Sometimes it is useful to obtain data from modified rows while they are
   being manipulated.  The INSERT, UPDATE,
   DELETE, and MERGE commands all have an
   optional RETURNING clause that supports this.  Use
   of RETURNING avoids performing an extra database query to
   collect the data, and is especially valuable when it would otherwise be
   difficult to identify the modified rows reliably.
  
   The allowed contents of a RETURNING clause are the same as
   a SELECT command's output list
   (see Section 7.3).  It can contain column
   names of the command's target table, or value expressions using those
   columns.  A common shorthand is RETURNING *, which selects
   all columns of the target table in order.
  
   In an INSERT, the data available to RETURNING is
   the row as it was inserted.  This is not so useful in trivial inserts,
   since it would just repeat the data provided by the client.  But it can
   be very handy when relying on computed default values.  For example,
   when using a serial
   column to provide unique identifiers, RETURNING can return
   the ID assigned to a new row:
CREATE TABLE users (firstname text, lastname text, id serial primary key);
INSERT INTO users (firstname, lastname) VALUES ('Joe', 'Cool') RETURNING id;
   The RETURNING clause is also very useful
   with INSERT ... SELECT.
  
   In an UPDATE, the data available to RETURNING is
   the new content of the modified row.  For example:
UPDATE products SET price = price * 1.10 WHERE price <= 99.99 RETURNING name, price AS new_price;
   In a DELETE, the data available to RETURNING is
   the content of the deleted row.  For example:
DELETE FROM products WHERE obsoletion_date = 'today' RETURNING *;
   In a MERGE, the data available to RETURNING is
   the content of the source row plus the content of the inserted, updated, or
   deleted target row.  Since it is quite common for the source and target to
   have many of the same columns, specifying RETURNING *
   can lead to a lot of duplicated columns, so it is often more useful to
   qualify it so as to return just the source or target row.  For example:
MERGE INTO products p USING new_products n ON p.product_no = n.product_no WHEN NOT MATCHED THEN INSERT VALUES (n.product_no, n.name, n.price) WHEN MATCHED THEN UPDATE SET name = n.name, price = n.price RETURNING p.*;
   If there are triggers (Chapter 37) on the target table,
   the data available to RETURNING is the row as modified by
   the triggers.  Thus, inspecting columns computed by triggers is another
   common use-case for RETURNING.