Why Does My PostgreSQL Select Query Order Change After Updating Data?

Unpredictable Row Order in PostgreSQL Select Queries After Data Modification

When executing a SELECT query without specifying an ORDER BY clause in PostgreSQL, the resulting rows are typically retrieved in the order they are stored physically on disk. However, this default ordering can change unexpectedly after performing data modifications such as updates or inserts.

Consider the following example:

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 updating row 5, the rows were returned in ascending order by id. However, after the update operation, the position of row 5 changed, resulting in a different ordering.

Explanation:

This behavior is due to PostgreSQL's internal mechanisms for handling data storage and retrieval.

• Physical Row Storage: Rows are physically stored on disk pages. When a row is updated, a new version of the row is written to a new page.
• Page Access: When PostgreSQL reads rows from a table, it typically retrieves pages from disk in a specific order. This order may vary based on factors such as index usage and page cache utilization.
• Default Ordering: Without an ORDER BY clause, PostgreSQL retrieves rows from pages in the order they are stored on those pages. Since rows are physically stored in the order of updates, the default ordering can change after data modifications.

Conclusion:

It's important to note that relying on the default ordering behavior in PostgreSQL can lead to unexpected results. To ensure consistent row ordering, always specify an explicit ORDER BY clause in your SELECT queries. Alternatively, if you need the row order to be consistent after data modifications, you can create an index on the desired sorting column.

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