Add stable sort

[nirolevy]

Overview

This Pull Request resolves an issue with inconsistent paging when running CSW GetRecords requests against a PostgreSQL backend that utilizes SortBy on a non-unique field (e.g., dc:date).

When multiple records share the same value for the sort field, PostgreSQL's tie-breaker is non-deterministic, causing records to be included in more than one page when pagination (LIMIT/OFFSET) is used.

The fix introduces a configuration option to enable a stable sort by automatically appending the unique record identifier (pycsw:Identifier) to the ORDER BY clause of the database query. This ensures a consistent and deterministic order for all records, guaranteeing reliable pagination.

Related Issue / Discussion

Fixes: #1155

Additional Information

A new configuration option, stable_sort, has been added to the [repository] section of the configuration file.

To enable the fix, set stable_sort to true (it defaults to false for backwards compatibility):

repository:
  database: # Your database connection string
  table: records
  stable_sort: true

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst\#contributions-and-licensing)

• I'd like to contribute bugfix to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines