索引

.gitignore

@@ -1,2 +1,3 @@
 .idea
 .vscode
+tmp*.sql

MySQL/common.sql

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-# InnoDB引擎页大小
+# InnoDB引擎页大小，默认16k
 show global status like 'Innodb_page_size';
 
-# 查询用户
+# Buffer Pool 默认大小是 128M
+show variables like '%innodb_buffer_pool%';
+
+# 查询MySQL用户
 select *
 from mysql.user;
 

PostgreSQL/common.sql

@@ -1,4 +1,57 @@
--- 修改用户的密码
+-- search_path
-alter user postgres with password '123456';
+SHOW search_path;
--- 查询所有用户
+
-select * from pg_catalog.pg_user;
+-- 查询表的 COLLATION VERSION
+SELECT
+    datname,
+    datcollversion,
+    pg_collation_actual_version((SELECT oid FROM pg_collation WHERE collname = 'default')) as system_version
+FROM pg_database
+WHERE datname IN ('fortern', 'postgres', 'kt_shop')
+ORDER BY datname;
+
+-- 重建索引
+REINDEX DATABASE postgres;
+ALTER DATABASE postgres REFRESH COLLATION VERSION;
+
+REINDEX DATABASE fortern;
+ALTER DATABASE fortern REFRESH COLLATION VERSION;
+
+REINDEX DATABASE kt_shop;
+ALTER DATABASE kt_shop REFRESH COLLATION VERSION;
+
+-- 重置某表的某列的自增序列 [表名]_[列名]_seq
+ALTER SEQUENCE person_id_seq RESTART WITH 4284858;
+
+-- ctid 会返回一个元组，元组中第一个数字是 page number, 第二个是 slot number。
+select r.ctid, r.* from study.r;
+-- 得到
+-- "(0,1)",1,101
+-- "(0,2)",2,102
+-- "(0,3)",3,103
+
+-- 删除一条
+delete from study.r where id = 2;
+-- 再重新查询，得到
+-- "(0,1)",1,101
+-- "(0,3)",3,103
+-- PostgreSQL 会保留空槽
+
+-- 插入新数据
+insert into study.r values (104, 4);
+-- 再重新查询，得到
+-- "(0,1)",1,101
+-- "(0,3)",3,103
+-- "(0,4)",4,104
+-- 可以看到使用了末尾的新槽位，而不是使用中间的新槽位
+
+-- 执行清理
+vacuum full study.r;
+-- 再次扫面，得到
+-- "(0,1)",1,101
+-- "(0,2)",3,103
+-- "(0,3)",4,104
+-- 看到空间已经回收
+
+-- 查看数据类型
+select pg_typeof(EXTRACT(EPOCH FROM current_timestamp(0)))

PostgreSQL/function.sql

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+-- ROW 关键字生成一个元组
+SELECT ROW (1, 2, 3);
+
+-- pg_column_size函数获取存储数据需要的字节数
+SELECT pg_column_size(ROW (1,2,3)); -- 36
+SELECT pg_column_size(ROW (1::int2, 2::int2, 3::int2)); -- 30

PostgreSQL/index.sql

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+-- 不阻塞的情况下创建索引
+create index concurrently idx_prism_timestamp on activities (timestamp);

PostgreSQL/time.sql

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+-- 设置会话时区
+SET TIME ZONE 'Asia/Shanghai';
+
+-- AT TIME ZONE 在 PostgreSQL 中是重载的，行为取决于左侧的类型：
+-- timestamp WITHOUT time zone AT TIME ZONE zone → 把该 wall time 视为 zone 的本地时间，然后转换为 UTC，结果为 timestamptz（显示为 UTC 偏移后的时间）。
+-- timestamptz AT TIME ZONE zone → 把该瞬间转换成 zone 的本地 wall time，结果为 timestamp WITHOUT time zone。
+select msg_time at time zone 'Asia/Shanghai' from msg where id = 1;
+select msg_time at time zone 'UTC+8' from msg where id = 1;
+
+select to_timestamp(1761850339) AT TIME ZONE 'Asia/Shanghai';
+
+-- 将当前时间转换为Unix时间戳
+select EXTRACT(EPOCH FROM current_timestamp(0)) - 60;

PostgreSQL/user.sql

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+-- 修改用户的密码
+alter user postgres with password '123456';
+-- 查询所有用户
+select * from pg_catalog.pg_user;

PostgreSQL/windows_fun.sql

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+-- 窗口函数
+-- row_number(): 对相等的值【不区分】，相当于行号，相等的值对应的排名不同，序号从1到n连续
+-- rank(): 相等的值排名相同，但若有相等的值，则序号从1到n不连续。符合一般人思维中的排名
+-- dense_rank(): 对相等的值排名相同，但序号从1到n连续
+
+-- 先按num分区，每个区内按id排序
+select id, num, id - row_number() over (partition by num order by id) as rnk
+from employee;
+
+-- 找出所有至少连续出现三次的数字
+-- 先按num分区，每个区内按id排序
+-- 一个分区内，如果数字连续，那么id会不断+1，row_number()也会不断+1，id - row_number()差值不变
+-- 在按照id - row_number()聚合，使用count(*)统计，count(*)的结果就是连续出现的次数
+select r.num, r.rnk, count(*)
+from (select num, id - row_number() over (partition by num order by id) as rnk from employee) as r
+group by r.num, r.rnk;
+
+select distinct r.num as ConsecutiveNums
+from (select num, id - row_number() over (partition by num order by id) as rnk from employee) as r
+group by r.num, r.rnk
+having count(*) >= 3
+limit 1;