Efficiency of querying a database with STI

Question

I was curious how rails queries a table that uses STI. For example, if my parent class is Book and I have two subclasses ComicBook and Novel if I do something like

Novel.all.each

since there's only one table, does the server shift through all of the comic books as well? Is appropriate indexing automatically added to prevent this? Thanks

Answer 1

Well, you can't do Novel.each, each isn't defined on model classes. You can however do Novel.all.each ... where ... is some block.

As for how the query works, just call to_sql on any ARel expression. Novel.all will return the collection of models itself, so you need to go a step further to ensure a valid ARel expresion is returned by calling scoped.

[1] pry(main)> Novel.scoped.to_sql
=> "SELECT \"books\".* FROM \"books\" WHERE \"books\".\"type\" IN ('Novel')"

Indexing by most columns that are queried against frequently is a good thing to consider. Yes, without the index your rdbms will have to look at all records in the table as part of the above condition check.