#RuPostgresLive 4: как писать и читать сложные SQL-запросы

#RuPostgresLive
Как писать и читать сложные SQL-запросы
Николай Самохвалов
Twitter: @postgresmen
ru@postgresql.org
2017-11-23

Подготовка
create table post (
id bigserial primary key,
person_id int8 not null,
created_at timestamptz not null,
something text
);
insert into post(user_id, created_at, something)
select
(random() * 10^5)::int8 as person_id, -- ~100k users
now() - interval '1 minute' * (random() * 60 * 24 * 365 * 2) as created_at,
(
select string_agg(substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW XYZ0123456789 ', (random() *
72)::integer + 1, 1), '')
from generate_series(1, 100 + i % 10 + (random() * 2000)::integer)
) as something -- 100-2100 bytes of something
from generate_series(1, 1000000) AS g(i) -- 1 million posts
;

Подготовка
delete from post
where (person_id, created_at) in (
select person_id, created_at
from post
group by person_id, created_at
having count(*) > 1
);
create index i_post_created_at on post using btree(created_at);
create unique index u_post_author_id_created_at on post(person_id, created_at);
create table person as
select distinct on (person_id)
person_id as id,
'person_' || person_id as name
from post
;
alter table person add primary key (id);
alter table post add constraint fk_post_person_id foreign key (person_id) references person(id);
analyze post;
analyze person;

Быстрый count(..). Таблица целиком
select count(*) from post; -- sloooow
explain select * from post; -- see “rows= …” for an estimation

Быстрый count(..). Поиск по индексу
select count(*) from post where person_id = 1;
select count(1) from post where person_id = 1;
Ещё (много интересного) — в статье Joe Nelson “Faster PostgreSQL Counting”
https://gist.github.com/begriffs/67839ff18176d5879e77954bfcd38f1f
https://www.citusdata.com/blog/2016/10/12/count-performance/

Немного про #nooffset
select *
from post
order by id desc
limit 10 offset 100000;

select *
from post
order by id desc

select *
from post
order by id desc
select *
from post
where id < :last_id
order by id desc
limit 10;

Как делать OFFSET правильно? Вариант 2
select pl.*
from (
select id
from post
order by id offset 500000
limit 1
) as t,
lateral (
select *
from post
where id >= t.id
order by id
limit 10
) as pl;
Хорошая статья про LATERAL JOIN:
https://medium.com/kkempin/postgresqls-lateral-join-bfd6bd0199df

Top-N постов
select
person.name,
p.*
from person
join lateral (
select *
from post
where person_id = person.id
order by id desc
limit 3
) as p on true
where person.id between 1 and 1000
order by person_id
;

DISTINCT
select distinct person_id
from post;

DISTINCT — recursive CTE
with recursive t as (
-- start from least persno ID
(
select person_id as _person_id
from post
order by 1
limit 1
)
union all
select person_id as _person_id
from t, lateral (
-- find the next person_id > current person_id
select person_id
from post
where person_id > t._person_id
order by 1
limit 1
) as p_id
)
select _person_id
from t;

Полезное
● Очень хорошая презентация о современном SQL и хороший сайт в
целом – Use the Index, Luke!
http://use-the-index-luke.com/blog/2015-02/modern-sql
● Про count(..)
https://gist.github.com/begriffs/67839ff18176d5879e77954bfcd38f1f
● Максим Богук “Как научить слона танцевать Рок-н-ролл”
https://pgday.ru/presentation/232/5964945ea4142.pdf
● Twitter: @postgresmen

#RuPostgresLive 4: как писать и читать сложные SQL-запросы

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