Question

I have a table with columns user_id, email, default. Default stores ‘Y’ or ‘N’ depending if the email is the users default email. Each user can have only one default email.

When a user is doing an update or insert on the table, in my SP I check if the user has passed isDefault as ‘Y’. If so, I need to update all the entries for that user to make default ‘N’. My question is: considering there are no locking issues (not more than one thread will request data from the table for a particular user) which one amongst the following queries is least time consuming:

update table
set default = 'N'
where user_id = 'abc'
and default = 'Y'

(Overhead of where default = ‘Y’ check)

OR

update table
set default = 'N'
where user_id = 'abc'

(Overhead of updating all records for the user)

Answer 1

There’s not a clear-cut answer here. Your efficiency will be best in the first case if default = ‘N’ for most records. In the second, it will be best if default = ‘Y’ in most cases.

So if most of your users only have 1 email address, use the 2nd query. If most users have at least 2, use the first.