postgresql timestamp 索引问题

CREATE INDEX idx_e_d ON public.expense USING btree (date);

explain select * from "public"."expense" where date > '2018-12-19'

Seq Scan on expense (cost=0.00..15133.15 rows=181367 width=41)

Filter: (date > '2018-12-19 00:00:00'::timestamp without time zone)

explain select * from "public"."expense" where date > '2018-12-20'

Bitmap Heap Scan on expense (cost=5245.18..14770.65 rows=162678 width=41)

Recheck Cond: (date > '2018-12-20 00:00:00'::timestamp without time zone)

-> Bitmap Index Scan on idx_e_d (cost=0.00..5204.51 rows=162678 width=0)

    Index Cond: (date > '2018-12-20 00:00:00'::timestamp without time zone)

日期差一天，不走索引，用的是阿里云的 postgresql 9.4 2 核 4g