postgres-how-to-use

本文介绍了如何在PostgreSQL中使用psql命令将特定SQL语句的执行结果导出到文件的方法,并提供了示例。此外,还涉及了PostgreSQL的一些常见问题及解决办法。

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参考:https://blog.youkuaiyun.com/u011024160/article/details/51372820?locationNum=9&fps=1
转载:
postgresql中可以导出某个sql的执行结果到文件中,方法是在psql中首先执行\o filename,然后执行要导出结果的sql语句即可,可以看到刚刚执行的sql执行结果已经在filename中了。方便,强大,赞!
举个例子:

psql test_db;
\o /tmp/test.out
select username from member where age > 18;

开放端口:
ubuntu ufw
centos firewall-cmd
https://www.jb51.net/os/Ubuntu/617627.html

Ident authentication failed for user “postgres”
https://blog.youkuaiyun.com/ohyoyo2014/article/details/24108429

ERROR:
There is 1 other session using the database.
select pg_terminate_backend(pid) from (select pid from pg_stat_activity where datname=‘数据库名’) a;

“root” execution of postgresql server is not permitted
http://www.itkeyword.com/doc/4824124756309341015/root-execution-of-the-postgresql-server-is-not-permitted
sudo -u postgres -i

ERROR:
systemctl 启动错误日志:
pg_ctl: directory “/usr/local/postgres/pgsql/data_pgsql” is not a database cluster directory

### PostgreSQL TABLESAMPLE Usage In PostgreSQL, `TABLESAMPLE` allows users to sample rows from tables efficiently without scanning the entire table. The syntax for using this clause involves specifying one of two methods: `BERNOULLI` or `SYSTEM`. Each method has distinct characteristics regarding how samples are selected. For instance, when applying Bernoulli sampling: ```sql SELECT * FROM employees TABLESAMPLE BERNOULLI (10); ``` Here, `(10)` indicates that approximately 10% of the rows will be sampled randomly based on row-level selection probability[^1]. Alternatively, system-based sampling operates differently by selecting blocks rather than individual records: ```sql SELECT * FROM employees TABLESAMPE SYSTEM (5); ``` The above command selects around 5% of disk pages containing data tuples, which may lead to more efficient performance but less precise representation compared with Bernoulli sampling[^2]. It's important to note that support for these features began officially after version 9.5 as part of ongoing enhancements within PostgreSQL development efforts[^3]. When working with large datasets where full scans might impact query execution times significantly, leveraging `TABLESAMPLE` provides a practical approach towards obtaining representative subsets quickly while reducing resource consumption. --related questions-- 1. How does block-level versus record-level sampling affect result accuracy? 2. What versions of PostgreSQL introduced official support for TABLESAMPLE functionality? 3. Can you provide scenarios illustrating optimal use cases for each type of sampling method?
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