plpgsql，如何将变量存储到文件中

Question

I want create a custom variable (string) that contain data from different tables. I can't do it in one query (special order to store, and dynamic columns).

Is there a way to store the variable into file?

Or using command line to save the result of one variable into file?

Answer 1

Because of tag plpgsql I assume that you are executing some queries inside a plpgsql function and the result saved in a variable should be send to a file.

In plain SQL you can save a text in a file using a copy command, something like this:

copy (select 'some text') to 'c:\data\sample.txt';

In a plpgsql function you should use execute command to select a value of a variable, example:

create or replace function copy_some_text_to_file(filename text)
returns void language plpgsql
as $$
declare
    var text:= 'some text';
begin
    execute format($fmt$
        copy (select '%s') to '%s'
        $fmt$, var, filename);
end $$;

select copy_some_text_to_file('c:\data\sample.txt')

If you want to save to file a variable with multiline string, you can use regexp_split_to_table() :

create or replace function copy_some_text_to_file(filename text)
returns void language plpgsql
as $$
declare
    var text:= 'first line\nsecond line\nthird line';
begin
    execute format($fmt$
        copy (select regexp_split_to_table(e'%s', e'\n')) to '%s'
        $fmt$, var, filename);
end $$;

Note, that the string cannot contain a single quotation mark, which must be properly escaped. While in the first version you can replace '%s' with %L (as Erwin suggested), you cannot do that in the second version.

Read more:

• COPY command
• Executing Dynamic Commands

Answer 2

Executing SQL from the Linux shell you can easily redirect the result to a file:

postgres@db:~$ psql mydb --tuples-only -c 'select 1' > /path/to/myfile.txt

Or execute SQL for a file:

postgres@db:~$ psql mydb -t -f /path/to/myscript.sql > /path/to/myfile.txt

Or from within psql:

\o /path/to/myfile.txt
\t
SELECT 1;
\o

Everything is explained in the manual for psql .