Postgresql 11.3 doesn't use gist index on polygon column

[Вадим Самохин <samokhinvadim@xxxxxxxxx>]

Hi there,

1. I have a table that looks like this:

create table zonez (p polygon);

2. I have an index that looks like this:

create index zonez__p2 on zonez using gist(p poly_ops);

3. I inserted several records in that table, it looks like:

postgres=# select * from zonez ;
                     p                    
-------------------------------------------
 ((1,1),(1,5),(5,5),(5,1))
 ((1,2),(1,5),(5,5),(5,1))
 ((1,3),(1,5),(5,5),(5,1))
 ((1,4),(1,5),(5,5),(5,1))
 ((1,6),(1,5),(5,5),(5,1))
 ((1,7),(1,5),(5,5),(5,1))
 ((1,7),(1,5),(5,5),(500000,1000))
 ((1,7),(1,5),(5,5),(52.654987,37.123789))
(8 rows)

4. I've "turned off" seqscan: set enable_seqscan = off;

5. I've issued vacuum analyze

6. But postgresql still doesn't want to use my index:

postgres=# explain analyze select * from zonez where '(2,2)'::point <@ p;
                                                     QUERY PLAN                                                      
---------------------------------------------------------------------------------------------------------------------
 Seq Scan on zonez  (cost=10000000000.00..10000000001.10 rows=1 width=101) (actual time=0.013..0.018 rows=2 loops=1)
   Filter: ('(2,2)'::point <@ p)
   Rows Removed by Filter: 6
 Planning Time: 0.069 ms
 Execution Time: 0.036 ms
(5 rows)

How come? Is it an index that should look different, or is it really more expensive than 10000000001.10?