How Does PostgreSQL Order Rows in Select Queries Without an Explicit ORDER BY Clause?

Unveiling the Elusive Default Ordering in Postgres Select Queries

In the realm of database querying, PostgreSQL offers the robust functionality of returning data from tables. However, the manner in which this data is ordered, particularly when rows are updated, can sometimes be perplexing. Let's delve into understanding the default ordering behavior employed by PostgreSQL in select queries.

Consider a table named check_user with the following data:

 id | name
----+------
  1 | x
  2 | y
  3 | z
  4 | a
  5 | c1\
  6 | c2
  7 | c3

Upon executing a select query without specifying an explicit order, PostgreSQL returns the rows in a seemingly random order:

<code class="postgres">postgres=# select * from check_user;
 id | name
----+------
  1 | x
  2 | y
  3 | z
  4 | a
  5 | c1\
  6 | c2
  7 | c3</code>

However, after updating a row (e.g., changing 'c1' to 'c1'):

<code class="postgres">postgres=# update check_user set name = 'c1' where name = 'c1\';
UPDATE 1</code>

Rerunning the select query produces an altered result:

<code class="postgres">postgres=# select * from check_user;
 id | name
----+------
  1 | x
  2 | y
  3 | z
  4 | a
  6 | c2
  7 | c3
  5 | c1</code>

The rows are now ordered differently, raising the question of what default ordering PostgreSQL employs. In essence, PostgreSQL resorts to an implicit ordering without any explicit specification by the user. This default ordering, however, is not predictable and can vary based on factors such as data retrieval methods (e.g., sequential scans or index utilization), memory page storage, and even environmental variables.

It is crucial to emphasize that this default ordering should not be relied upon for consistent results. It is considered undefined behavior, and relying on it can lead to unexpected outcomes. To ensure predictable ordering, it is recommended to explicitly specify the desired ordering criteria in select queries using the ORDER BY clause:

<code class="postgres">postgres=# select * from check_user ORDER BY name;
 id | name
----+------
  1 | a
  2 | c1
  3 | c2
  4 | c3
  5 | x
  6 | y
  7 | z</code>

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