我还没有能够得到这个查询来搜索索引而不是执行完整扫描 - 我有另一个查询使用date_part（'day'，datelocal）对几乎相同的表（该表只有少一点的数据，但相同的结构）并且会触及我在datelocal列上创建的索引（这是一个没有时区的时间戳）。查询（这个在表上执行并行seq扫描并执行内存快速排序）：
SELECT
date_part('hour', datelocal) AS hour,
SUM(CASE WHEN gender LIKE 'male' THEN views ELSE 0 END) AS male,
SUM(CASE WHEN gender LIKE 'female' THEN views ELSE 0 END) AS female
FROM reportimpression
WHERE datelocal >= '2-1-2019' AND datelocal < '2-28-2019'
GROUP BY date_part('hour', datelocal)
ORDER BY date_part('hour', datelocal)
这是另一个命中我的datelocal索引：
SELECT
date_part('day', datelocal) AS day,
SUM(CASE WHEN gender LIKE 'male' THEN views ELSE 0 END) AS male,
SUM(CASE WHEN gender LIKE 'female' THEN views ELSE 0 END) AS female
FROM reportimpressionday
WHERE datelocal >= '2-1-2019' AND datelocal < '2-28-2019'
GROUP BY date_trunc('day', datelocal), date_part('day', datelocal)
ORDER BY date_trunc('day', datelocal)
对此我的头脑！关于如何加速第一个或者至少让它达到指数的任何想法？我尝试在datelocal字段上创建一个索引，在datelocal，gender和views上创建一个复合索引，在date_part（'hour'，datelocal）上创建一个表达式索引，但这些都没有效果。
架构：
-- Table Definition ----------------------------------------------
CREATE TABLE reportimpression (
datelocal timestamp without time zone,
devicename text,
network text,
sitecode text,
advertisername text,
mediafilename text,
gender text,
agegroup text,
views integer,
impressions integer,
dwelltime numeric
);
-- Indices -------------------------------------------------------
CREATE INDEX reportimpression_datelocal_index ON reportimpression(datelocal timestamp_ops);
CREATE INDEX reportimpression_viewership_index ON reportimpression(datelocal timestamp_ops,views int4_ops,impressions int4_ops,gender text_ops,agegroup text_ops);
CREATE INDEX reportimpression_test_index ON reportimpression(datelocal timestamp_ops,(date_part('hour'::text, datelocal)) float8_ops);
-- Table Definition ----------------------------------------------
CREATE TABLE reportimpressionday (
datelocal timestamp without time zone,
devicename text,
network text,
sitecode text,
advertisername text,
mediafilename text,
gender text,
agegroup text,
views integer,
impressions integer,
dwelltime numeric
);
-- Indices -------------------------------------------------------
CREATE INDEX reportimpressionday_datelocal_index ON reportimpressionday(datelocal timestamp_ops);
CREATE INDEX reportimpressionday_detail_index ON reportimpressionday(datelocal timestamp_ops,views int4_ops,impressions int4_ops,gender text_ops,agegroup text_ops);
解释（分析，缓冲）输出：
Finalize GroupAggregate (cost=999842.42..999859.67 rows=3137 width=24) (actual time=43754.700..43754.714 rows=24 loops=1)
Group Key: (date_part('hour'::text, datelocal))
Buffers: shared hit=123912 read=823290
I/O Timings: read=81228.280
-> Sort (cost=999842.42..999843.99 rows=3137 width=24) (actual time=43754.695..43754.698 rows=48 loops=1)
Sort Key: (date_part('hour'::text, datelocal))
Sort Method: quicksort Memory: 28kB
Buffers: shared hit=123912 read=823290
I/O Timings: read=81228.280
-> Gather (cost=999481.30..999805.98 rows=3137 width=24) (actual time=43754.520..43777.558 rows=48 loops=1)
Workers Planned: 1
Workers Launched: 1
Buffers: shared hit=123912 read=823290
I/O Timings: read=81228.280
-> Partial HashAggregate (cost=998481.30..998492.28 rows=3137 width=24) (actual time=43751.649..43751.672 rows=24 loops=2)
Group Key: date_part('hour'::text, datelocal)
Buffers: shared hit=123912 read=823290
I/O Timings: read=81228.280
-> Parallel Seq Scan on reportimpression (cost=0.00..991555.98 rows=2770129 width=17) (actual time=13.097..42974.126 rows=2338145 loops=2)
Filter: ((datelocal >= '2019-02-01 00:00:00'::timestamp without time zone) AND (datelocal < '2019-02-28 00:00:00'::timestamp without time zone))
Rows Removed by Filter: 6792750
Buffers: shared hit=123912 read=823290
I/O Timings: read=81228.280
Planning time: 0.185 ms
Execution time: 43777.701 ms
好吧，你的查询都在不同的表上（reportimpression与reportimpressionday），所以两个查询的比较实际上不是比较。你们两个都是ANALYZE吗？各种列统计也可能发挥作用。索引或表格膨胀可能不同。所有行的大部分是否符合2019年2月的要求？等等。
在黑暗中一击，比较两个表的百分比：
SELECT tbl, round(share * 100 / total, 2) As percentage
FROM (
SELECT text 'reportimpression' AS tbl
, count(*)::numeric AS total
, count(*) FILTER (WHERE datelocal >= '2019-02-01' AND datelocal < '2019-03-01')::numeric AS share
FROM reportimpression
UNION ALL
SELECT 'reportimpressionday'
, count(*)
, count(*) FILTER (WHERE datelocal >= '2019-02-01' AND datelocal < '2019-03-01')
FROM reportimpressionday
) sub;
reportimpression的那个更大吗？然后它可能会超过索引预期有助于的数量。
通常，你的索引reportimpression_datelocal_index on（datelocal）看起来不错，而reportimpression_viewership_index甚至允许仅索引扫描，如果autovacuum击败表上的写入负载。 （虽然impressions和agegroup只是为此付出了代价，如果没有它会更好）。
你有我的查询26.6 percent, and day is 26.4 percent。对于如此大的百分比，索引通常根本没用。顺序扫描通常是最快的方式。如果底层表格更大，那么只有仅索引扫描才有意义。 （或者你有严重的表膨胀，而且索引更少，这使得索引再次具有吸引力。）
您的第一个查询可能只是跨越临界点。尝试缩小时间范围，直到看到仅索引扫描。你不会看到（位图）索引扫描超过大约5％的所有行符合条件（取决于许多因素）。
尽管如此，请考虑以下修改过的查询：
SELECT date_part('hour', datelocal) AS hour
, SUM(views) FILTER (WHERE gender = 'male') AS male
, SUM(views) FILTER (WHERE gender = 'female') AS female
FROM reportimpression
WHERE datelocal >= '2019-02-01'
AND datelocal < '2019-03-01' -- '2019-02-28' -- ?
GROUP BY 1
ORDER BY 1;
SELECT date_trunc('day', datelocal) AS day
, SUM(views) FILTER (WHERE gender = 'male') AS male
, SUM(views) FILTER (WHERE gender = 'female') AS female
FROM reportimpressionday
WHERE datelocal >= '2019-02-01'
AND datelocal < '2019-03-01'
GROUP BY 1
ORDER BY 1;
'2-1-2019'这样的本地化日期格式时，请使用带有显式格式说明符的to_timestamp()。否则，这取决于区域设置，并且在从具有不同设置的会话调用时可能会中断（静默）。而是使用不依赖于区域设置的ISO日期/时间格式。datelocal < '2-28-2019'也排除了2月28日的全部。请改用datelocal < '2019-03-01'。SELECT列表中相同的表达式进行分组和排序会更便宜。所以也在那里使用date_trunc()。不需要使用不同的表达式。如果您需要结果中的datepart，请将其应用于分组表达式，如：
SELECT date_part('day', date_trunc('day', datelocal)) AS day
...
GROUP BY date_trunc('day', datelocal)
ORDER BY date_trunc('day', datelocal);
代码更嘈杂，但速度更快（也可能更容易针对查询规划器进行优化）。FILTER子句。它更干净，更快。看到：
How can I simplify this game statistics query?
For absolute performance, is SUM faster or COUNT?