postgresql - Use text output from a function as new query

Question

In continuing from a previous case that was assisted by @Erwin Brandstetter and @Craig Ringer, I have fixed my code to become as follows. Note, that my function myresult() outputs now text, and not a table (as indeed, as was pointed out in the former case, there is no point in outputting a table object, since we would need to define all its columns ahead, which basically defies the entire purpose):

CREATE OR REPLACE FUNCTION myresult(mytable text, myprefix text)
RETURNS text AS 
$func$
DECLARE
   myoneliner text;
BEGIN
   SELECT INTO myoneliner  
          'SELECT '
        || string_agg(quote_ident(column_name::text), ',' ORDER BY column_name)
        || ' FROM ' || quote_ident(mytable)
   FROM   information_schema.columns
   WHERE  table_name = mytable
   AND    column_name LIKE myprefix||'%'
   AND    table_schema = 'public';  -- schema name; might be another param

   RAISE NOTICE 'My additional text: %', myoneliner;
   RETURN myoneliner;
END
$func$ LANGUAGE plpgsql;

Call:

select myresult('dkj_p_k27ac','enri');   

Upon running the above procedure I get a text string, which is basically a query. I'll refer to it up next as 'oneliner-output', just for simplicity.
The 'oneline-output' looks as follows (I just copy/paste it from the one output cell that I've got into here):

"SELECT enrich_d_dkj_p_k27ac,enrich_lr_dkj_p_k27ac,enrich_r_dkj_p_k27ac FROM dkj_p_k27ac"

• Note that the double quotes from both sides of the statement were part of the myresult() output. I didn't add them.

I understand much better now the problematic idea of thinking to construct a single function that would both create the 'oneliner-output' AND execute it. I am able to copy/paste the 'oneliner-output' into a new Postgres query window and execute it as a normal query just fine, receiving the desired columns and rows in my Data Output window.
I would like, however, to automate this step, so to avoid the copy/paste step. Is there a way in Postgres to use the text output (the 'oneliner-output') that I receive from myresult() function, and execute it? Can a second function be created that would receive the output of myresult() and use it for executing a query?

Along these lines, while I know that the following scripting (here below) works and actually outputs exactly the desired columns and rows:

-- DEALLOCATE stmt1; -- use this line after the first time 'stmt1' was created
prepare stmt1 as SELECT enrich_d_dkj_p_k27ac,enrich_lr_dkj_p_k27ac,enrich_r_dkj_p_k27ac FROM dkj_p_k27ac;
execute stmt1;

• I was thinking maybe something like the following scripting could potentially work, after doing the correct tweaking? Not sure how though.

  prepare stmt1 as THE_OUTPUT_OF_myresult();
  execute stmt1;
  

Attempt with a refcursor

CREATE OR REPLACE FUNCTION show_mytable(ref refcursor) RETURNS refcursor AS $$
BEGIN
   OPEN ref FOR SELECT enrich_d_dkj_p_k27ac,enrich_lr_dkj_p_k27ac,enrich_r_dkj_p_k27ac FROM dkj_p_k27ac;   -- Open a cursor 
   RETURN ref;    -- Return the cursor to the caller
END;
$$ LANGUAGE plpgsql;

Call:

BEGIN;
SELECT show_mytable('roy');
FETCH ALL IN "roy"; 

This procedure, actually works and spits out the desired columns and rows, and yet again, I have to provide the exact SELECT statement.

I basically would like to be able and provide it instead as the output of my myresult() function. Something like this:

CREATE OR REPLACE FUNCTION show_mytable(ref refcursor) RETURNS refcursor AS $$
BEGIN
   OPEN ref FOR myresult();   -- Open a cursor 
   RETURN ref;    -- Return the cursor to the caller
END;
$$ LANGUAGE plpgsql;

Call:

BEGIN;
SELECT show_mytable('roy');
FETCH ALL IN "roy"; 

Answer 1

The trick with PREPARE doesn't work, since it does not take a * text string* (a value) like CREATE FUNCTION does, but a valid statement (code).

To convert data into executable code you need to use dynamic SQL, i.e. EXECUTE in a plpgsql function or DO statement. This works without problem as long as the return type does not depend on the outcome of the first function myresult(). Else you are back to catch 22 as outlined in my previous answer:

• How to execute a string result of a stored procedure in postgres

The crucial part is to declare the return type (row type in this case) somehow. You can create a TABLE, TEMP TABLE or TYPE for the purpose. Or you can use a prepared statement or a refcursor.

Solution with prepared statement

You have been very close. The missing piece of the puzzle is to prepare the generated query with dynamic SQL.

Function to prepare statement dynamically

Create this function once. It's a optimized and safe version of your function myresult():

CREATE OR REPLACE FUNCTION f_prep_query (_tbl regclass, _prefix text)
  RETURNS void AS 
$func$
BEGIN
   IF EXISTS (SELECT 1 FROM pg_prepared_statements WHERE name = 'stmt_dyn') THEN
      DEALLOCATE stmt_dyn;
   END IF;                 -- you my or may not need this safety check 

   EXECUTE (
     SELECT 'PREPARE stmt_dyn AS SELECT '
         || string_agg(quote_ident(attname), ',' ORDER BY attname)
         || ' FROM ' || _tbl
      FROM   pg_catalog.pg_attribute
      WHERE  attrelid = _tbl
      AND    attname LIKE _prefix || '%'
      AND    attnum > 0
      AND    NOT attisdropped
     );
END
$func$  LANGUAGE plpgsql;

I use regclass for the table name parameter _tbl to make it unambiguous and safe against SQLi. Details:

• Table name as a PostgreSQL function parameter

The information schema does not include the oid column of system catalogs, so I switched to pg_catalog.pg_attribute instead of information_schema.columns. That's faster, too. There are pros and cons for this:

• How to check if a table exists in a given schema

If a prepared statement with the name stmt_dyn already existed, PREPARE would raise an exception. If that is acceptable, remove the check on the system view pg_prepared_statements and the following DEALLOCATE.
_{More sophisticated algorithms are possible to manage multiple prepared statements per session, or take the name of the prepared statement as additional parameter, or even use an MD5 hash of the query string as name, but that's beyond the scope of this question.}

Be aware that PREPARE operates outside the scope of transactions, once PREPARE succeeds, the prepared statement exists for the lifetime of the session. If the wrapping transaction is aborted, PREPARE is unaffected. ROLLBACK cannot remove prepared statements.

Dynamic query execution

Two queries, but only one call to the server. And very efficient, too.

SELECT f_prep_query('tbl'::regclass, 'pre'::text);
EXECUTE stmt_dyn;

Simpler and much more efficient for most simple use cases than creating a temp table or a cursor and selecting / fetching from that (which would be other options).

SQL Fiddle.