Question

I’ve a problem with an enormous table in a database of an RoR application.
This is a part of my MySql database:

TABLE DEVICES:

create_table "devices", :force => true do |t|
t.string "mac"
end

TABLE EVENTS:

create_table "events", :force => true do |t|
    t.integer  "id"
    t.integer  "device_id"
    t.string   "data_type"
    t.integer  "element_id"
    ...
end

A device has_many events, so this table is very big.

the problem is when i want to find a record in this table by this query:

SELECT SQL_NO_CACHE 
    element_id, 
    created_at, 
    device_id, 
    multimedia_id 
FROM events 
WHERE device_id = N 
  AND data_type = 'S' 
  AND element_id = N 
LIMIT N

the problem is:
No index was used here. In this case, that meant scanning 930447 rows..

In fact, the primary key in “id”.

What can i do?
I had to think about:

1. change the primary key but RoR have no Composite Primary Key.
2. divide the events table in many table, one for each device. How? What will the association become? (Table has_many ?)
3. can i link a table by a string? for example "Events".where(...) ?

can anyone help me?

Sorry for my English. bye.
Davide Lentini

Answer 1

Just because you should not use composite primary key doesn’t mean you can’t add a composite index (in a new migration):

def self.up
  add_index :events, [:device_id, :data_type, :element_id], :name => 'my_events_index'
end

def self.down
  remove_index :events, :name => 'my_events_index'
end