Matching indexe for timestamp

[Job <Job@xxxxxxxxxxxxxxxxxxxx>]

Hello,

 

on this table:

 

                                          Table "public.gruorari_tmp"
  Column   |          Type          |                                Modifiers
-----------+------------------------+--------------------------------------------------------------------------
 id        | numeric(1000,1)        | not null default function_get_next_sequence('gruorari_tmp_id_seq'::text)
 idgrucate | numeric(1000,1)        |
 dalle     | time without time zone |
 alle      | time without time zone |
 gg_sett   | integer                |
 azione    | character varying      |
Indexes:
    "keygruorari_tmp" PRIMARY KEY, btree (id)
    "gruorari_tmp_alle_idx" btree (alle)
    "gruorari_tmp_dalle_alle_idx" btree (dalle, alle)
    "gruorari_tmp_dalle_idx" btree (dalle)
    "gruorari_tmp_gg_sett_idx" btree (gg_sett)
    "gruorari_tmp_idgrucate_idx" btree (idgrucate)

 

i have a specific condition (i report example value):

"and ( gruorari_tmp.id is null or ( 2 = gg_sett and '16:00:00'::time between gruorari_tmp.dalle and gruorari_tmp.alle ) )"

 

But in the query planner, at that point, Postgresql 9.6.1 seems not to use any index (single on dalle / alle field and combindex index on dalle+alle) but it use seqscan:

 

Seq Scan on gruorari_tmp  (cost=0.00..5.90 rows=290 width=68) (actual time=0.014..0.062 rows=290 loops=1)
                                 ->  Hash  (cost=164.06..164.06 rows=1 width=29) (actual time=0.770..0.770 rows=1 loops=1)

And it seems to be the main cost for the whole query.

Which kind of index should i use for that condition/fields?

Thank you!

/F