简单查看postgresql查询的算法改进

假设我有一个这样的表：

Table "public.summary"
   Column    |       Type        |                      Modifiers
-------------+-------------------+------------------------------------------------------
 ts          | integer           | not null default nextval('summary_ts_seq'::regclass)
 field1      | character varying | not null
 otherfield  | character varying | not null
 country     | character varying | not null
 lookups     | integer           | not null


Indexes:
    "summary_pk" PRIMARY KEY,btree (ts,field1,otherfield,country)
    "ix_summary_country" btree (country)
    "ix_summary_field1" btree (field1)
    "ix_summary_otherfield" btree (otherfield)
    "ix_summary_ts" btree (ts)

我想要的查询是：

select summary.field1,summary.country,summary.ts,sum(summary.lookups) as lookups,from summary
where summary.country = 'za' and
    summary.ts = 1275177600
group by summary.field1,summary.ts
order by summary.ts,lookups desc,summary.field1
limit 100;

(英语：在特定(ts,国家)的前100名field1’s,其中’topness’是总和
任何匹配行的查找,无论其他字段的值如何)

我能做些什么来加速这个吗？在算法上
这似乎是一个全表扫描的东西,但我可能会遗漏一些东西.