Mysql – Update Large Table

MySQLperformancequery-performance

I have these two tables:

user_profile:
user_profile_id_int int primary key 
user_profile_id varchar

user_activity:
user_profile_id_int int key
user_profile_id varchar

both fields in user_profile are ok but I don't have the user_profile_id_int from user_activity and I want to update it using the map I have in user_profile. I wrote a query like this:

update user_activity a
set user_profile_id_int =
(select user_profile_id_int from user_profile b
where a.user_profile_id = b.user_profile_id);

It was running on a really strong server for 1 day and I stopped it. But to record has been updated.

So 2 questions here:

  1. Is there a better and faster way to do this?
  2. Is this way ok?

Best Answer

Answer to question 1

This should do it for you

UPDATE user_activity a
INNER JOIN user_profile b 
USING (user_profile_id)
SET a.user_profile_id_int = b.user_profile_id_int;

This will work in MySQL. If you are not used to that JOIN syntax, do this:

UPDATE user_activity a
INNER JOIN user_profile b 
ON a.user_profile_id = b.user_profile_id
SET a.user_profile_id_int = b.user_profile_id_int;

Both should work.

Answer to question 2

Your query, in theory, works. However, look at what it is doing:

A table scan on user_activity, an indexed lookup of the user_profile_id_int using the PRIMARY KEY of user_profile, and an in-place update of the current row in user_activity.

The query is hitting two tables and two primary keys, back and forth, on a per-row basis. All steps slow each other down. Thus, you get a longer running query.

CAVEAT

Adding a compound index on user_profile should speed things up:

ALTER TABLE user_profile ADD INDEX (user_profile_id,user_profile_id_int);
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