A minute bucket with only a queued dispatch (no attempt) made the histogram
SUM(...) FILTER (attempt_id IS NOT NULL) return NULL over zero matching rows,
violating the hist_NN NOT NULL constraint and aborting the whole backfill with
SQLSTATE[23502]. COALESCE the hist backfill selects to 0, matching the live drain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>