Which is more expensive? For loop or database call?

Question

In general, which is more expensive? A double-nested for loop and one call to a database or a call to a database for each of N items in only one for loop?

Not looking for an answer down to microseconds, just a general idea of which direction I should take.

TIA.

Answer 1

In general, the fewer times you hit the database, the better. There are a number of reasons for this, including:

• The database will be able to optimize better if it can try to fetch everything all at once
• You remove all of the overhead of communicating with the database multiple times, which can be quite significant

Answer 2

In general, anything done in memory (for loop) is faster than the same thing done over a network (database call). However:

for i = 1 to num_users     get user from database end 

will be slower than

get users 1 to num_users from database (in one query) 

because it's the number of times you ask the database for something that really matters.