Oracle - Table alias and NULL evaluation in join

Question

I was just trying to make an example to explain how NULL in Oracle can lead to 'unexpected' behaviours, but I've found something I did not expect...

setup:

create table tabNull (val varchar2(10), descr varchar2(100));
insert into tabNull values (null, 'NULL VALUE');
insert into tabNull values ('A', 'ONE CHAR');

This gives what I expected:

SQL> select * from tabNull T1 inner join tabNull T2 using(val);

VAL        DESCR                DESCR
---------- -------------------- --------------------
A          ONE CHAR             ONE CHAR

If I remove table aliases, I get:

SQL> select * from tabNull inner join tabNull using(val);

VAL        DESCR                DESCR
---------- -------------------- --------------------
A          ONE CHAR             ONE CHAR
A          ONE CHAR             ONE CHAR

and this is quite surprising to me.

A reason can be found in the execution plans for the two queries; with table aliases, Oracle makes an HASH JOIN and then checks for T1.val = T2.val:

------------------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |     1 |   118 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN         |         |     1 |   118 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| TABNULL |     2 |   118 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| TABNULL |     2 |   118 |     3   (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."VAL"="T2"."VAL")

Without aliases, it first filters one occurrence of the table for not null values, thus picking only one row, and then it makes a CARTESIAN with the second occurrence, thus giving two rows; even if it's correct, I would expect the result of a cartesian, but I don't have any row with DESCR = 'NULL VALUE'.

--------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |         |     2 |   118 |     6   (0)| 00:00:01 |
|   1 |  MERGE JOIN CARTESIAN|         |     2 |   118 |     6   (0)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL  | TABNULL |     1 |    59 |     3   (0)| 00:00:01 |
|   3 |   BUFFER SORT        |         |     2 |       |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL | TABNULL |     2 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("TABNULL"."VAL" IS NOT NULL)

Is this somehow correct / expected? Isn't the result value of the cartesian even stranger than the number of returned rows? Am I misunderstanding the plans, or missing something so big that I can't see?

Answer 1

Sorry, I don't think this is really an answer. It's mostly just a comment / reply to this in your posting:

Isn't the result value of the cartesian even stranger than the number of returned rows?

Every step of an plan has a "projection" that is the list of columns / expressions that are output from the step. What is happening is that the identical aliases is causing Oracle's projection to combine what should be two columns projected into only one column.

This is easier to see if you use two separate tables in your example and add a pair of uniquely named columns to see what is going on, like this:

create table tabNull1 (val varchar2(10), descr varchar2(100), t1_real_descr varchar2(100) );
insert into tabNull1 values (null, 'T1-NULL VALUE', 'T1-NULL VALUE');
insert into tabNull1 values ('A', 'T1-ONE CHAR', 'T1-ONE CHAR');


create table tabNull2 (val varchar2(10), descr varchar2(100), t2_real_descr varchar2(100) );
insert into tabNull2 values (null, 'T2-NULL VALUE', 'T2-NULL VALUE');
insert into tabNull2 values ('A', 'T2-ONE CHAR', 'T2-ONE CHAR');

select * from tabNull1 t inner join tabNull2 t using(val);

VAL    DESCR            T1_REAL_DESCR     DESCR_1       T2_REAL_DESCR                                                                                        
------ ---------------- ----------------- ------------- -----------------
A      T2-ONE CHAR      T1-NULL VALUE     T2-ONE CHAR   T2-ONE CHAR
A      T2-ONE CHAR      T1-ONE CHAR       T2-ONE CHAR   T2-ONE CHAR

As you can see, your theory about the Cartesian join was correct.

Answer 2

The USING keyword is new to me, but according to what I read it's just a new way of simplifying SQL join syntax. (See Oracle USING Keyword)

select * from tabNull T1 inner join tabNull T2 using(val);
is equivalent to:
select * from tabNull T1 inner join tabNull T2 on T1.val = T2.val;

select * from tabNull inner join tabNull using(val);
is equivalent to:
select * from tabNull inner join tabNull on tabNull.val = tabNull.val;

The problem is that in the second query the table names in the join tabNull.val = tabNull.val are not unique.

This is bad syntax which would have resulted in an error if traditional join syntax had been used.

My best guess is that Oracle performed a full cross-product on the two tables (which doubled all the rows), and then eliminated the nulls because USING must use equijoins (i.e., equals "=") and null isn't equal to anything.

Answer 3

According to http://docs.oracle.com/javadb/10.10.1.2/ref/rrefsqljusing.html using(val) translates here as ON tabnull.val=tabnull.val So

select tabNull.*, tabNull.descr from tabNull inner join tabNull 
on tabNull.val = tabNull.val;

Next to build a plan Oracle must [virtually] assign different aliases for every JOIN member but sees no reason to use second alias at any place in SELECT and ON. So

select t1.*, t1.descr from tabNull t1 inner join tabNull t2 
on t1.val = t1.val;

Plan

--------------------------------------------------------------------------------                        
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time     |                        
--------------------------------------------------------------------------------                        
|   0 | SELECT STATEMENT     |         |     2 |    28 |     4   (0)| 00:00:01 |                        
|   1 |  MERGE JOIN CARTESIAN|         |     2 |    28 |     4   (0)| 00:00:01 |                        
|*  2 |   TABLE ACCESS FULL  | TABNULL |     1 |    14 |     2   (0)| 00:00:01 |                        
|   3 |   BUFFER SORT        |         |     2 |       |     2   (0)| 00:00:01 |                        
|   4 |    TABLE ACCESS FULL | TABNULL |     2 |       |     2   (0)| 00:00:01 |                        
--------------------------------------------------------------------------------                        


Predicate Information (identified by operation id):                                                     
---------------------------------------------------                                                     

   2 - filter("T1"."VAL" IS NOT NULL)    

Answer 4

EDIT: I say below that the syntax is illegal; on further thought, that's BS on my part, I don't know that for a fact (I can't point to where in the language definition aliases are required for a self-join). I still believe the explanation below is probably correct, whether it is for the "bug" or for the "undefined behavior" I mention below.

*

The syntax is illegal (you knew that - you were just curious to see what would happen, and if you can understand the output). I agree with jarlh that you should have received an error message. Clearly Oracle didn't code it that way.

Since this is not valid syntax, what you are seeing can't be called a bug (so I disagree with Nick's comment). The behavior is "undefined" - when you use syntax that is not supported by the Oracle language definition, you may get any kind of crazy results, for which Oracle is not taking any responsibility.

OK, with that out of the way, is there any explanation for what you are seeing? I believe it is indeed a Cartesian join, and not a union as Nick suggested.

Let's put ourselves in the optimizer's shoes. It sees the first table in the FROM list, it scans it, so far so good.

Then it reads the second table, and it has a list of columns like this:

tabNULL.val, tabNULL.descr, tabNULL.val, tabNULL.descr

The join condition is tabNULL.val = tabNULL.val

The optimizer is dumb, it is not smart. It, unlike you, doesn't realize at this point that tabNULL is meant to stand for two different incarnations of the table. It thinks tabNULL.val on both sides of the equation are THE SAME value and they both refer to the first "incarnation" of the table. The only case when that fails is if tabNULL.val is NULL, so it REWRITES the query with the clause becoming tabNULL.val IS NOT NULL.

Only the FIRST table is checked for tabNULL.val IS NOT NULL; the optimizer doesn't "know" tabNULL.val appears again in the list and it may have a DIFFERENT meaning! Then the join happens; at this point there are no other conditions left, so BOTH rows in the second incarnation of the table will produce rows in the join, for A, ONE CHAR from the first table.

Then, in the projection, again only the FIRST tabNULL.val will be read and will populate BOTH columns in the output. You ask the query engine to return the value tabNULL.val twice, and in your mind it's from different places, but there is only one memory location labeled tabNULL.val, and it stores what came from the first table.

Of course, very few know with any certainty what the optimizer and the query engine do, but in this case I think this is a pretty safe guess.