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Wrong result on a simple SQL statement

cadwinvalOct 2 2020

20201002-bug-reproducer.txt (1.44 KB)Hello
I think that I've found a bug in Oracle 19 (probably reproducible on Oracle 12 but I haven't tested).
Please find enclosed a self-contained reproducer which creates a single table, insert 4 lines and runs a select query.
It gives the expected result if and if I use the optimizer hint "NO_QUERY_TRANSFORMATION" or if I drive "r_m_s_id" non nullable or if I drive the select query much more complicated by duplicating some code and using a coalesce or if I remove one inner join but those workarounds aren't satisfying (performance considerations).
I assume that Oracle SQL transforms the query to optimize the execution plan but it seems to treat a nullable column as always null.
By the way, PostgreSQL 9.6 always returns the expected result.
Has someone else experienced the same problem? Does it work in Oracle 20?

Comments

21205
show it
SQL> declare
  2     ignore_column_name varchar2 (100) := 'ENAME';
  3     CURSOR C1 is
  4     SELECT column_name
  5     FROM user_tab_columns
  6     WHERE table_name = 'EMP'
  7  --   AND column_name NOT IN (ignore_column_name)
  8  ;
  9  begin
 10     for r in c1
 11     loop
 12        dbms_output.put_line ('result: '||r.column_name);
 13     end loop;
 14  end;
 15  /
result: EMPNO
result: ENAME
result: JOB
result: MGR
result: HIREDATE
result: SAL
result: COMM
result: DEPTNO

PL/SQL procedure successfully completed.

SQL> 
SQL> 
SQL> declare
  2     ignore_column_name varchar2 (100) := 'ENAME';
  3     CURSOR C1 is
  4     SELECT column_name
  5     FROM user_tab_columns
  6     WHERE table_name = 'EMP'
  7     AND column_name NOT IN (ignore_column_name)
  8  ;
  9  begin
 10     for r in c1
 11     loop
 12        dbms_output.put_line ('result: '||r.column_name);
 13     end loop;
 14  end;
 15  /
result: EMPNO
result: JOB
result: MGR
result: HIREDATE
result: SAL
result: COMM
result: DEPTNO

PL/SQL procedure successfully completed.

SQL> 
646894
Hello,

the only difference between your program and mine is that..

ignore_column_name variable is declared but not initialized before the cursor. I mean..

declare
ignore_columns_name varchar2(100);
cursor c1 is
select column_name
from user_tab_columns
where column_name not in (ignore_column_name);
begin
ignore_column_name := 'LNAME';
for r in c1
loop
column := column||','||r.column_name;
end loop;
##REST OF THE CODE##
end;
/

the thing is .. ignore_column_name is not hardcoded, instead it is generated from a different part of the code. so I cannot initialise it like u have shown it. Can u help me? is it possible to achieve this?

Thanks
BluShadow
Azeroth wrote:
Hello,

the only difference between your program and mine is that..

ignore_column_name variable is declared but not initialized before the cursor. I mean..

the thing is .. ignore_column_name is not hardcoded, instead it is generated from a different part of the code. so I cannot initialise it like u have shown it. Can u help me? is it possible to achieve this?

Thanks
It doesn't make a difference if it's initialized before the cursor declaration or not, just as long as it's initialized before the cursor is opened.
SQL> declare
  2    ignore_column_name varchar2 (100);
  3    CURSOR C1 is
  4    SELECT column_name
  5    FROM user_tab_columns
  6    WHERE table_name = 'EMP'
  7    AND column_name NOT IN (ignore_column_name);
  8  begin
  9    ignore_column_name := 'ENAME';
 10    for r in c1
 11    loop
 12       dbms_output.put_line ('result: '||r.column_name);
 13    end loop;
 14  end;
 15  /
result: EMPNO
result: JOB
result: MGR
result: HIREDATE
result: SAL
result: COMM
result: DEPTNO

PL/SQL procedure successfully completed.

SQL>
646894
Hello,

This was a really helpful reply.. it cleared my only suspicion but not my problem. Here I am pasting the entire code.. I will highlight the part thats not working.. Please let me know what you think the problem is.

CREATE OR REPLACE PROCEDURE
test_etl1
(
table1 IN user_tab_columns.table_name%TYPE,
table2 IN user_tab_columns.table_name%TYPE
) IS
col_list VARCHAR2(3000);
new_col_list VARCHAR2(3000);
column_name VARCHAR2(100);
num number;
table3 VARCHAR2(100);
column_list VARCHAR2(3000);
stmt VARCHAR2(3000);
CURSOR C1 IS
SELECT column_list
FROM ignore_columns
WHERE table_name = table1;
CURSOR C2 IS
SELECT column_name
FROM user_tab_columns
WHERE table_name = table3
AND column_name NOT IN (new_col_list);
BEGIN
col_list := null;
FOR R1 in C1
LOOP
col_list := R1.column_list;
END LOOP;
num := length(col_list);

WHILE num <> 1
LOOP
column_name := SUBSTR(col_list, 0,INSTR(col_list,' '));

IF (NVL(SUBSTR(col_list, INSTR(col_list, ' '), LENGTH(col_list)),'1') != '1')
THEN
col_list := SUBSTR(col_list, INSTR(col_list, ' '), LENGTH(col_list));
col_list := LTRIM(col_list);
new_col_list := new_col_list||''''||RTRIM(column_name)||''''||',';
ELSE
num := 1;
END IF;
END LOOP;

new_col_list := RTRIM(new_col_list,',');

table3 := upper(table1);
FOR R2 IN C2
LOOP
* column_list := column_list||''''||R2.column_name||''''||',';*
END LOOP;
column_list := RTRIM(column_list,',');
stmt := 'CREATE TABLE error_table AS SELECT '||column_list||' FROM '||table1||' MINUS SELECT '||column_list||'FROM '||table2;

END;

Table ignore_columns contains a single row.
table_name = patient_v
column_list = 'LAST_NAME'; with the quotes

Thanks
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Added on Oct 2 2020
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