||Oracle Tips by Burleson
Chapter 7 - Tuning and Performance
OCP Instructors Guide for
Oracle DBA Certification
Using Explain Plan to
Determine Access Paths
Operation 3.1 – The nested loop join method is being used to join
the two tables. The results from the join in Step 3.1 are used as
the outer-table in Join 2.1. Oracle is only able to join two tables
at a time. The rows are read again and the join column is used to
find a match in the inner-table (PUBLISHER_APP)
Operation 2.1 – Once again, Oracle knows the
values being passed from the nested loop join (Step 3.1). Since it
is being passed values to look for matching rows, it uses the index
(PK_PUBLISHER_APP) which is built on the join column. Remember, the
inner-table of a nested loop join can use an index on a join column
(it knows the value being passed from the outer-table).
Operation 3.2 – Oracle is using the ROWID
retrieved in Step 4.1 to probe the inner-table (PK_PUBLISHER_APP).
Explain Plan Output
Because of space considerations, I am unable to
cover every access path that can be selected by a particular SQL
statement. In addition, the Oracle reference manuals do a pretty
good (actually a very good) job of describing the various access
paths a statement can take.
The Oracle8 and Oracle8i Concepts manual and
the Oracle9i Database Performance Guide and Reference are the
manuals to turn to when trying to learn more about Oracle access
paths. The Oracle9i
Database and Performance Reference Guide is the better of the three.
The manual goes into great detail on what the access path actually
does, factors that affect it, why the optimizer would select it and
what impact it has on performance.
The above text is
an excerpt from:
OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
by Christopher T. Foot
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