The window-bounded pick joins (4a3b1256) let the optimizer choose, under
understated statistics, a nested-loops RANGE seek over the window with the pick
time as a residual, once per bucket (O(buckets x window rows)); the cached plan
was then reused for wide reads: gate 9 1d@1min 20 s, here 47 s / 3.6M reads.
Each pick join now carries WITH (FORCESEEK (PK_Sample (TagId, SourceTimeUtc))):
an exact-key seek on both columns per bucket under any statistics, and no
hash/merge join over the tag's history can exist (forcing one is error 8622), so
the window bound is dropped from the join. Output identical (EXCEPT both ways,
fwd/rev, incl. a mixed Double/Int64/Boolean/String/DateTime/outage tag).
Measured (6M-row tag + 86k mixed-kinds tag; ms / logical reads), understated
stats: 1d@1min 47,223/3.6M -> 253/25.6k; mixed 1d@1min 20,164/1.66M -> 137/24k;
accurate stats: 1d@1min 121 -> 160 ms, 1d@1h 79 -> 117 ms. Rejected: OUTER
APPLY TOP(1) FORCESEEK (identical), OPTION (RECOMPILE) (still 20 s under the bad
stats, and a compile per read).
StartEndJoinBackTests rewritten: understated statistics + compile-small/reuse-wide
(18,645 reads; the merged query read 1,645,845 / 23 s and failed it), accurate
statistics with 300k history rows either side (22,616), forced HASH/MERGE
refused (8622 — also failed against the merged query).
Docs: perf table + explanation; SDK-parity N2 (*ActualTime Calculated bit) and
N3 (Good String => BadAggregateInvalidInputs, SDK skips it).
- SqlServer StartEnd join-back: every pick join also carries the window predicate
(forward >= @start AND < @end, reverse > @start AND <= @end). Keyed on the pick
time alone a hash/merge/adaptive join read the tag's whole history (review:
43,657 vs 361 logical reads), and parameter sniffing reused that plan for every
tag. Forced OPTION (HASH JOIN), 1d@1h on 6M rows: 152,524 -> 6,756 reads.
- Usable picks (x/y) re-check the exact u predicate in the join, so a concurrent
HistoryUpdate Replace that turned the picked sample Bad/Empty reads as absent
(BadNoData), never a Bad row's value under Delta.
- StartEndJoinBackTests (integration): 300k history rows either side of a 1 h
window, forced HASH and MERGE joins, results identical to the unhinted plan and
Start/End correct, session logical reads < 1,000 (183 bounded; 8,055 with the
bound removed — the test fails then).
- TagIndexTests: the refused-ADD-INDEX test asserts the Warning naming the ALTER.
- Historian.md: re-measured table incl. long-history windows (output identical by
EXCEPT both ways in all 7 cases) and the forced-join read counts.
Joseph Doherty