Why Does PostgreSQL Return Unordered Data After Row Updates?

Why Does PostgreSQL Return Unordered Data After Row Updates?

When querying a database without specifying an explicit ordering, it's common to expect the results to be returned in a consistent order. However, in PostgreSQL, updates to rows can unexpectedly alter the order of the results.

In the example provided:

postgres=# select * from check_user;
 id | name
----+------
  1 | x
  2 | y
  3 | z
  4 | a
  5 | c1\
  6 | c2
  7 | c3
(7 rows)

postgres=# update check_user set name = 'c1' where name = 'c1\';
UPDATE 1
postgres=# select * from check_user;
 id | name
----+------
  1 | x
  2 | y
  3 | z
  4 | a
  6 | c2
  7 | c3
  5 | c1
(7 rows)

Before the update, the rows were ordered by ID. However, after updating the row with the escaped name, the order of the results changed. This can be confusing and may lead to unexpected results.

Default Ordering in PostgreSQL

When an ORDER BY clause is not specified in a SELECT query, PostgreSQL uses an undefined internal ordering for the returned rows. This ordering is based on factors such as the order rows are stored on disk, page read order, and index usage.

Importance of Explicit Ordering

It's essential to avoid relying on the default ordering of PostgreSQL results. This behavior is unpredictable and can change based on various factors. For this reason, always specify an ORDER BY clause in your queries to ensure consistent and predictable result ordering.

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