CODEWITHSUNDEEP
sqlpostgresqlsql-order-byunion
qed
qed

Reputation: 23104

Is order preserved after UNION in PostgreSQL?

Here is the code:

CREATE TABLE audit_trail (
      old_email TEXT NOT NULL,
      new_email TEXT NOT NULL
);

INSERT INTO audit_trail(old_email, new_email)
  VALUES ('[email protected]', '[email protected]'),
         ('[email protected]', '[email protected]'),
         ('[email protected]', '[email protected]'),
         ('[email protected]', '[email protected]'),
         ('[email protected]', '[email protected]');


WITH RECURSIVE all_emails AS (
  SELECT  old_email, new_email
    FROM audit_trail
    WHERE old_email = '[email protected]'
  UNION
  SELECT at.old_email, at.new_email
    FROM audit_trail at
    JOIN all_emails a
      ON (at.old_email = a.new_email)
)
SELECT * FROM all_emails;

        old_email         |        new_email
--------------------------+--------------------------
 [email protected]     | [email protected]
 [email protected]     | [email protected]
 [email protected] | [email protected]
(3 rows)

select old_email, new_email into iter1
from audit_trail where old_email = '[email protected]';
select * from iter1;
--       old_email       |      new_email
-- ----------------------+----------------------
--  [email protected] | [email protected]
-- (1 row)

select a.old_email, a.new_email into iter2
from audit_trail a join iter1 b on (a.old_email = b.new_email);
select * from iter2;
--       old_email       |        new_email
-- ----------------------+--------------------------
--  [email protected] | [email protected]
-- (1 row)

select * from iter1 union select * from iter2;
--       old_email       |        new_email
-- ----------------------+--------------------------
--  [email protected] | [email protected]
--  [email protected] | [email protected]
-- (2 rows)

As you can see the recursive code gives the result in right order, but the non-recursive code does not.
They both use union, why the difference?

Upvotes: 12

Views: 8327

Answers (3)

Erwin Brandstetter
Erwin Brandstetter

Reputation: 656521

Basically, your query is incorrect to begin with. Use UNION ALL, not UNION or you would incorrectly remove duplicate entries. (There is nothing to say the trail cannot switch back and forth between the same emails.) And UNION is highly likely to reorder rows.

The Postgres implementation for UNION ALL typically returns values in the sequence as appended - as long as you do not add ORDER BY at the end or do anything else with the result. But there is no formal guarantee, and since the advent of Parallel Append plans in Postgres 11, this can actually break. See this related post:

Be aware though, that each SELECT returns rows in arbitrary order unless ORDER BY is appended. There is no natural order in tables.

So this usually works:

SELECT * FROM iter1
UNION ALL  -- union all!
SELECT * FROM iter2;

To get a reliable sort order, and "simulate the record of growth", you can track levels like this:

WITH RECURSIVE all_emails AS (
   SELECT  *, 1 AS lvl
   FROM    audit_trail
   WHERE   old_email = '[email protected]'

   UNION ALL  -- union all!
   SELECT t.*, a.lvl + 1
   FROM   all_emails  a
   JOIN   audit_trail t ON t.old_email = a.new_email
)
TABLE  all_emails
ORDER  BY lvl;

db<>fiddle here
Old sqlfiddle

Aside: if old_email is not defined UNIQUE in some way, you can get multiple trails. You would need a unique column (or combination of columns) to keep it unambiguous. If all else fails you can (ab-)use the internal tuple ID ctid for the purpose of telling trails apart. But you should rather use your own columns. (Added example in the fiddle.)

Consider:

Upvotes: 24

Rohil Patel
Rohil Patel

Reputation: 468

Order is preserved if one can pass after all unions statement as below:

select "ClassName","SectionName","Students","OrderNo" from table
UNION 
select '----TOTAL----' as "ClassName",'----' as "SectionName",sum("Total Students"),9999 as "OrderNo" from table
ORDER BY "OrderNo"

Upvotes: -2

Gordon Linoff
Gordon Linoff

Reputation: 1269643

Ordering is never preserved after any operation in any reasonable database. If you want the result set in a particular order, use ORDER BY. Period.

This is especially true after a UNION. UNION removes duplicates and that operation is going to change the ordering of the rows, in all likelihood.

Upvotes: 5

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