Firebird and dummy conditions - firebird

Question

I have two, similar queries. In one of case have additional dummy where conditions (1=1, 0=0, true):
SELECT t1.*
FROM table1 t1
JOIN table2 t2 ON t2.fk_t1 = t1.id
JOIN table3 t3 ON t3.id = t1.fk_t3
WHERE
0 = 0 AND /* with this in 1st case, without this line in 2nd case */
t3.field = 6
AND EXISTS (SELECT 1 FROM table2 x WHERE x.fk2_t2 = t2.id)
All necessary fields are indexed.
For each case, Firebird (both versions 2.1 and 3.0) works differently, and statistics of reads see like this:
1st case (with 0=0):
Query Time
------------------------------------------------
Prepare : 32,00 ms
Execute : 1 046,00 ms
Avg fetch time: 61,53 ms
Operations
------------------------------------------------
Read : 8 342
Writes : 1
Fetches: 1 316 042
Marks : 0
Enhanced Info:
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
| Table Name | Records | Indexed | Non-Indexed | Updates | Deletes | Inserts | Backouts | Purges | Expunges |
| | Total | reads | reads | | | | | | |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|TABLE2 | 0 | 4804 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
|TABLE1 | 0 | 0 | 96884 | 0 | 0 | 0 | 0 | 0 | 0 |
|TABLE3 | 0 | 387553 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
And in 2nd case (without dummy condition):
Query Time
------------------------------------------------
Prepare : 16,00 ms
Execute : 515,00 ms
Avg fetch time: 30,29 ms
Operations
------------------------------------------------
Read : 7 570
Writes : 1
Fetches: 648 103
Marks : 0
Enhanced Info:
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
| Table Name | Records | Indexed | Non-Indexed | Updates | Deletes | Inserts | Backouts | Purges | Expunges |
| | Total | reads | reads | | | | | | |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|TABLE2 | 0 | 506 | 152655 | 0 | 0 | 0 | 0 | 0 | 0 |
|TABLE1 | 0 | 467 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
|TABLE3 | 0 | 1885 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
Queries have different execution plans.
PLAN JOIN (T2 NATURAL, T1 INDEX (T1_ID_IDX), T3 INDEX (T3_ID_IDX))
PLAN JOIN (T1 NATURAL, T3 INDEX (T3_ID_IDX1), T2 INDEX (T2_FK_T1_IDX))
It's strange for me. Why query with same sense of conditions works so different? How work FB optimizer and how write quick and optimal queries? How understand this?
P.S. https://github.com/FirebirdSQL/firebird/issues/6941