Using postgres functions inside a where clause can make a query non-sargable.

Database Structure

tracks has_many artists

Non-Sargable Query

Using a LOWER function prevents DBMS engine from using indexes.

Track.joins(:artists).where('LOWER(tracks.display_name) LIKE ?', "eric clapton%").explain

Gather  (cost=1000.85..56714.32 width=4061)
   Workers Planned: 2
   ->  Nested Loop  (cost=0.85..55713.62 rows=3 width=4061)
         ->  Nested Loop  (cost=0.42..55708.37 rows=3 width=4069)
               ->  Parallel Seq Scan on tracks  (cost=0.00..55365.17 rows=41 width=4061)
                     Filter: (lower((display_name)::text) ~~* 'eric clapton%'::text)
               ->  Index Scan using index_artist_relations_on_artist_item_type_and_artist_item_id on artist_relations  (cost=0.42..8.36 rows=1 width=16)
                     Index Cond: (((artist_item_type)::text = 'Track'::text) AND (artist_item_id = tracks.id))
         ->  Index Only Scan using idx_35952_primary on artists  (cost=0.43..1.75 rows=1 width=8)
               Index Cond: (id = artist_relations.artist_id)

Sargable Query

Removing LOWER function allows DBMS engine to use indexes, resulting in faster execution.

Track.joins(:artists).where('tracks.display_name ILIKE ?', "eric clapton%").explain

Nested Loop  (cost=1497.60..2695.43 width=4061)
   ->  Nested Loop  (cost=1497.17..2684.94 rows=6 width=4069)
         ->  Bitmap Heap Scan on tracks  (cost=1496.75..1873.05 rows=97 width=4061)
               Recheck Cond: ((display_name)::text ~~* 'eric clapton%'::text)
               ->  Bitmap Index Scan on index_tracks_on_display_name  (cost=0.00..1496.73 rows=97 width=0)
                     Index Cond: ((display_name)::text ~~* 'eric clapton%'::text)
         ->  Index Scan using index_artist_relations_on_artist_item_type_and_artist_item_id on artist_relations  (cost=0.42..8.36 rows=1 width=16)
               Index Cond: (((artist_item_type)::text = 'Track'::text) AND (artist_item_id = tracks.id))
   ->  Index Only Scan using idx_35952_primary on artists  (cost=0.43..1.75 rows=1 width=8)
         Index Cond: (id = artist_relations.artist_id)