索引

PostgreSQL/common.sql

@@ -1,7 +1,5 @@
--- 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
@@ -33,5 +31,27 @@ select r.ctid, r.* from study.r;
 -- "(0,3)",3,103
 
 -- 删除一条
-delete from study.r where id = 101;
+delete from study.r where id = 2;
--- 得到
+-- 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

@@ -0,0 +1,6 @@
+-- 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

@@ -0,0 +1,2 @@
+-- 不阻塞的情况下创建索引
+create index concurrently idx_prism_timestamp on activities (timestamp);

PostgreSQL/time.sql

@@ -8,3 +8,6 @@ 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

@@ -0,0 +1,4 @@
+-- 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;