Thursday Jun 26, 2008

Why are some of the tables in my query missing from the plan?

In 10gR2, we introduced a new transformation, table elimination (alternately called "join elimination"), which removes redundant tables from a query. A table is redundant if its columns are only referenced to in join predicates, and it is guaranteed that those joins neither filter nor expand the resulting rows. There are several cases where Oracle will eliminate a redundant table. We will discuss each case in turn.[Read More]

Tuesday Dec 11, 2007

Outerjoins in Oracle

Since release 6, Oracle has supported a restricted form of left outerjoin, which uses Oracle-specific syntax. In 9i, Oracle introduced support for ANSI SQL 92/99 syntax for inner joins and various types of outerjoin. The Oracle syntax for left outerjoin and that of ANSI SQL 92/99 are not equivalent, as the latter is more expressive.

There appears to be some confusion about equivalence between ANSI outer join and Oracle outer join syntax. The following examples explain the equivalences and in-equivalences of these two syntaxes.

[Read More]
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The Oracle Optimizer blog is written by members of the Optimizer development team. The goal of this blog is to provide an insight into the workings of the Optimizer and the statistics it relies on. The views expressed on this blog are our own and do not necessarily reflect the views of Oracle and its affiliates. The views and opinions expressed by visitors on this blog are theirs solely and may not reflect ours.

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