Reviewed as part of a batch pass (read-only). The optimization itself is sound — for a 1–9
digit fraction, raw * nanosMultiplier(9 - len) reproduces the old zero-padding exactly and
drops the two throwaway strings. Nice.

One correctness gap, though: the change assumes nanos.length <= 9, and that invariant does
not hold on every path into parseNanos.

• The modern parser (castStringToDTInterval, microPattern = (\.\d{1,9})) does guarantee
  1–9 digits, so that path is fine.
• But parseNanos is also called from parseDayTimeLegacy (line 507, m.group(9)), whose
  regex dayTimePatternLegacy = "^([+|-])?((\d+) )?((\d+):)?(\d+):(\d+)(\.(\d+))?$" captures
  the fraction with \d+ (unbounded), not \d{1,9}. This path is reachable when
  spark.sql.legacy.fromDayTimeString.enabled = true.

Failure scenario (legacy flag on): a fractional part of 10+ digits whose numeric value is
<= 999999999, i.e. with leading zeros — e.g. "0 0:0:0.0000000001" (fraction
"0000000001", length 10, value 1):

• Old code: nanos.length (10) < 9 is false, so alignedStr = nanos; toLongWithRange = 1
  (in range) → returns 0 micros, no error.
• New code: raw = toLongWithRange(..., "0000000001", 0, 999999999) = 1 (passes the range
  check), then nanosMultiplier(9 - 10) = nanosMultiplier(-1) → ArrayIndexOutOfBoundsException
  (caught at the case e: Exception and rethrown as INVALID_INTERVAL_FORMAT.DAY_TIME_PARSING).

So on the legacy path this input flips from "parses (to 0)" to "throws" — which contradicts
the "No user-facing change / result is identical" note. (For a 10+ digit fraction whose value
exceeds 1e9, both old and new throw the same out-of-range error before the multiplier, so those
are unaffected.)

Minimal fix that preserves the old semantics exactly and avoids the negative index:

val nanoSecond =
  if (nanos.length >= maxNanosLen) raw
  else raw * nanosMultiplier(maxNanosLen - nanos.length)

(For len == 9 this equals raw * 1, and for len > 9 it reproduces the old
alignedStr = nanos branch.) A regression test for the legacy-flag path would be worth adding.

Thanks for the careful review, @HyukjinKwon! You're right: the only path where the fraction can exceed 9 digits is the legacy parseDayTimeLegacy (its dayTimePatternLegacy captures the fraction with an unbounded \d+), reachable with spark.sql.legacy.fromDayTimeString.enabled=true -- and there the new code hit nanosMultiplier(-1).

I applied your suggested guard so a fraction of length >= maxNanosLen returns raw directly, which reproduces the old alignedStr = nanos behavior exactly (for len == 9 it equals raw * nanosMultiplier(0)). I also added a regression test to the "from day-time string - legacy" case covering a 10+ digit leading-zero fraction (e.g. 0 0:0:0.0000000001), which threw an ArrayIndexOutOfBoundsException before the fix and now parses to 0 micros as before.

Rebased on latest master and force-pushed. Mind taking another look?