Manual Concatenation and LNNVL

A quick example about things to consider when considering rewriting a SQL OR condition to a UNION/UNION ALL, as pointed out to me by Jonathan Lewis referencing this post on this recent OTN forum thread.

At the heart of the issue are:

• how to deal with the possibility of duplicates
• the potential impact of NULLS

The impact of NULLS is best described by Jonathan in the article already mentioned so I’m going to use a noddy example to illustrate:

• the problem of duplicates
• why suggesting a UNION is not an appropriate generic approach (although it may work for specific examples)
• why that generic approach should be a UNION ALL plus LNNVL

So… if, for whatever reason, we want to rewrite this statement:

select *
from   t2, t1
where  t2.col1 = t1.par
or     t2.col1 = t1.child;

If this is our setup that includes a noddy duplicate:

SQL> create table t1
  2  (par   number
  3  ,child number);

Table created.

SQL> create table t2
  2  (col1 number
  3  ,col2 varchar2(1));

Table created.

SQL> insert into t1 values (1,1);

1 row created.

SQL> insert into t1 values (1,1);

1 row created.

SQL> insert into t2 values (1,'A');

1 row created.

SQL> select * from t1;

       PAR      CHILD
---------- ----------
         1          1
         1          1

2 rows selected.

SQL> select * from t2;

      COL1 C
---------- -
         1 A

1 row selected.

SQL> 

Such that our target statement returns this:

SQL> select *
  2  from   t2, t1
  3  where  t2.col1 = t1.par
  4  or     t2.col1 = t1.child;

      COL1 C        PAR      CHILD
---------- - ---------- ----------
         1 A          1          1
         1 A          1          1

2 rows selected.

SQL> 

We can see that rewriting with a UNION is not equivalent to the original:

SQL> select *
  2  from   t2, t1
  3  where  t2.col1 = t1.par
  4  union
  5  select *
  6  from   t2, t1
  7  where  t2.col1 = t1.child;

      COL1 C        PAR      CHILD
---------- - ---------- ----------
         1 A          1          1

1 row selected.

SQL> 

And if we use a UNION ALL:

SQL> select *
  2  from   t2, t1
  3  where  t2.col1 = t1.par
  4  union all
  5  select *
  6  from   t2, t1
  7  where  t2.col1 = t1.child;

      COL1 C        PAR      CHILD
---------- - ---------- ----------
         1 A          1          1
         1 A          1          1
         1 A          1          1
         1 A          1          1

4 rows selected.

Then we need LNNVL to eliminate the duplicates from the second branch, the lower half of the statement:

SQL> select *
  2  from   t2, t1
  3  where  t2.col1 = t1.par
  4  union all
  5  select *
  6  from   t2, t1
  7  where  t2.col1 = t1.child
  8  and    lnnvl(t1.par=t1.child);

      COL1 C        PAR      CHILD
---------- - ---------- ----------
         1 A          1          1
         1 A          1          1

2 rows selected.

SQL>