Two sources off every ladder now read at the very same instant on thirty
days: on fifteen the name decides, on the other fifteen the first name's
systolic is under the floor and the second takes the day. A dedicated
case asserts which source each of those days shows, under a UTC and a
Berlin session; the parity cases compare them with the reference as
well. Dropping the name from the window's order turns 26 of 59 cases
red on Postgres 16 and 18.
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.
Marc-André Bombeck