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Implementation details have been anonymized to protect intellectual property. The methodology, metrics, and results are real. For the full technical breakdown, reach out at tomas@omnimetrix.io.
Loop 12: Group Boundary Simplification
BEFORE
AFTER
Loop 12: Group Boundary Simplification
What changed
Replaced a descending row-number window function used for group boundary detection with a simpler max-timestamp comparison. Instead of identifying the final record in each group via a descending sort, the system checks whether each record's timestamp equals the group maximum. This targets the most expensive remaining window function: a full descending sort across every partition.
Why we expected it to work
The descending row number is only ever checked for equality with 1 (identifying the final record). A max-timestamp comparison produces the same result without requiring a full sort pass, reducing the window function count and associated overhead. Initial benchmarks showed an 11.7% improvement.
Results
| Metric | Before | After | Delta |
|---|---|---|---|
| Execution time | 712ms | 629ms | -11.7% |
| Output correctness | -- | -- | Incorrect |
Why it didn't work
When multiple records share the exact same timestamp at a group boundary, the original approach assigns the boundary marker to exactly one record (arbitrary but singular). The max-timestamp approach marks all tied records, producing incorrect downstream output. This is not a rare edge case -- the test data contains batched records that arrive simultaneously. Output correctness is non-negotiable; the 11.7% gain does not justify incorrect results.
Cumulative impact
Original: 1,829ms →After Loop 09: ~500ms (72.7% total reduction, unchanged)