修改
似乎可以将纯物化作为列存储在表中并编制索引;但是，我的具体用例（semver.satisfies）需要更通用的解决方案：
create table Submissions (
version text
created_at timestamp
)
create index Submissions_1 on Submissions (created_at)
我的查询将如下所示：
select * from Submissions
where
created_at <= '2016-07-12' and
satisfies(version, '>=1.2.3 <4.5.6')
order by created_at desc
limit 1;
我无法实际使用相同的记忆技术。
原始
我有一个表格，用于存储文本数据及其创建日期：
create table Submissions (
content text,
created_at timestamp
);
create index Submissions_1 on Submissions (created_at);
给定校验和和参考日期，我想得到Submission字段与校验和匹配的最新content：
select * from Submissions
where
created_at <= '2016-07-12' and
expensive_chksm(content) = '77ac76dc0d4622ba9aa795acafc05f1e'
order by created_at desc
limit 1;
这很有效，但速度很慢。 Postgres最终做的是对每一行进行校验和，然后执行order by：
Limit (cost=270834.18..270834.18 rows=1 width=32) (actual time=1132.898..1132.898 rows=1 loops=1)
-> Sort (cost=270834.18..271561.27 rows=290836 width=32) (actual time=1132.898..1132.898 rows=1 loops=1)
Sort Key: created_at DESC
Sort Method: top-N heapsort Memory: 25kB
-> Seq Scan on installation (cost=0.00..269380.00 rows=290836 width=32) (actual time=0.118..1129.961 rows=17305 loops=1)
Filter: created_at <= '2016-07-12' AND expensive_chksm(content) = '77ac76dc0d4622ba9aa795acafc05f1e'
Rows Removed by Filter: 982695
Planning time: 0.066 ms
Execution time: 1246.941 ms
没有order by，它是一个亚毫秒级的操作，因为Postgres知道我只想要第一个结果。唯一的区别是我希望Postgres从最近的日期开始搜索。
理想情况下，Postgres会：
- 按
过滤created_at- 按
created_at排序，降序- 返回校验和匹配的第一行
我尝试使用内嵌视图编写查询，但explain analyze表示它将被重写为我上面已有的内容。
答案 0 :(得分：2)
您可以同时为两个字段创建索引：
create index Submissions_1 on Submissions (created_at DESC, expensive_chksm(content));
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.15..8.16 rows=1 width=40) (actual time=0.004..0.004 rows=0 loops=1)
-> Index Scan using submissions_1 on submissions (cost=0.15..16.17 rows=2 width=40) (actual time=0.002..0.002 rows=0 loops=1)
Index Cond: ((created_at <= '2016-07-12 00:00:00'::timestamp without time zone) AND ((content)::text = '77ac76dc0d4622ba9aa795acafc05f1e'::text))
Planning time: 0.414 ms
Execution time: 0.036 ms
在索引中使用DESC也很重要。
对于存储和比较版本，您可以使用int []
create table Submissions (
version int[],
created_at timestamp
);
INSERT INTO Submissions SELECT ARRAY [ (random() * 10)::int2, (random() * 10)::int2, (random() * 10)::int2], '2016-01-01'::timestamp + ('1 hour')::interval * random() * 10000 FROM generate_series(1, 1000000);
create index Submissions_1 on Submissions (created_at DESC, version);
EXPLAIN ANALYZE select * from Submissions
where
created_at <= '2016-07-12'
AND version <= ARRAY [5,2,3]
AND version > ARRAY [1,2,3]
order by created_at desc
limit 1;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.42..13.24 rows=1 width=40) (actual time=0.074..0.075 rows=1 loops=1)
-> Index Only Scan using submissions_1 on submissions (cost=0.42..21355.76 rows=1667 width=40) (actual time=0.073..0.073 rows=1 loops=1)
Index Cond: ((created_at <= '2016-07-12 00:00:00'::timestamp without time zone) AND (version <= '{5,2,3}'::integer[]) AND (version > '{1,2,3}'::integer[]))
Heap Fetches: 1
Planning time: 3.019 ms
Execution time: 0.100 ms
到a_horse_with_no_name评论： where子句中条件的顺序与索引用法无关。最好先将可用于等式表达式的那个放在索引中，然后放入范围表达式。 -
BEGIN;
create table Submissions (
content text,
created_at timestamp
);
CREATE FUNCTION expensive_chksm(varchar) RETURNS varchar AS $$
SELECT $1;
$$ LANGUAGE sql;
INSERT INTO Submissions SELECT (random() * 1000000000)::text, '2016-01-01'::timestamp + ('1 hour')::interval * random() * 10000 FROM generate_series(1, 1000000);
INSERT INTO Submissions SELECT '77ac76dc0d4622ba9aa795acafc05f1e', '2016-01-01'::timestamp + ('1 hour')::interval * random() * 10000 FROM generate_series(1, 100000);
create index Submissions_1 on Submissions (created_at DESC, expensive_chksm(content));
-- create index Submissions_2 on Submissions (expensive_chksm(content), created_at DESC);
EXPLAIN ANALYZE select * from Submissions
where
created_at <= '2016-07-12' and
expensive_chksm(content) = '77ac76dc0d4622ba9aa795acafc05f1e'
order by created_at desc
limit 1;
使用Submission1：
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.43..10.98 rows=1 width=40) (actual time=0.018..0.019 rows=1 loops=1)
-> Index Scan using submissions_1 on submissions (cost=0.43..19341.43 rows=1833 width=40) (actual time=0.018..0.018 rows=1 loops=1)
Index Cond: ((created_at <= '2016-07-12 00:00:00'::timestamp without time zone) AND ((content)::text = '77ac76dc0d4622ba9aa795acafc05f1e'::text))
Planning time: 0.257 ms
Execution time: 0.033 ms
使用Submission2：
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=4482.39..4482.40 rows=1 width=40) (actual time=29.096..29.096 rows=1 loops=1)
-> Sort (cost=4482.39..4486.98 rows=1833 width=40) (actual time=29.095..29.095 rows=1 loops=1)
Sort Key: created_at DESC
Sort Method: top-N heapsort Memory: 25kB
-> Bitmap Heap Scan on submissions (cost=67.22..4473.23 rows=1833 width=40) (actual time=15.457..23.683 rows=46419 loops=1)
Recheck Cond: (((content)::text = '77ac76dc0d4622ba9aa795acafc05f1e'::text) AND (created_at <= '2016-07-12 00:00:00'::timestamp without time zone))
Heap Blocks: exact=936
-> Bitmap Index Scan on submissions_1 (cost=0.00..66.76 rows=1833 width=0) (actual time=15.284..15.284 rows=46419 loops=1)
Index Cond: (((content)::text = '77ac76dc0d4622ba9aa795acafc05f1e'::text) AND (created_at <= '2016-07-12 00:00:00'::timestamp without time zone))
Planning time: 0.583 ms
Execution time: 29.134 ms
PostgreSQL 9.6.1
答案 1 :(得分：1)
您可以将子查询用于时间戳和订购部分，然后在外面运行chksum：
bundles.Add(new StyleBundle("~/Content/my/path/sharedcss").Include(
"~/Content/my/path/bootstrap.css",
"~/Content/my/path/font-awesome.css",
"~/Content/my/path/AjaxLoadAnimation.css"));
答案 2 :(得分：0)
如果您总是要查询checksum，那么另一种方法是在表中使用另一个名为校验和的列，例如：
create table Submissions (
content text,
created_at timestamp,
checksum varchar
);
然后，只要行insert/update（或写inserted/updated）为您执行此操作，您就可以trigger校验和，并直接在checksum列查询以获得快速结果
答案 3 :(得分：0)
试试这个
select *
from Submissions
where created_at = (
select max(created_at)
from Submissions
where expensive_chksm(content) = '77ac76dc0d4622ba9aa795acafc05f1e')