在PostgreSql 8.4查询中

explain analyze SELECT 
    max( kuupaev||kellaaeg ) as res
  from ALGSA 
  where laonr=1 and kuupaev <='9999-12-31' and 
     kuupaev||kellaaeg <= '9999-12-3123 59'

需要3秒钟来运行：

"Aggregate  (cost=3164.49..3164.50 rows=1 width=10) (actual time=2714.269..2714.270 rows=1 loops=1)"
"  ->  Seq Scan on algsa  (cost=0.00..3110.04 rows=21778 width=10) (actual time=0.105..1418.743 rows=70708 loops=1)"
"        Filter: ((kuupaev <= '9999-12-31'::date) AND (laonr = 1::numeric) AND ((kuupaev || (kellaaeg)::text) <= '9999-12-3123 59'::text))"
"Total runtime: 2714.363 ms"

如何在PostgreSQL
8.4.4中加快速度？表结构如下。algsa表在kuupaev上有索引，也许可以使用？或者可以更改查询以添加其他一些索引以使其快速。表中的现有列无法更改。

CREATE TABLE firma1.algsa
(
  id serial NOT NULL,
  laonr numeric(2,0),
  kuupaev date NOT NULL,
  kellaaeg character(5) NOT NULL DEFAULT ''::bpchar,
  ... other columns
  CONSTRAINT algsa_pkey PRIMARY KEY (id),
  CONSTRAINT algsa_id_check CHECK (id > 0)
)
);

CREATE INDEX algsa_kuupaev_idx  ON firma1.algsa  USING btree  (kuupaev);

更新

试过了 analyze verbose firma1.algsa;

INFO:  analyzing "firma1.algsa"
INFO:  "algsa": scanned 1640 of 1640 pages, containing 70708 live rows and 13 dead rows; 30000 rows in sample, 70708 estimated total rows
Query returned successfully with no result in 1185 ms.

但查询运行时间仍为2.7秒。

为什么有30000 rows in sample 。是不是太多了，应该减少吗？