Unexpected query results in Oracle db - sql

Question

We have Oracle 12.2.0.1.0 database. We create a simple table like this:
CREATE TABLE TABLE1 (DATE1 TIMESTAMP (6));
INSERT INTO TABLE1 VALUES (TIMESTAMP'2018-05-30 00:00:00');
INSERT INTO TABLE1 VALUES (TIMESTAMP'2018-05-30 00:00:00');
When we query with the following two select statements, we get different results. The first one returns two rows as expected, while the second one doesn't.
SELECT T1.*, NVL(T2.DATE1, TIMESTAMP'1900-01-01 00:00:00')
FROM TABLE1 T1
LEFT JOIN TABLE1 T2
ON 1 = 0
WHERE T1.DATE1 > NVL(T2.DATE1, TIMESTAMP'1900-01-01 00:00:00');
SELECT T1.*, NVL(T2.DATE1, TIMESTAMP'1900-01-01 00:00:00')
FROM TABLE1 T1
LEFT JOIN TABLE1 T2
ON T1.DATE1 || '---' = '-'
WHERE T1.DATE1 > NVL(T2.DATE1, TIMESTAMP'1900-01-01 00:00:00');
T1 and T2 are the same TABLE1. We are joining it on itself.
Please advise why that is so. Thanks.

Answer 1

It seems the optimizer gets confused with so many levels of obfuscating the join condition.
The first query results in the following execution plan:
SQL_ID 9k6g3m0xs31w7, child number 1
-------------------------------------
select t1.*, nvl(t2.date1, timestamp'1900-01-01 00:00:00') from table1
t1 left join table1 t2 on 1 = 0 where t1.date1 > nvl(t2.date1,
timestamp'1900-01-01 00:00:00')
Plan hash value: 963482612
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| 2 |00:00:00.01 | 7 |
|* 1 | TABLE ACCESS FULL| TABLE1 | 1 | 2 | 26 | 3 (0)| 2 |00:00:00.01 | 7 |
-----------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$F7AF7B7D / T1#SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("T1"."DATE1">TIMESTAMP' 1900-01-01 00:00:00.000000000')
So the planner correctly sees that the self join is unnecessary and replaces the NVL() condition on the joined table with a condition on the column itself.
Apparently this "replacing" the condition does not work correctly in 12.2.
The second query results in the following plan:
SQL_ID 3twykk3kcyyxy, child number 1
-------------------------------------
select t1.*, nvl(t2.date1, timestamp'1900-01-01 00:00:00') from table1
t1 left join table1 t2 on t1.date1 || '---' = '-' where t1.date1 >
nvl(t2.date1, timestamp'1900-01-01 00:00:00')
Plan hash value: 736255932
----------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 8 (100)| 0 |00:00:00.01 | 7 | | | |
|* 1 | FILTER | | 1 | | | | 0 |00:00:00.01 | 7 | | | |
| 2 | MERGE JOIN OUTER | | 1 | 1 | 26 | 8 (25)| 2 |00:00:00.01 | 7 | | | |
| 3 | SORT JOIN | | 1 | 2 | 26 | 4 (25)| 2 |00:00:00.01 | 7 | 2048 | 2048 | 2048 (0)|
| 4 | TABLE ACCESS FULL | TABLE1 | 1 | 2 | 26 | 3 (0)| 2 |00:00:00.01 | 7 | | | |
|* 5 | SORT JOIN | | 2 | 2 | 26 | 4 (25)| 0 |00:00:00.01 | 0 | 1024 | 1024 | |
| 6 | VIEW | VW_LAT_C83A7ED5 | 2 | 2 | 26 | 3 (0)| 0 |00:00:00.01 | 0 | | | |
|* 7 | FILTER | | 2 | | | | 0 |00:00:00.01 | 0 | | | |
| 8 | TABLE ACCESS FULL| TABLE1 | 0 | 2 | 26 | 3 (0)| 0 |00:00:00.01 | 0 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$F7AF7B7D
4 - SEL$F7AF7B7D / T1#SEL$1
6 - SEL$BCD4421C / VW_LAT_AE9E49E8#SEL$AE9E49E8
7 - SEL$BCD4421C
8 - SEL$BCD4421C / T2#SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("T1"."DATE1">NVL("ITEM_1",TIMESTAMP' 1900-01-01 00:00:00.000000000'))
5 - access(INTERNAL_FUNCTION("T1"."DATE1")>NVL("ITEM_1",TIMESTAMP' 1900-01-01 00:00:00.000000000'))
7 - filter(INTERNAL_FUNCTION("T1"."DATE1")||'---'='-')
So the optimizer replaced the reference to the table column with some ITEM1 placeholder - and the step access(INTERNAL_FUNCTION("T1"."DATE1")>NVL("ITEM_1",TIMESTAMP' 1900-01-01 00:00:00.000000000')) messes things up.
With 12.1 the plan is essentially the same, the only difference is that the access() part is missing in the predicates, so I guess that replacement is somewhat buggy in 12.2 (to be precise my version is: 12.2.0.1.0)