Add version to auditable_index

[fatkodima]

This will optimize:

• two scopes from

  audited/lib/audited/audit.rb

  Lines 55 to 56 in c48893a

  	scope :from_version, ->(version){ where('version >= ?', version) }
  	scope :to_version, ->(version){ where('version <= ?', version) }

  
  as they always will be used with auditable_id and auditable_type (like in user.audits.from_version(100)), so new index will be used to speed this up;
• and here

  audited/lib/audited/audit.rb

  Line 174 in c48893a

  max = self.class.auditable_finder(auditable_id, auditable_type).descending.first.try(:version) || 0

  Notice also, that now no auditable object will be loaded in this line

Probably, this index should be a UNIQUE index, but there are possible race conditions, at least at previously mentioned line.

Simple data for testing:

INSERT
INTO audits (auditable_id, auditable_type, version)
SELECT i % 1000 + 1, 'User', random() * 1000
FROM generate_series(1, 10000000) AS i;

With old index:

explain analyze select max(version) from audits where auditable_id = 12 and auditable_type = 'User'

Aggregate  (cost=16718.01..16718.02 rows=1 width=4) (actual time=49.106..49.107 rows=1 loops=1)
  ->  Index Scan using auditable_index on audits  (cost=0.43..16694.02 rows=9598 width=4) (actual time=1.541..47.316 rows=10000 loops=1)
        Index Cond: (((auditable_type)::text = 'User'::text) AND (auditable_id = 12))
Planning time: 0.190 ms
Execution time: 49.131 ms

With new index:

explain analyze select max(version) from audits where auditable_id = 525 and auditable_type = 'User'

    ->  Limit  (cost=0.43..1.36 rows=1 width=4) (actual time=0.064..0.064 rows=1 loops=1)
          ->  Index Only Scan Backward using auditable_index on audits  (cost=0.43..8997.91 rows=9735 width=4) (actual time=0.063..0.063 rows=1 loops=1)
                Index Cond: ((auditable_type = 'User'::text) AND (auditable_id = 525) AND (version IS NOT NULL))
                Heap Fetches: 1
Planning time: 2.660 ms
Execution time: 0.082 ms

So, 3 orders of magnitude faster.

Closes #189

[tbrisker]

Great work @fatkodima !