Question

Possible Duplicate:
Dynamically-generated table-name in PostgreSQL COPY command

I am using the copy to file function in postgresql, which works fine.

Current code looks like this

COPY table 
TO 'filename.txt'

This works fine, but I would like to add date to the filename.

So I tried

COPY table 
TO 'filename' || CURRENT_DATE || '.txt'

without success.

I have also tried putting it in a query

COPY table 
TO (select 'filename' || CURRENT_DATE || '.txt')

Anyone willing to help me out?

EDIT

I think the problem is, that the function COPY TO expects a string, as the destination. And I put in a query, returning a resultset…

Answer 1

As you suspected, the problem is that PostgreSQL is that expecting a string-literal filename, in the form 'filename' or E'filename', rather than an expression (such as a subquery) that would generate a filename as varchar or text or whatnot. So far as I’m aware, the only work-around is to dynamically generate the entire COPY statement, and run it in PL/pgSQL using the EXECUTE command. You can create a function like this:

CREATE OR REPLACE FUNCTION copy_table_to_file(filename TEXT) RETURNS VOID AS
$$
  BEGIN
    EXECUTE 'COPY table TO ''' || filename || '''';
  END;
$$ LANGUAGE 'plpgsql';

and then invoke it like this:

SELECT copy_table_to_file('filename' || CURRENT_DATE || '.txt');

A few notes:

• If desired, you can make the table-name be a parameter as well.
• As you can see from the above-linked documentation, the EXECUTE command supports a USING clause to pass in data; but I’ve just tested, and it seems that something like EXECUTE 'COPY table TO $1' USING filename; does not work. I assume that this is for more or less the same reason as your original question: the filename has to be specified as a string-literal, not as a substituted parameter. So, be careful with the filename you pass into the function, and make sure it never contains any single-quotes or backslashes or whatnot, or else you’ll accidentally perform a SQL injection attack on yourself.
• You may be interested in this previous, related question: Dynamically-generated table-name in PostgreSQL COPY command