PostgreSQL 10 High Performance - Third Edition

Combining indexes

However, this doesn't mean the v index is useless. If you are running something that is selective based on the k index, the one on v can be combined with that. With the index in place, this earlier example now uses a complicated query plan:

EXPLAIN ANALYZE SELECT count(*) FROM t WHERE k>9000 AND v=5;
QUERY PLAN                                                        
-------------------------
Aggregate  (cost=787.97..787.98 rows=1 width=0) (actual time=40.545..40.546 rows=1 loops=1)
       ->  Bitmap Heap Scan on t  (cost=170.38..765.03 rows=9176 width=0) (actual time=4.107..24.606 rows=9097 loops=1)
             Recheck Cond: (v = 5)
             Filter: (k > 9000)
             ->  Bitmap Index Scan on i  (cost=0.00..168.08 rows=10110 width=0) (actual time=3.255..3.255 rows=9993 loops=1)
                   Index Cond: (v = 5)
     Total runtime: ...