Speed up working with timezones

Optimize timezone-aware query functions (timestamp_floor\_\*\_tz, format_timestamp_tz)

Problem: queries using timezone functions on a non-Moscow/non-UTC zone (e.g.
Europe/Bucharest) ran 10-20x slower than their non-timezone equivalents. For
Moscow/UTC a lookup table handles the conversion; for any other zone the code
falls back to live cctz calls.

Cause, confirmed by flamegraph and code:

1. NDatetime::ToCivilTime performed two independent timezone lookups per call.
   cctz::convert(tp, tz) is defined as tz.lookup(tp).cs, and the function then
   called tz.lookup(tp) again for the offset/is_dst fields. Both pieces of data
   are already present in a single absolute_lookup result.
2. TimestampFloorDayTZInternal (and Month/Quarter/Year variants) located the
   start of the period via a binary search that called ToCivilTime on every
   iteration — \~18 iterations for the day case. Combined with (1) that is \~38
   cctz lookups per call, and timestamp_floor_day_tz appears twice per row in a
   typical WHERE clause.

* Changelog entry

Type: fix

1. ToCivilTime now does a single tz.lookup(tp) and reuses both cs and the
   offset/is_dst fields. Behavior is identical; affects all callers.
2. TimestampFloor\{Day,Month,Quarter,Year\}TZ compute the period start directly
   (subtract the civil time-of-day, or build the first second of the period and
   convert it back), then verify the result. The binary search is kept as a
   fallback for the rare days that do not start at midnight (DST transitions at
   00:00, historical offset shifts), so results are unchanged.
commit_hash:295ee82832ab2a4a35920067e7c063d6992bb083

1 files changed, 2 insertions, 1 deletions

diff --git a/library/cpp/timezone_conversion/convert.cpp b/library/cpp/timezone_conversion/convert.cpp
index 490bb4e2700..2bec10eccfa 100644
--- a/library/cpp/timezone_conversion/convert.cpp
+++ b/library/cpp/timezone_conversion/convert.cpp
@@ -72,7 +72,8 @@ namespace NDatetime {
 
     TSimpleTM ToCivilTime(const TInstant& absoluteTime, const TTimeZone& tz) {
         TTimePoint tp = TSystemClock::from_time_t(absoluteTime.TimeT());
-        return CivilToTM(cctz::convert(tp, tz), tz.lookup(tp));
+        auto al = tz.lookup(tp);
+        return CivilToTM(al.cs, al);
     }
 
     TSimpleTM CreateCivilTime(const TTimeZone& tz, ui32 year, ui32 mon, ui32 day, ui32 h, ui32 m, ui32 s) {