在psql中，为什么有些命令没有效果? [英] In psql, why do some commands have no effect?

问题描述

有时我在 psql 中的命令似乎没有效果.知道为什么吗?

Sometimes my commands in psql seem to be having no effect. Any idea why?

以下是数据库library_development中所有表的列表:

The below is the list of all tables in the database library_development:

library_development=> d

               List of relations
 Schema |       Name        | Type  |  Owner
--------+-------------------+-------+----------
 public | Pavan             | table | postgres
 public | schema_migrations | table | sai
(2 rows)

在此之后，我使用删除了表 Pavan:

library_development-> drop table Pavan

但表格没有被删除，其显示如图所示:

library_development=> d
               List of relations
 Schema |       Name        | Type  |  Owner
--------+-------------------+-------+----------
 public | Pavan             | table | postgres
 public | schema_migrations | table | sai
(2 rows)

还有:

1. 我在 Windows 中使用 PostgreSQL.是否有任何命令可以清除控制台(如 Oracle 中的 cl scr)?

1. I am using PostgreSQL in Windows. Is there any command to clear the console (Like cl scr present in Oracle)?

在使用 DML 脚本时，我需要在 Postgresql 中执行提交"的概念吗?

Is there any concept of a "commit" I need to perform in Postgresql when working with DML scripts?

推荐答案

语句以分号结尾.

在 psql 中，按下不带分号的 Enter 将语句继续到下一行，将您写入的内容添加到查询缓冲区中，而不是执行它.您会注意到提示从 dbname=> 更改为 dbname-> 以指示您在续行上.

In psql, pressing enter without a semicolon continues the statement onto the next line, adding what you wrote to the query buffer rather than executing it. You will notice that the prompt changes from dbname=> to dbname-> to indicate that you're on a continuation line.

regress=> DROP TABLE sometable
regress-> 
Query buffer reset (cleared).
regress=> DROP TABLE sometable;
ERROR:  table "sometable" does not exist
regress=> 

注意我在没有分号的情况下按 Enter 后，提示更改为 regress-# 并且不执行任何操作.没有表sometable，所以如果语句运行了会报错.

Notice how after I press enter without a semicolon, the prompt changes to regress-# and no action is taken. There is no table sometable, so if the statement had run an error would be reported.

接下来，看下一行 的用法?这会清除查询缓冲区.请注意，当缓冲区被清除时，提示会变回 regress=#，因为不再缓冲部分语句.

Next, see the use of on the next line? That clears the query buffer. Notice that the prompt changes back to regress=# when the buffer is cleared, as there's no partial statement buffered anymore.

这显示了如何跨行拆分语句:

This shows how statements can be split across lines:

regress=> DROP TABLE
regress-> sometable
regress-> ;
ERROR:  table "sometable" does not exist

令人困惑的是，像 d 这样的 psql 反斜杠命令是以换行符结束的，而不是以分号结束的，所以它们确实在您运行时运行按回车键.当您想(比如说)在编写语句时查看表定义时，这很方便，但对新手来说有点混乱.

The confusing thing is that psql backslash commands like d are newline-terminated, not semicolon terminated, so they do run when you press enter. That's handy when you want to (say) view a table definition while writing a statement, but it's a bit confusing for newcomers.

至于您的其他问题:

1. 如果 psql 中有一个用于 Windows 的清除屏幕"命令，我还没有找到.在 Linux 上，我只使用 control-L，与任何其他使用 readline 的程序相同.在 Windows <代码>!cls 会起作用.

1. If there's a "clear screen" command in psql for Windows I haven't found it yet. On Linux I just use control-L, same as any other readline-using program. In Windows ! cls will work.

PostgreSQL 中的 DDL 是事务性的.你可以BEGIN一个事务，发出一些DDL，然后COMMIT事务让它生效.如果您不在显式事务中执行 DDL，则它会立即生效.

DDL in PostgreSQL is transactional. You can BEGIN a transaction, issue some DDL, and COMMIT the transaction to have it take effect. If you don't do your DDL in an explicit transaction then it takes effect immediately.

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