PostgreSQL：在timestamp：：DATE 上创建索引

我正在创建一个汇总表，总结一天中的所有事件.

INSERT INTO graph_6(
  day,
  event_type,
  (SELECT COUNT(*) FROM event e 
                  WHERE event_type = e.event_type 
                  AND creation_time::DATE = sq.day)
FROM event_type
CROSS JOIN
    (SELECT generate_series(
                (SELECT '2014-01-01'::DATE),
                (SELECT '2014-01-02'::DATE),
                '1 day') as day) sq;

creation_time列被编入索引:

CREATE INDEX event_creation_time_date_idx ON event USING BTREE(creation_time);

但是，即使只查询两天的数据和少量事件(2014年1月1日至2日)，查询也会运行相当长的时间.

查询中的EXPLAIN非常糟糕——它对event表进行顺序扫描，根本不使用索引:

->  Seq Scan on event e_1  (cost=0.00..12557.39 rows=531 width=38)
Filter: ... AND ((creation_time)::date = (generate_series(($12)::timestamp with time zone, ($13)::timestamp with time zone, '1 day'::interval))))

我认为这是因为我们比较了一个铸造值——creation_time::DATE，而不是creation_time.我试着为演员编制索引:

CREATE INDEX event_creation_time_date_idx ON event USING BTREE(creation_time::DATE);

但有一个错误:

ERROR: syntax error at or near "::"

Is there a way to utilize PostgreSQL indices on a timezone column casted to DATE?