Weird rounding issue

问题

Something very weird is happening with decimals and floating numbers and I can't understand why or where (ruby/rails/postgresql).

Given a purchases table with a decimal column - total:

p1 = Purchase.where(total: 5.99).first_or_create
p2 = Purchase.where(total: 5.99).first_or_create

[p1.id, p2.id] # => [1, 2]

p3 = Purchase.where(total: 5.99.to_d).first_or_create
p4 = Purchase.where(total: 5.99.to_d).first_or_create

[p3.id, p4.id] # => [1, 1]

Both Ruby and postgresql have no problem representing 5.99 exactly, no matter if decimals or floats:

5.99.to_s         # => "5.99"
5.99.to_d.to_s    # => "5.99"
5.99 == 5.99.to_d # => true

SELECT CAST(5.99 AS DECIMAL) AS decimal, CAST(5.99 AS FLOAT) AS float;
  #  decimal | float 
  # ---------+-------
  #     5.99 |  5.99
  # (1 row)

SELECT CAST(5.99 AS DECIMAL) = CAST(5.99 AS FLOAT) AS equal;
  #  equal 
  # -------
  #  t
  # (1 row)

To top it all off, this doesn't happen with some other values:

p5 = Purchase.where(total: 5.75).first_or_create
p6 = Purchase.where(total: 5.75).first_or_create
p7 = Purchase.where(total: 5.75.to_d).first_or_create

[p5.id, p6.id, p7.id] # => [3, 3, 3]