Indexing on JSONB field not working

I have an index on JSONB fields like this,

CREATE INDEX float_number_index_path2

    ON public.assets USING btree

    (((_doc #> '{floatValue}'::text[])::double precision) ASC NULLS LAST)

    TABLESPACE pg_default;

However query doesn’t use it,

explain select id, _doc->>'floatValue' from assets where (_doc #> '{floatValue}'::text[])::double precision < 3.0 limit 3;

Limit  (cost=0.00..3.24 rows=3 width=53)

  ->  Seq Scan on assets  (cost=0.00..936605.40 rows=867607 width=53)

        Filter: (((_doc #> '{floatValue}'::text[]))::double precision < '3'::double precision)

The version of the database,

"PostgreSQL 11.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.2 20140120 (Red Hat 4.8.2-16), 64-bit"

However, the index works for text field in JSONB.

Let me know if I can provide more information.

Zhihong Zhang

zhihong@gmail.com