QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------------- Nested Loop (cost=0.56..4001768.10 rows=479020 width=26) (actual time=2.303..15371.237 rows=479020 loops=1) Output: path.pathid, batch.filename Buffers: shared hit=2403958 read=7539 -> Seq Scan on public.batch (cost=0.00..11727.20 rows=479020 width=85) (actual time=0.340..160.142 rows=479020 loops=1) Output: batch.path, batch.filename Buffers: shared read=6937 -> Index Scan using idx_path on public.path (cost=0.56..8.32 rows=1 width=16) (actual time=0.030..0.031 rows=1 loops=479020) Output: path.pathid, path.path Index Cond: (path.path = batch.path) Buffers: shared hit=2403958 read=602 Total runtime: 15439.043 ms
As you can see, more than twice as fast, and a very high hit ratio on the path table, even if we start from a cold cache (I did, here, both PostgreSQL and OS). We have an excellent hit ratio because the batch table contains few different path (several files in a directory), and is already quite clustered, as it comes from a backup, which is of course performed directory by directory.