INSERT INTO ... FROM SELECT ... RETURNING id mappings

This would be simpler for UPDATE, where additional rows joined into the update are visible to the RETURNING clause:

• Return pre-UPDATE Column Values Using SQL Only - PostgreSQL Version

The same is currently not possible for INSERT. Per documentation:

The expression can use any column names of the table named by table_name

table_name being the target of the INSERT command.

You can use (data-modifying) CTEs to get this to work.
Assuming title to be unique per query, else you need to do more:

WITH sel AS (   SELECT id, title   FROM   posts   WHERE  id IN (1,2)   -- select rows to copy   ),    ins AS (   INSERT INTO posts (title)   SELECT title FROM sel   RETURNING id, title )SELECT ins.id, sel.id AS from_idFROM   insJOIN   sel USING (title);

If title is not unique per query (but at least id is unique per table):

WITH sel AS (   SELECT id, title, row_number() OVER (ORDER BY id) AS rn   FROM   posts   WHERE  id IN (1,2)   -- select rows to copy   ORDER  BY id   ),    ins AS (   INSERT INTO posts (title)   SELECT title FROM sel ORDER  BY id  -- ORDER redundant to be sure   RETURNING id )SELECT i.id, s.id AS from_idFROM  (SELECT id, row_number() OVER (ORDER BY id) AS rn FROM ins) iJOIN   sel s USING (rn);

This second query relies on the undocumented implementation detail that rows are inserted in the order provided. It works in all current versions of Postgres and is probably not going to break.

SQL Fiddle.

if id column of posts is serial type, it's generated like nextval('posts_id_seq'::regclass),you can manually call this function for every new row

withsel as (  SELECT id, title, nextval('posts_id_seq'::regclass) new_id  FROM   posts  WHERE  id IN (1,2)),ins as (  INSERT INTO posts (id, title)  SELECT new_id, title  FROM sel)SELECT id, new_idFROM sel

it'l works with any data, include non-unique title

The simplest solution IMHO would be to simply add a column to your table where you could put id of the row that was cloned.