Question

这是我的疑问：

SELECT
    i::date AS day,
    (SELECT COUNT(*) FROM genericevent WHERE event = 'chat_message' AND eventDate::date = i::date AND extra1 = 'public') AS message_public_total,
    (SELECT COUNT(*) FROM genericevent WHERE event = 'chat_message' AND eventDate::date = i::date AND extra1 = 'public' AND extra2 = 'clean') AS message_public_clean
FROM generate_series('2013-08-01', '2013-08-27', INTERVAL '1 day') i

我有一个索引，我作为人类认为完全可用于此查询（实际上它应该导致仅索引扫描）：

CREATE INDEX idx__genericevent__event__extra1__date
  ON genericevent
  USING btree
  (event COLLATE pg_catalog."default", extra1 COLLATE pg_catalog."default", eventDate);

然而，正如EXPLAIN ed，PostgreSQL并不认为如此。它使用此索引中的event和extra1，但不使用eventDate（请参阅Index Cond行）：

"Function Scan on generate_series i  (cost=0.00..145219698.17 rows=1000 width=8)"
"  SubPlan 1"
"    ->  Aggregate  (cost=72274.87..72274.88 rows=1 width=0)"
"          ->  Bitmap Heap Scan on genericevent  (cost=11367.74..72271.51 rows=1345 width=0)"
"                Recheck Cond: (((event)::text = 'chat_message'::text) AND ((extra1)::text = 'public'::text))"
"                Filter: ((eventDate)::date = (i.i)::date)"
"                ->  Bitmap Index Scan on idx__genericevent__event__extra1__date  (cost=0.00..11367.40 rows=269012 width=0)"
"                      Index Cond: (((event)::text = 'chat_message'::text) AND ((extra1)::text = 'public'::text))"
"  SubPlan 2"
"    ->  Aggregate  (cost=72944.79..72944.80 rows=1 width=0)"
"          ->  Bitmap Heap Scan on genericevent  (cost=11367.50..72943.80 rows=396 width=0)"
"                Recheck Cond: (((event)::text = 'chat_message'::text) AND ((extra1)::text = 'public'::text))"
"                Filter: (((extra2)::text = 'clean'::text) AND ((eventDate)::date = (i.i)::date))"
"                ->  Bitmap Index Scan on idx__genericevent__event__extra1__date  (cost=0.00..11367.40 rows=269012 width=0)"
"                      Index Cond: (((event)::text = 'chat_message'::text) AND ((extra1)::text = 'public'::text))"

我认为它可能需要eventDate::date强制转换。如何更改查询或索引以提高性能？

为了完整性，这是表格：

CREATE TABLE genericevent
(
  id bigint NOT NULL,
  eventDate timestamp with time zone NOT NULL,
  event character varying(50) NOT NULL,
  extra1 character varying(100),
  extra2 character varying(100),
  CONSTRAINT genericevent_pkey PRIMARY KEY (id)
)

Answer 1

您需要使用时间戳才能工作，而不是日期。

在纸面上，您可以将索引更改为表达式，以便将日期截断为指定日期。但是如果时间戳有一个时区，这将不起作用，因为它会因为服务器时区改变的理论潜力而变得不稳定。

实际上，您需要将等式子句更改为等效的不等式，例如：类似的东西：

eventDate >= i and eventDate < i + interval '1 day'

但在继续重写查询之前，请注意您只需在Clodoaldo Neto的查询中添加适当的where子句：

select
    i::date as day,
    count(*) as message_public_total,
    count(extra2 = 'clean' or null) as message_public_clean
from
    genericevent
    right join
    generate_series(
        '2013-08-01', '2013-08-27', interval '1 day'
    ) i on eventdate::date = i::date
where
    event = 'chat_message'
    and extra1 = 'public'
    and eventDate >= '2013-08-01'
    and eventDate < '2013-08-27' + interval '1 day'
group by 1

或者：

select
    i::date as day,
    count(*) as message_public_total,
    count(extra2 = 'clean' or null) as message_public_clean
from
    genericevent
    right join
    generate_series(
        '2013-08-01', '2013-08-27', interval '1 day'
    ) i on eventdate >= i and eventDate < i + interval '1 day'
where
    event = 'chat_message'
    and extra1 = 'public'
--    and eventDate >= '2013-08-01'
--    and eventDate < '2013-08-27' + interval '1 day'
group by 1

Answer 2

此等效查询只扫描一个，而不是像你的那样扫描。

select
    i::date as day,
    count(*) as message_public_total,
    count(extra2 = 'clean' or null) as message_public_clean
from
    genericevent
    right join
    generate_series(
        '2013-08-01', '2013-08-27', interval '1 day'
    ) i on eventdate::date = i::date
where
    event = 'chat_message'
    and extra1 = 'public'
group by 1

然后索引将是

create index idx on genericevent (
    eventDate::date,
    event,
    extra1   
)

我将日期放在第一位，因为我猜它具有最高cardinality。

转换为日期时未使用PostgreSQL时间戳索引

2 个答案: