I'm trying to speed up this query in Postgres 9.4:

SELECT "groupingsFrameHash", COUNT(*) AS nb
FROM "public"."zrac_c1e350bb-a7fc-4f6b-9f49-92dfd1873876"
GROUP BY "groupingsFrameHash"
HAVING COUNT(*) > 1
ORDER BY nb DESC LIMIT 10

I have an index on groupingsFrameHash. I don't need exact results, a fuzzy approximation is good enough.

Here is the query plan:

Limit  (cost=17207.03..17207.05 rows=10 width=25) (actual time=740.056..740.058 rows=10 loops=1)
  ->  Sort  (cost=17207.03..17318.19 rows=44463 width=25) (actual time=740.054..740.055 rows=10 loops=1)
        Sort Key: (count(*))
        Sort Method: top-N heapsort  Memory: 25kB
        ->  GroupAggregate  (cost=14725.95..16246.20 rows=44463 width=25) (actual time=615.109..734.740 rows=25977 loops=1)
              Group Key: "groupingsFrameHash"
              Filter: (count(*) > 1)
              Rows Removed by Filter: 24259
              ->  Sort  (cost=14725.95..14967.07 rows=96446 width=25) (actual time=615.093..705.507 rows=96026 loops=1)
                    Sort Key: "groupingsFrameHash"
                    Sort Method: external merge  Disk: 3280kB
                    ->  Seq Scan on "zrac_c1e350bb-a7fc-4f6b-9f49-92dfd1873876"  (cost=0.00..4431.46 rows=96446 width=25) (actual time=0.007..33.813 rows=96026 loops=1)
Planning time: 0.080 ms
Execution time: 740.877 ms

I don't understand why it needs to do a Seq Scan.