Block time 

To calculate the block_time from the received_time variable of block, I calculated the difference between previous and current row.

```sql
SELECT
    height,
    received_time, 
    received_time - LAG ( received_time, 1, 0 ) OVER ( ORDER BY height ) block_time 
FROM
    block 
LIMIT 10
OFFSET 1; --- select all rows except top row 
```
