PL/pgSQL to copy data from tables dynamically - postgresql

Question

I'm trying to write some SQL to copy data from all of my PostgreSQL tables in a given database based on what's in the information_schema. It should output data files to my local machine ready for import to another machine. Ultimately, I'm going to tweak this so that I dump only select portions of tables (some of the tables I'm dumping have millions of records and I only want a small subset of data for testing purposes).
Here's what I have so far...
--Copy all tables...
DO
$$
DECLARE
formatstring text;
rec record;
BEGIN
RAISE NOTICE 'Copying tables...';
formatstring = 'COPY (select * from %I) to ''C:\Media\Code\%s.csv'';';
FOR rec IN
select table_name from information_schema.tables where table_schema = 'public' order by table_name
LOOP
RAISE NOTICE 'Table: %', rec.table_name;
RAISE NOTICE format(formatstring, rec.table_name, rec.table_name);
EXECUTE format(formatstring, rec.table_name, rec.table_name);
END LOOP;
END;
$$
LANGUAGE plpgsql;
However, I am getting this exception...
ERROR: unrecognized exception condition "format"
CONTEXT: compilation of PL/pgSQL function "inline_code_block" near line 12
********** Error **********
ERROR: unrecognized exception condition "format"
SQL state: 42704
Context: compilation of PL/pgSQL function "inline_code_block" near line 12
The escaping of the single quotes seems fine (already checked this question: Insert text with single quotes in PostgreSQL). Indeed I can do the following and it works, with text being inserted into the formatting:
select format('COPY (select * from %I) to ''C:\Media\Code\%s.csv'';', 'system_user', 'system_user');
Can anyone assist with this issue? I can easily write a script or code that will generate the copy commands for me, but it would be great to do it all within a simple bit of SQL.

Answer 1

The cause is a syntax error in your 3nd RAISE statement. There are several valid formats, but you cannot feed an expression to RAISE directly. It has to be a string literal - with the option of string interpolation.
While being at it, I would simplify a couple of other things, too:
DO
$do$
DECLARE
_formatstring text := $$COPY %1$I TO 'C:\Media\Code\%1$s.csv'$$;
_sql text;
_tbl text;
BEGIN
RAISE NOTICE 'Copying tables...';
FOR _tbl IN
SELECT table_name
FROM information_schema.tables
WHERE table_schema = 'public'
ORDER BY table_name
LOOP
_sql := format(_formatstring, _tbl);
RAISE NOTICE 'Table: %', _tbl;
RAISE NOTICE '%', _sql; -- fixed!!
EXECUTE _sql;
END LOOP;
END
$do$ LANGUAGE plpgsql;
Major points
The plain table name with COPY instead of SELECT * FROM tbl.
The use of nested dollar-quotes.
Format specifiers %1$I and %1$s for the format() function, so we only need to supply the table name once.
You can assign variables at declaration time.
The scalar variable instead of a record in the FOR loop - we only need the one column anyway.