try to fix pro-rata yet again

db/helpers.go

@@ -1532,6 +1532,36 @@ GROUP BY
 	return insert, nil
 }
 
+// UpdateSummaryPresenceByWindow recomputes AvgIsPresent using CreationTime/DeletionTime overlap with the window.
+func UpdateSummaryPresenceByWindow(ctx context.Context, dbConn *sqlx.DB, summaryTable string, windowStart, windowEnd int64) error {
+	if err := ValidateTableName(summaryTable); err != nil {
+		return err
+	}
+	if windowEnd <= windowStart {
+		return fmt.Errorf("invalid presence window: %d to %d", windowStart, windowEnd)
+	}
+	duration := float64(windowEnd - windowStart)
+	startExpr := `CASE WHEN "CreationTime" IS NOT NULL AND "CreationTime" > 0 AND "CreationTime" > ? THEN "CreationTime" ELSE ? END`
+	endExpr := `CASE WHEN "DeletionTime" IS NOT NULL AND "DeletionTime" > 0 AND "DeletionTime" < ? THEN "DeletionTime" ELSE ? END`
+	query := fmt.Sprintf(`
+UPDATE %s
+SET "AvgIsPresent" = CASE
+  WHEN %s > %s THEN (CAST((%s - %s) AS REAL) / ?)
+  ELSE 0
+END
+`, summaryTable, endExpr, startExpr, endExpr, startExpr)
+	query = dbConn.Rebind(query)
+	args := []interface{}{
+		windowEnd, windowEnd,
+		windowStart, windowStart,
+		windowEnd, windowEnd,
+		windowStart, windowStart,
+		duration,
+	}
+	_, err := execLog(ctx, dbConn, query, args...)
+	return err
+}
+
 // RefineCreationDeletionFromUnion walks all snapshot rows in a period and tightens CreationTime/DeletionTime
 // by using the first and last observed samples and the first sample after disappearance.
 func RefineCreationDeletionFromUnion(ctx context.Context, dbConn *sqlx.DB, summaryTable, unionQuery string) error {