Order of returned rows with IN statement

WITH ORDINALITY in Postgres 9.4+

Introduced with Postgres 9.4. The manual:

When a function in the FROM clause is suffixed by WITH ORDINALITY, a bigint column is appended to the output which starts from 1 and increments by 1 for each row of the function's output. This is most useful in the case of set returning functions such as unnest().

SELECT u.*
FROM   unnest('{13589, 16674, 13588}'::int[]) WITH ORDINALITY AS x(id, order_nr)
JOIN   users u USING (id)
ORDER  BY x.order_nr;

array or set?

x IN (set) statements are rewritten internally in Postgres to x = ANY (array), which is equivalent:

SELECT users.id FROM users WHERE users.id = ANY ('{13589, 16674, 13588}')

You can see for yourself with EXPLAIN.

Postgres 9.3 or earlier

For now, to preserve the order of elements, you could:

SELECT u.*
FROM  (
   SELECT arr, generate_subscripts(arr, 1) AS order_nr
   FROM  (SELECT '{13589, 16674, 13588}'::int[]) t(arr)
   ) x
JOIN   users u ON u.id = x.arr[x.order_nr]
ORDER  BY x.order_nr;

db<>fiddle here

Further reading:

  • How to preserve the original order of elements in an unnested array?

If you want to rewrite the condition slightly you could do something like this:

with id_list (id, sort_order) as (
   values 
      (13589, 1), 
      (16674, 2),
      (13588, 3)
)
select u.id
from users u
  join id_list il on il.id = u.id
order by il.sort_order;

You can also put the values clause directly into the join:

select u.id
from users u
  join (
   values 
      (13589, 1), 
      (16674, 2),
      (13588, 3)
  ) as il(id, sort_order) on il.id = u.id
order by il.sort_order;

Tags:

Postgresql

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