Select your classic report and then go to Attributes > Heading. Then choose the heading type as PL/SQL Function body and put the following code in it:

DECLARE
c NUMBER;
d NUMBER;
col_cnt INTEGER;
f BOOLEAN;
rec_tab DBMS_SQL.DESC_TAB;
col_num NUMBER;
v_columns varchar2(1000);
PROCEDURE print_rec(rec in DBMS_SQL.DESC_REC) IS
BEGIN
DBMS_OUTPUT.NEW_LINE;
DBMS_OUTPUT.PUT_LINE('col_type = '
|| rec.col_type);
DBMS_OUTPUT.PUT_LINE('col_maxlen = '
|| rec.col_max_len);
DBMS_OUTPUT.PUT_LINE('col_name = '
|| rec.col_name);
DBMS_OUTPUT.PUT_LINE('col_name_len = '
|| rec.col_name_len);
DBMS_OUTPUT.PUT_LINE('col_schema_name = '
|| rec.col_schema_name);
DBMS_OUTPUT.PUT_LINE('col_schema_name_len = '
|| rec.col_schema_name_len);
DBMS_OUTPUT.PUT_LINE('col_precision = '
|| rec.col_precision);
DBMS_OUTPUT.PUT_LINE('col_scale = '
|| rec.col_scale);
DBMS_OUTPUT.PUT('col_null_ok = ');
IF (rec.col_null_ok) THEN
DBMS_OUTPUT.PUT_LINE('true');
ELSE
DBMS_OUTPUT.PUT_LINE('false');
END IF;
END;
BEGIN
c := DBMS_SQL.OPEN_CURSOR;

DBMS_SQL.PARSE(c, nvl(:P17_QUERY, 'select null from dual where 1 = 2'), DBMS_SQL.NATIVE);

d := DBMS_SQL.EXECUTE(c);

DBMS_SQL.DESCRIBE_COLUMNS(c, col_cnt, rec_tab);

col_num := rec_tab.first;
IF (col_num IS NOT NULL) THEN
LOOP
v_columns := v_columns ||':'|| (rec_tab(col_num).col_name);
col_num := rec_tab.next(col_num);
EXIT WHEN (col_num IS NULL);
END LOOP;
END IF;

DBMS_SQL.CLOSE_CURSOR(c);
v_columns := ltrim(v_columns, ':');
return v_columns;
END;

Here is the app example: SQL Command.

Dynamic actions will be executed in the sequence they created. But executing JavaScript code is a client-side action, and the PL/SQL one is server-side. DA will execute the JS first and then will immediately execute the PL/SQL. So both will execute at almost the same time.

To execute PL/SQL code after JS, you can use the following trick:

From JS code, set a value of a hidden item (create a new hidden item) and write a DA to execute PL/SQL code on the change value of that hidden item.

Using this method, the JS code will execute first, and when that code sets a value of a hidden item, then only PL/SQL code will run.