Why Does Postgres SELECT Query Order Change After Row Updates?

Default Ordering in Postgres SELECT Queries After Row Updates

When retrieving data from a table in Postgres without specifying an ORDER BY clause, it's generally assumed that the rows will be returned in the order they were inserted. However, this isn't always the case, especially after updating rows.

In Postgres, updates do not modify rows in place but instead create new versions. The old version is marked as deleted, and the new version is inserted into the table. This process can result in a change in the physical order of the rows on disk.

As a consequence, when a SELECT query is executed after an update, Postgres might read the modified row's new version from a different page or block on the disk than where the original row was located. This can lead to the retrieval of rows in an unintended order.

To ensure consistent ordering in SELECT queries, it's crucial to always include an explicit ORDER BY clause that specifies the desired column(s) for sorting. This will override the default behavior and force Postgres to return rows in the specified order, regardless of any updates that may have occurred.

Therefore, it's important to remember that the default ordering in Postgres is undefined and should never be relied upon for any specific functionality. Always explicitly specify the desired ordering using ORDER BY to avoid unexpected results.

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