Time Required For Query Execution?
The execution speed of a database server depends on a great many factors, for example 


concurrent load, what else is happening - that is if the database is
busy doing something else your query may have to wait  
CPU speed, if the basic computing power of the database is slow your query will be executed slower than it would be otherwise
available disk IO bandwidth, if
the data is not cached in RAM and has to be fetched from a disk the
speed of the disk will affect performance
structure of the data, is it indexed to allow the DB to find your row more quickly?
network speed, how long does it take for the query to arrive at the DB and for the result to return to you app?


However to seek a single row out of 800,000 rows on a single table assuming the presence of an index on the column "id" that is specifying the row of data you want, it is likely to take a tiny fraction of a second on modern hardware. In fact the network round trip to your database server is likely to take longer than the database operation.