检查DB中是否存在记录：单步还是两步？

Question

Suppose you want to get a record from database which returns a large amount of data and requires multiple joins.

So my question would be, is it better to use a single query to check if data exists and get the result if it exists? Or do a more simple query to check if data exists then id record exists, query once again to get the result knowing that it exists.

Example:

3 tables a, b and ab(junction table)

select * from 
from a, b, ab 
where condition
and condition 
and condition 
and condition etc...

or

select id 
from a, b ab 
where condition

then if exists do the query above.

So I don't know if there is any reason to do the second. Any ideas how this affects DB performance or does it matter at all?

Answer 1

Usually the slowest part in a query is the filter, then it's the indexed search, joins, etc. If it's a lot of data to be transfered - the transfer is also time-consuming. It will be twice slower if you check the existence then extract the data. Just extract the data. If it is there you get it, and if not - you get nothing.

When extracting from multiple tables, JOIN is faster and more flexible.