Return rows of a table that actually changed in an UPDATE

Only update rows that actually change

That saves expensive updates and expensive checks after the UPDATE.

To update every column with the new value provided (if anything changes):

UPDATE accounts a
SET   (status,   field1,   field2)  -- short syntax for  ..
  = (m.status, m.field1, m.field2)  -- .. updating multiple columns
FROM   merge_accounts m
WHERE  m.uid = a.uid
AND   (a.status IS DISTINCT FROM m.status OR
       a.field1 IS DISTINCT FROM m.field1 OR 
       a.field2 IS DISTINCT FROM m.field2)
RETURNING a.*;

Due to PostgreSQL's MVCC model any change to a row writes a new row version. Updating a single column is almost as expensive as updating every column in the row at once. Rewriting the rest of the row comes at practically no cost, as soon as you have to update anything.

Details:

• How do I (or can I) SELECT DISTINCT on multiple columns?
• UPDATE a whole row in PL/pgSQL

Shorthand for whole rows

If the row types of accounts and merge_accounts are identical and you want to adopt everything from merge_accounts into accounts, there is a shortcut comparing the whole row type:

UPDATE accounts a
SET   (status,   field1,   field2)
  = (m.status, m.field1, m.field2)
FROM   merge_accounts m
WHERE  a.uid = m.uid
AND    m IS DISTINCT FROM a
RETURNING a.*;

This even works for NULL values. Details in the manual.
But it's not going to work for your home-grown solution where (quoting your comment):

merge_accounts is identical, save that all non-pk columns are array types

It requires compatible row types, i.e. each column shares the same data type or there is at least an implicit cast between the two types.

For your special case

UPDATE accounts a
SET   (status, field1, field2)
    = (COALESCE(m.status[1], a.status)  -- default to original ..
     , COALESCE(m.field1[1], a.field1)   -- .. if m.column[1] IS NULL
     , COALESCE(m.field2[1], a.field2))
FROM   merge_accounts m
WHERE  m.uid = a.uid
AND   (m.status[1] IS NOT NULL AND a.status IS DISTINCT FROM m.status[1]
    OR m.field1[1] IS NOT NULL AND a.field1 IS DISTINCT FROM m.field1[1]
    OR m.field2[1] IS NOT NULL AND a.field2 IS DISTINCT FROM m.field2[1])
RETURNING a.*

m.status IS NOT NULL works if columns that shouldn't be updated are NULL in merge_accounts.
m.status <> '{}' if you operate with empty arrays.
m.status[1] IS NOT NULL covers both options.

Related:

• Return pre-UPDATE column values using SQL only

if you aren't relying on side-effectts of the update, only update the records that need to change

UPDATE accounts
SET status = merge_accounts.status,
    field1 = merge_accounts.field1,
    field2 = merge_accounts.field2,
    etc.
FROM merge_accounts WHERE merge_accounts.uid =accounts.uid
 AND NOT (status IS NOT DISTINCT FROM merge_accounts.status 
      AND field1 IS NOT DISTINCT FROM merge_accounts.field1 
      AND field2 IS NOT DISTINCT FROM merge_accounts.field2
      )
RETURNING accounts.*