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Marc-André Bombeck
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The blood-pressure series chose each UTC day's source with DISTINCT ON over the day's floor-passing systolic rows and joined the choices back to the readings. That is the shape the analytics pick had, with the same weakness: on a measurements table without planner statistics the planner estimated one row on each side and ran the join as a nested loop that rescanned the pick for every reading. The ten-year series over 6,000 freshly written rows from one source took 0.57 s for the statement alone, 0.86 s with a second source on every third day; the plan visited about 4,900 rows per row stored, so the cost grows with the square of the history. The pick is now one window over one scan: the day's rows ordered systolic first, then by ladder rank, then by source name, and the first row's source taken only when it is a systolic one. The scan starts at the window's first UTC day, so the pick still sees the whole edge day, and a day without a floor-passing systolic still shows nothing. Both fixtures above read in about 10 ms, with or without statistics. A parity suite runs the old statement as the reference against the new one across overlapping and off-ladder sources, half readings, sub-floor rows, tombstones, both DST changes, window starts on and either side of UTC midnight, and four session zones. The unanalysed-table guard now covers the series' ten-year, quarter and week reads.