Oracle 5.0 Reference Manual page 694

the condition is null-rejected for the second outer join operation but is not null-rejected for the
WHERE
first one.
If the condition is null-rejected for an outer join operation in a query, the outer join operation is
WHERE
replaced by an inner join operation.
For example, the preceding query is replaced with the query:
SELECT * FROM T1 LEFT JOIN T2 ON T2.A=T1.A
INNER JOIN T3 ON T3.B=T1.B
WHERE T3.C > 0
For the original query, the optimizer would evaluate plans compatible with only one access order
T1,T2,T3. For the replacing query, it additionally considers the access sequence T3,T1,T2.
A conversion of one outer join operation may trigger a conversion of another. Thus, the query:
SELECT * FROM T1 LEFT JOIN T2 ON T2.A=T1.A
LEFT JOIN T3 ON T3.B=T2.B
WHERE T3.C > 0
will be first converted to the query:
SELECT * FROM T1 LEFT JOIN T2 ON T2.A=T1.A
INNER JOIN T3 ON T3.B=T2.B
WHERE T3.C > 0
which is equivalent to the query:
SELECT * FROM (T1 LEFT JOIN T2 ON T2.A=T1.A), T3
WHERE T3.C > 0 AND T3.B=T2.B
Now the remaining outer join operation can be replaced by an inner join, too, because the condition
is null-rejected and we get a query without outer joins at all:
T3.B=T2.B
SELECT * FROM (T1 INNER JOIN T2 ON T2.A=T1.A), T3
WHERE T3.C > 0 AND T3.B=T2.B
Sometimes we succeed in replacing an embedded outer join operation, but cannot convert the
embedding outer join. The following query:
SELECT * FROM T1 LEFT JOIN
(T2 LEFT JOIN T3 ON T3.B=T2.B)
ON T2.A=T1.A
WHERE T3.C > 0
is converted to:
SELECT * FROM T1 LEFT JOIN
(T2 INNER JOIN T3 ON T3.B=T2.B)
ON T2.A=T1.A
WHERE T3.C > 0,
That can be rewritten only to the form still containing the embedding outer join operation:
SELECT * FROM T1 LEFT JOIN
(T2,T3)
ON (T2.A=T1.A AND T3.B=T2.B)
WHERE T3.C > 0.
When trying to convert an embedded outer join operation in a query, we must take into account the join
condition for the embedding outer join together with the
SELECT * FROM T1 LEFT JOIN
(T2 LEFT JOIN T3 ON T3.B=T2.B)
ON T2.A=T1.A AND T3.C=T1.C
WHERE T3.D > 0 OR T1.D > 0
Optimizing Statements
SELECT
674
condition. In the query:
WHERE