The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.

Optimal indexes for this query:

CREATE INDEX objects_idx_identifier ON OBJECTS (IDENTIFIER);
CREATE INDEX properties_idx_object_target ON PROPERTIES (OBJECT,TARGET);
CREATE INDEX relations_idx_child ON RELATIONS (CHILD);

The optimized query:

SELECT
        PARENT.IDENTIFIER AS PARENT_IDENTIFIER,
        PARENT.CLASS AS PARENT_CLASS,
        CHILD.TYPE AS CHILD_TYPE,
        CHILD.IDENTIFIER AS CHILD_IDENTIFIER,
        PROPERTY.IDENTIFIER AS PROPERTY_IDENTIFIER,
        PROPERTY.DESCRIPTION AS PROPERTY_DESCRIPTION,
        PROPERTY.TYPE AS PROPERTY_TYPE,
        PROPERTY.PP AS PROPERTY_PP,
        PROPERTY.STATUS AS PROPERTY_STATUS,
        PROPERTY.TARGET AS PROPERTY_TARGET 
    FROM
        OBJECTS AS CHILD 
    LEFT OUTER JOIN
        RELATIONS 
            ON RELATIONS.CHILD = CHILD.IDENTIFIER 
    LEFT OUTER JOIN
        OBJECTS AS PARENT 
            ON RELATIONS.PARENT = PARENT.IDENTIFIER 
    INNER JOIN
        PROPERTIES AS PROPERTY 
            ON PROPERTY.OBJECT = CHILD.IDENTIFIER 
    WHERE
        PROPERTY.TARGET IS NOT NULL 
    ORDER BY
        CASE 
            WHEN PARENT.IDENTIFIER IS NULL THEN 1 
            ELSE 0 END,
PARENT.IDENTIFIER,
CHILD.IDENTIFIER,
PROPERTY.TARGET,
PROPERTY.IDENTIFIER