presto-main/src/main/java/io/prestosql/sql/gen/ExpressionProfiler.java/ExpressionProfiler/stop
class ExpressionProfiler:    public void stop(int batchSize)
    {
        verify(previousTimestamp != NOT_INITALIZED, "start() is not called");
        verify(batchSize > 0, "batchSize must be positive");

        long now = ticker.read();
        long delta = now - previousTimestamp;
        totalExecutionTimeNanos += delta;
        samples += batchSize;
        if ((totalExecutionTimeNanos / samples) < expensiveExpressionThresholdNanos) {
            isExpressionExpensive = false;
        }
        previousTimestamp = NOT_INITALIZED;
    }