PostgreSQL 1000W 条数据，执行 SELECT 查询，很缓慢，如何优化？

我帮我朋友做了个 iOS 关于定位的 app 后端, 用 NodeJS 来做的 http api, PostgreSQL 来做的数据存储。

现在用户量上来了，经纬度数据有 1000W 条了， 每分钟最多有 400 个请求。

数据库表结构:

location=# \d locations
                                   Table "public.locations"
  Column   |           Type           |                       Modifiers
-----------+--------------------------+--------------------------------------------------------
 id        | integer                  | not null default nextval('locations_id_seq'::regclass)
 phone     | text                     | not null
 longitude | text                     | not null
 latitude  | text                     | not null
 date      | text                     | not null
 createdAt | timestamp with time zone |
 updatedAt | timestamp with time zone |
Indexes:
    "locations_pkey" PRIMARY KEY, btree (id)
    "createdAt_idx" btree ("createdAt")
    "phone_idx" btree (phone)

执行查询：

location=# EXPLAIN ANALYZE select * from "locations" where "phone" = '15828354000' order by "createdAt" desc limit 1;
                                                                        QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.43..5.22 rows=1 width=74) (actual time=4848.720..4848.720 rows=1 loops=1)
   ->  Index Scan Backward using "createdAt_idx" on locations  (cost=0.43..663383.70 rows=138739 width=74) (actual time=4848.718..4848.718 rows=1 loops=1)
         Filter: (phone = '15828354000'::text)
         Rows Removed by Filter: 2032111
 Planning time: 0.109 ms
 Execution time: 4848.746 ms

也就是说没次用户查询一次他最近的位置都会耗时 4.8s

我在想要不要用 Redis 来对这个接口做个缓存呢？