Dynamic SQL and RETURN type

^{(I saved the best for last, keep reading!)}
You want to execute dynamic SQL. In principal, that’s simple in plpgsql with the help of EXECUTE. You don’t need a cursor. In fact, most of the time you are better off without explicit cursors.

The problem you run into: you want to return records of yet undefined type. A function needs to declare its return type in the RETURNS clause (or with OUT or INOUT parameters). In your case you would have to fall back to anonymous records, because number, names and types of returned columns vary. Like:

CREATE FUNCTION data_of(integer)
  RETURNS SETOF record AS ...

However, this is not particularly useful. You have to provide a column definition list with every call. Like:

SELECT * FROM data_of(17)
AS foo (colum_name1 integer
      , colum_name2 text
      , colum_name3 real);

But how would you even do this, when you don’t know the columns beforehand?
You could use less structured document data types like json, jsonb, hstore or xml. See:

• How to store a data table in database?

But, for the purpose of this question, let’s assume you want to return individual, correctly typed and named columns as much as possible.

Simple solution with fixed return type

The column datahora seems to be a given, I’ll assume data type timestamp and that there are always two more columns with varying name and data type.

Names we’ll abandon in favor of generic names in the return type.
Types we’ll abandon, too, and cast all to text since every data type can be cast to text.

CREATE OR REPLACE FUNCTION data_of(_id integer)
  RETURNS TABLE (datahora timestamp, col2 text, col3 text)
  LANGUAGE plpgsql AS
$func$
DECLARE
   _sensors text := 'col1::text, col2::text';  -- cast each col to text
   _type    text := 'foo';
BEGIN
   RETURN QUERY EXECUTE '
      SELECT datahora, ' || _sensors || '
      FROM   ' || quote_ident(_type) || '
      WHERE  id = $1
      ORDER  BY datahora'
   USING  _id;
END
$func$;

The variables _sensors and _type could be input parameters instead.

Note the RETURNS TABLE clause.

Note the use of RETURN QUERY EXECUTE. That is one of the more elegant ways to return rows from a dynamic query.

I use a name for the function parameter, just to make the USING clause of RETURN QUERY EXECUTE less confusing. $1 in the SQL-string does not refer to the function parameter but to the value passed with the USING clause. (Both happen to be $1 in their respective scope in this simple example.)

Note the example value for _sensors: each column is cast to type text.

This kind of code is very vulnerable to SQL injection. I use quote_ident() to protect against it. Lumping together a couple of column names in the variable _sensors prevents the use of quote_ident() (and is typically a bad idea!). Ensure that no bad stuff can be in there some other way, for instance by individually running the column names through quote_ident() instead. A VARIADIC parameter comes to mind …

Simpler since PostgreSQL 9.1

With version 9.1 or later you can use format() to further simplify:

RETURN QUERY EXECUTE format('
   SELECT datahora, %s  -- identifier passed as unescaped string
   FROM   %I            -- assuming the name is provided by user
   WHERE  id = $1
   ORDER  BY datahora'
  ,_sensors, _type)
USING  _id;

Again, individual column names could be escaped properly and would be the clean way.

Variable number of columns sharing the same type

After your question updates it looks like your return type has

• a variable number of columns
• but all columns of the same type double precision (alias float8)

Use an ARRAY type in this case to nest a variable number of values. Additionally, I return an array with column names:

CREATE OR REPLACE FUNCTION data_of(_id integer)
  RETURNS TABLE (datahora timestamp, names text[], values float8[])
  LANGUAGE plpgsql AS
$func$
DECLARE
   _sensors text := 'col1, col2, col3';  -- plain list of column names
   _type    text := 'foo';
BEGIN
   RETURN QUERY EXECUTE format('
      SELECT datahora
           , string_to_array($1)  -- AS names
           , ARRAY[%s]            -- AS values
      FROM   %s
      WHERE  id = $2
      ORDER  BY datahora'
    , _sensors, _type)
   USING  _sensors, _id;
END
$func$;

Various complete table types

To actually return all columns of a table, there is a simple, powerful solution using a polymorphic type:

CREATE OR REPLACE FUNCTION data_of(_tbl_type anyelement, _id int)
  RETURNS SETOF anyelement
  LANGUAGE plpgsql AS
$func$
BEGIN
   RETURN QUERY EXECUTE format('
      SELECT *
      FROM   %s  -- pg_typeof returns regtype, quoted automatically
      WHERE  id = $1
      ORDER  BY datahora'
    , pg_typeof(_tbl_type))
   USING  _id;
END
$func$;

Call (important!):

SELECT * FROM data_of(NULL::pcdmet, 17);

Replace pcdmet in the call with any other table name.

How does this work?

anyelement is a pseudo data type, a polymorphic type, a placeholder for any non-array data type. All occurrences of anyelement in the function evaluate to the same type provided at run time. By supplying a value of a defined type as argument to the function, we implicitly define the return type.

PostgreSQL automatically defines a row type (a composite data type) for every table created, so there is a well defined type for every table. This includes temporary tables, which is convenient for ad-hoc use.

Any type can be NULL. Hand in a NULL value, cast to the table type: NULL::pcdmet.

Now the function returns a well-defined row type and we can use SELECT * FROM data_of() to decompose the row and get individual columns.

pg_typeof(_tbl_type) returns the name of the table as object identifier type regtype. When automatically converted to text, identifiers are automatically double-quoted and schema-qualified if needed, defending against SQL injection automatically. This can even deal with schema-qualified table-names where quote_ident() would fail. See:

• Table name as a PostgreSQL function parameter