Question

I’m trying to figure out why one of our
migration scripts is taking forever we are trying to do an update that
joins from another table to get a relevant piece of data.

Each table (A, B) has about 100,000 rows.

# now populate the ACHIEVEMENT_INSTANCE.OBJECTIVE_INSTANCE_ID
update A a, B b
set a.INSTANCE_ID = b.INSTANCE_ID
where a.ID = b.ID;

It seems like we’re dealing with an INNER JOIN that is creating some
type of Cartesian product between the 2 tables 100,000 X 100,000 which
is taking FOREVER (probably wayyyy to long).

According to MySQL update uses an inner join by default not sure if we
could use some other type of JOIN that wouldn’t be so shitty.

MySQL documentation UPDATE:

UPDATE [LOW_PRIORITY] [IGNORE] table_references
   SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
   [WHERE where_condition]

The table_references clause lists the
tables involved in the join. Its
syntax is described in Section
12.2.8.1, “JOIN Syntax”. Here is an example: UPDATE items,month SET
items.price=month.price WHERE
items.id=month.id; The preceding
example shows an inner join that uses
the comma operator, but multiple-table
UPDATE statements can use any type of
join allowed in SELECT statements,
such as LEFT JOIN.

Answer 1

As Greg stated in the comments:

Do you have an index on a.ID and b.ID?

We did not have indexes on those columns. Once we added them the query took 30 seconds:

create index id_idx on A(id);
create index id_idx on B(id);