Why Does a Postgres SELECT Query Return Rows in an Unforeseen Order After a Row Update?

Unforeseen Row Ordering in Postgres SELECT Queries After Row Updates

Postgres is a widely used relational database management system, which often raises questions regarding the default behavior of its operations. In this scenario, the inquiry pertains to the unexpected ordering of rows in the result of a SELECT query following an update operation on a row.

When performing a SELECT query without explicitly specifying an ORDER BY clause, Postgres retrieves rows from the database in an arbitrary order. This order primarily depends on the physical storage and retrieval patterns of the database. To illustrate this concept, 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)

In the above table, the rows are initially ordered by their id values. However, after updating a row with the same name as another row:

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)

The order of the rows has changed, with the updated row now appearing at the bottom of the result. This is because Postgres typically does not update rows in place but instead marks them as deleted and inserts new rows.

Consequently, when subsequent SELECT queries are executed, Postgres retrieves rows from the fastest available source, which may or may not align with the original order. To ensure predictable ordering, it is imperative to explicitly specify an ORDER BY clause in SELECT queries.

In summary, Postgres does not maintain a pre-defined default ordering for rows in result sets unless explicitly instructed via an ORDER BY clause. Instead, it retrieves rows based on internal storage and retrieval patterns, which can result in unordered results after row updates. It is crucial to rely on explicit ordering mechanisms to ensure consistent ordering in SELECT queries.

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