diff --git a/tests/functional/arno/optimizer/test_opt_sort_by_index_18.py b/tests/functional/arno/optimizer/test_opt_sort_by_index_18.py
index bd1e8d8c..12ff791e 100644
--- a/tests/functional/arno/optimizer/test_opt_sort_by_index_18.py
+++ b/tests/functional/arno/optimizer/test_opt_sort_by_index_18.py
@@ -4,89 +4,125 @@
 ID:          optimizer.sort-by-index-18
 TITLE:       ORDER BY ASC using index (single) and WHERE clause
 DESCRIPTION:
-  WHERE X = 1 ORDER BY Y
-  Index for both X and Y should be used when available.
+    WHERE X = 1 ORDER BY Y
+    Index for both X and Y should be used when available.
 FBTEST:      functional.arno.optimizer.opt_sort_by_index_18
+NOTES:
+    [17.11.2024] pzotov
+    Re-implemented after https://github.com/FirebirdSQL/firebird/commit/26e64e9c08f635d55ac7a111469498b3f0c7fe81
+    ( Cost-based decision between ORDER and SORT plans (#8316) ).
+    Execution plan was replaced with explained. Plans are splitted for versions up to 5.x and 6.x+.
+    Discussed with dimitr, letters 16.11.2024.
+
+    Checked on 6.0.0.532; 5.0.2.1567; 4.0.6.3168; 3.0.13.33794.
 """
 
 import pytest
 from firebird.qa import *
 
-init_script = """CREATE TABLE Table_53 (
-  ID1 INTEGER,
-  ID2 INTEGER
-);
+init_sql = """
+    recreate table table_53 (
+      id1 integer,
+      id2 integer
+    );
 
-SET TERM ^^ ;
-CREATE PROCEDURE PR_FillTable_53
-AS
-DECLARE VARIABLE FillID INTEGER;
-DECLARE VARIABLE FillID1 INTEGER;
-BEGIN
-  FillID = 1;
-  WHILE (FillID <= 50) DO
-  BEGIN
-    FillID1 = (FillID / 10) * 10;
-    INSERT INTO Table_53
-      (ID1, ID2)
-    VALUES
-      (:FillID1, :FillID - :FillID1);
-    FillID = FillID + 1;
-  END
-  INSERT INTO Table_53 (ID1, ID2) VALUES (0, NULL);
-  INSERT INTO Table_53 (ID1, ID2) VALUES (NULL, 0);
-  INSERT INTO Table_53 (ID1, ID2) VALUES (NULL, NULL);
-END
-^^
-SET TERM ; ^^
+    set term ^ ;
+    create procedure pr_filltable_53
+    as
+        declare k integer;
+        declare i integer;
+    begin
+        k = 1;
+        while (k <= 50) do
+        begin
+            i = (k / 10) * 10;
+            insert into table_53 (id1, id2) values (:i, :k - :i);
+            k = k + 1;
+        end
+        insert into table_53 (id1, id2) values (0, null);
+        insert into table_53 (id1, id2) values (null, 0);
+        insert into table_53 (id1, id2) values (null, null);
+    end
+    ^
+    set term ;^
+    commit;
 
-COMMIT;
+    execute procedure pr_filltable_53;
+    commit;
 
-EXECUTE PROCEDURE PR_FillTable_53;
-
-COMMIT;
-
-CREATE ASC INDEX I_Table_53_ID1_ASC ON Table_53 (ID1);
-CREATE DESC INDEX I_Table_53_ID1_DESC ON Table_53 (ID1);
-CREATE ASC INDEX I_Table_53_ID2_ASC ON Table_53 (ID2);
-CREATE DESC INDEX I_Table_53_ID2_DESC ON Table_53 (ID2);
-
-COMMIT;
+    create asc index i_table_53_id1_asc on table_53 (id1);
+    create desc index i_table_53_id1_desc on table_53 (id1);
+    create asc index i_table_53_id2_asc on table_53 (id2);
+    create desc index i_table_53_id2_desc on table_53 (id2);
+    commit;
 """
 
-db = db_factory(init=init_script)
+db = db_factory(init = init_sql)
 
-test_script = """SET PLAN ON;
-SELECT
-  t53.ID2,
-  t53.ID1
-FROM
-  Table_53 t53
-WHERE
-  t53.ID1 = 30
-ORDER BY
-t53.ID2 ASC;"""
+act = python_act('db', substitutions = [(r'record length: \d+, key length: \d+', 'record length: NN, key length: MM')])
 
-act = isql_act('db', test_script)
+#-----------------------------------------------------------
 
-expected_stdout = """PLAN (T53 ORDER I_TABLE_53_ID2_ASC INDEX (I_TABLE_53_ID1_ASC))
+def replace_leading(source, char="."):
+    stripped = source.lstrip()
+    return char * (len(source) - len(stripped)) + stripped
 
-         ID2          ID1
-============ ============
-
-           0           30
-           1           30
-           2           30
-           3           30
-           4           30
-           5           30
-           6           30
-           7           30
-           8           30
-9           30"""
+#-----------------------------------------------------------
 
 @pytest.mark.version('>=3')
-def test_1(act: Action):
-    act.expected_stdout = expected_stdout
-    act.execute()
+def test_1(act: Action, capsys):
+
+    # opt_clause = '' if act.is_version('<5') else 'optimize for first rows' if act.is_version('<6') else ''
+    opt_clause = ''
+
+    test_sql = f"""
+        select
+            t53.id2,
+            t53.id1
+        from table_53 t53
+        where
+            t53.id1 = 30
+        order by
+            t53.id2 asc
+        {opt_clause}
+    """
+
+    with act.db.connect() as con:
+        cur = con.cursor()
+        ps = None
+        try:
+            ps = cur.prepare(test_sql)
+
+            # Print explained plan with padding eash line by dots in order to see indentations:
+            # print(test_sql)
+            print( '\n'.join([replace_leading(s) for s in ps.detailed_plan.split('\n')]) )
+            print('')
+        except DatabaseError as e:
+            print(e.__str__())
+            print(e.gds_codes)
+        finally:
+            if ps:
+                ps.free()
+
+    expected_stdout_5x = """
+        Select Expression
+        ....-> Filter
+        ........-> Table "TABLE_53" as "T53" Access By ID
+        ............-> Index "I_TABLE_53_ID2_ASC" Full Scan
+        ................-> Bitmap
+        ....................-> Index "I_TABLE_53_ID1_ASC" Range Scan (full match)
+    """
+
+    expected_stdout_6x = """
+        Select Expression
+        ....-> Sort (record length: NN, key length: MM)
+        ........-> Filter
+        ............-> Table "TABLE_53" as "T53" Access By ID
+        ................-> Bitmap
+        ....................-> Index "I_TABLE_53_ID1_ASC" Range Scan (full match)
+    """
+
+    act.expected_stdout = expected_stdout_5x if act.is_version('<6') else expected_stdout_6x
+
+    act.stdout = capsys.readouterr().out
     assert act.clean_stdout == act.clean_expected_stdout