Data Handling and ADS

I am very grateful for this forum, it helps me a lot as a new researcher in this area. I have several questions:

1. ADS-B is known to have an increasing data losses related to distance and number of aircraft in range. How does this data losses handled in OpenSky database? 2. Suppose an aircraft is detected by two ADS-B receivers. if I get it correctly, the data from both receivers will be stored in the database and with a correct query I can select data from a specific receiver, right? 3. The transmission rate of airborne position is 2 message/second, so I expect the position will be updated every 0.5 seconds. However, if I substract between two consecutive 'lastposupdate', most of the value is around 1s (figure attached). Is this due to the even/odd transmission?

Best regards,

Fazlur