Question

I have a model called A with columns col and col2. The col1 has an unique index. (I am using Postgres database.)

class A < ActiveRecord::Base
  # id, col1, col2
end

Data:

id  col1   col2
==  ====   ====
1   2       3
2   3       4

Let us try to find the first row matching the given value in col2 OR col1.

a = A.first(:conditions=> ["col2 = :id OR col1 = :id", {:id => 3}])
# select * from A where (col2 = 3 OR col1 = 3) LIMIT 1

The SQL above returns the row#2 rather than row#1. I suspect the query optimizer chooses to execute col1 = 3 first as col1 has an unique index.

How do I override this behavior? How do I instruct Postgres optimizer to use the existing order for the OR condition?

Answer 1

If you don’t specify an explicit order then first row doesn’t make any sense; the result set from a specific query at a specific moment in time will, of course, have a first row but there is no guarantee that running the same query again with the same data will produce the same first row. Tables in a relational database are not ordered so there is no specified existing order at all. You’re making a bad assumption; the implementation may implicitly order the records by the PK on the disk but there’s no guarantee of that and no guarantee that the next version will behave the same; furthermore, there’s no guarantee that the database will return rows in disk-order, the database is free to order the rows in any way it wants to unless you have specified a specific order with an ORDER BY clause.

If you want an order then you should include an ORDER BY clause; you probably want something more like this:

a = A.where("col2 = :id OR col1 = :id", {:id => 3}).order(:id).first

And if you want to look at col2 before col1 then you should say so:

a =  A.where('col2 = :id', :id => 3).order(:id).first \
  || A.where('col1 = :id', :id => 3).order(:id).first

col2 = :id OR col1 = :id and col1 = :id OR col2 = :id are equivalent boolean expressions so the database is free to check col2 = :id before, after, or at the same time as col1 = :id and it is free to access the rows in any order it pleases.