Rails + validates_overlap gem：何时向数据库添加索引？

Question

I'm using the validates_overlap gem ( https://github.com/robinbortlik/validates_overlap ) in a Rails app. Here is the Model code:

  validates :start_time, :end_time, overlap: { scope: "device_id", exclude_edges: ["start_time", "end_time"] }

And here is the SQL it triggers:

SELECT  1 AS one FROM "bookings"  WHERE 
((bookings.end_time IS NULL OR bookings.end_time > '2014-04-11 13:00:00.000000') AND
(bookings.start_time IS NULL OR bookings.start_time < '2014-04-11 16:00:00.000000') AND
bookings.device_id  = 20) LIMIT 1

I just want to know if I should be adding an index in my postgres database that covers start_time, end_time and device_id, or something similar? eg something like this:

add_index :bookings, [:device_id, :start_time, :end_time], unique: true

Answer 1

Adding the above index to ensure database consistency would make no sense. After all you are validating the Range AND excluding the actual edges (the unique index would check exactly the edges!).

Adding a non unique index to speed up the validation is a good idea. If so you should analyze your data and app queries. The easiest approach is to simply add a single index for each column. Postgres can still use these for the multicolumn query (see heroku devcenter ). Only if it really matters (or you do not query the columns in other combinations) a multicolumn index is necessary. If so the device_id should be first in index Rule of thumb: index for equality first—then for ranges .