WPF FinOps latest-database-sizes read plans for 148 ms over every database_size_stats chunk to execute in 2.6 ms #4245

Problem

DatabaseSizeLatestSql, the WPF viewer's FinOps "latest database sizes" read, spends almost all of its time planning. It has no collection_time bound, so the planner considers every retained database_size_stats chunk to find the newest snapshot.

Measured

From #4227's field comment: a production SQL Server store, 2026-09-25, the busiest server, read-only EXPLAIN (ANALYZE, BUFFERS). The read took 2.6 ms to execute and 148 ms to plan, with 80 buffers, over 71 database_size_stats chunks.

Until #4240, FinOps refreshed every 30 s, so this ran twice a minute per open tab. #4240 moves the tab to a refresh on activation, which cuts how often it runs. Each activation still pays the planning cost, and that cost grows with retention and with the chunk count.

Fix shape

• Bound the read by collection_time so the planner can exclude old chunks, without changing the result. The newest snapshot must still come back for a server whose collection stopped days ago. One option: probe each server's newest collection_time first (a one-chunk read), then read that snapshot with the bound.
• Measure planning before and after on a seeded store with 30+ days of chunks.
• Check Lite: DuckDB has no chunks, so this is likely Darling only. Say so with the reason.

Related

• Design: size raw chunk interval, compress_after and the hourly CAGG refresh window from ingest rate; a large store keeps 1–2 days of raw uncompressed and it outgrows RAM #4211 (the raw chunk-sizing design): smaller chunks mean more chunks, and every read without a chunk-excluding bound pays per chunk at planning time.
• WPF FinOps Server Inventory re-runs a 7-day raw query_stats DISTINCT for every server on the 30 s timer (~43 × 44 k buffers per refresh); query_stats_db_hourly answers it in 32 ms / 175 buffers #4227 / FinOps refresh: take the FinOps tab off the 30s timer, merge its doubled query_stats and bounds reads (#4227) #4240: the FinOps work this was split from. It was the optional last job of lane V3c, which ran out of budget before reaching it.