally (documented TODO in state.rs:102).

Desired behavior

1. On connect — detect pg_ash via SELECT 1 FROM pg_extension WHERE extname = 'pg_ash'
2. If installed — query ash.samples for the configured window (default 10 min, up to 24h via zoom)
3. Merge — history snapshots pre-populate the ring buffer; live pg_stat_activity polls append new samples
4. Seamless — left side of timeline = history, right side = live. No gap, no jump.
5. Zoom — when user zooms out beyond ring buffer capacity, re-query ash.samples for larger window

Implementation plan

Sampler changes (src/ash/sampler.rs)

Add query_ash_history():

select
    wait_event_type,
    wait_event,
    query_id,
    left(query, 80) as q,
    count(*) as count,
    bucket_ts  -- truncated to bucket_secs
from ash.samples
where sample_ts >= now() - interval '10 minutes'
group by 1, 2, 3, 4, bucket_ts
order by bucket_ts

Note: ash.samples schema — verify column names against pg_ash v1.2 (sample_ts, wait_event_type, wait_event, query_id, query).

State changes (src/ash/state.rs)

Activate ViewMode::History when pg_ash_installed = true. On zoom out past ring buffer, trigger re-query.

mod.rs event loop

// On startup, if pg_ash installed:
let history = query_ash_history(&client, window_secs).await?;
for snap in history {
    snapshots.push_back(snap);
}
// Then continue live polling as normal

CI tests required

• Unit test: query_ash_history() SQL correctness (mock rows)
• Integration test (ignored by default, needs pg_ash): populate ash.samples, verify ring buffer pre-populated
• State test: pg_ash_installed = true activates history mode

AI test required

New file: tests/ai/slash-ash-pgash.md

• Setup: install pg_ash, run pgbench for 5 min to populate history
• Steps: connect, run /ash, verify left side of timeline shows pre-existing history (not empty)
• GIF: record 10s showing bars from minute 0, new bars scrolling in live
• Post GIF to PR as evidence

References

• pg_ash repo: https://github.com/NikolayS/pg_ash
• ash.samples schema: see pg_ash v1.2 sql/ directory
• Current stub: src/ash/state.rs:102 (TODO: history mode (pg_ash Layer 2))
• PR feat(ash): Active Session History TUI — /ash command #756 (base implementation): feat(ash): Active Session History TUI — /ash command #756

Scope

New PR on top of main (after #756 merges). Do not modify existing /ash live behavior — history is additive.