RE: View Mechanics
- From: "Powell, Mark" <mark.powell2@xxxxxx>
- To: Oracle-L List <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 2 Sep 2011 17:36:13 +0100
That is strange. Oracle pushing the wehre field = '123' into the view text and
driving off that table would be what I would normally expect. The resulting
query ran in sqlplus should not run better.
Do you have an session level settings changes that need to be taken into
account?
Have you made any database parameter setting changes for an of the parameters
that effect the optimizer or set any underbar paramters?
Are any events set in this database?
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Lange, Kevin G
Sent: Friday, September 02, 2011 11:38 AM
To: Oracle-L List
Subject: RE: View Mechanics
What we have seen is this:
In the application, running a select from the view in the form of
Select * from app_view where field_a = '123';
Run a lot slower than running the EXACT code that makes up the view adding
the line
and field_a = '123'
In the underlying tables field_a is an indexed column. And the columns
that join Table_1 to Table_2 to Table_3 are all indexed. Therefore I
would expect the view to use the same path the sql behind the view when
selecting from it.
What we saw when explaining the select from the view is that all three tables
are doing Full Table Scans. When running the sql outside the view, the
appropriate indexes are being used.
I just thought that, all things equal, the view would use the same methods to
get the data that the SQl behind the view did. I had not
seen this drastic a difference before.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Powell, Mark
Sent: Friday, September 02, 2011 7:48 AM
To: Oracle-L List
Subject: RE: View Mechanics
I am not sure I understand your wording but it is not uncommon to create a view
whose SQL query would be a multiple table join of all rows and if you run
select * from the view the performance would be pretty bad.
However, when used in the application or by the end-user tool a where clause
using an indexed column is provided in the query against the view which in turn
when merged into the view code by the CBO results in a much more selective
plan/performance.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Lange, Kevin G
Sent: Thursday, September 01, 2011 6:12 PM
To: Oracle-L List
Subject: View Mechanics
I am just curious if its common for an Explain Plan of a view to have a totally
different response than an explain plan of the SQL behind the view.
Reason being, we have a very simple view made of of 3 tables that, when you
explain it, it shows full table scans on all three tables. But, in the mean
time, if you explain plan the SQL behind the view, you get index range scans on
all three tables.
Just makes no sense to me (which seems to be happneing more and more these
days).
System: Oracle 10.2.0.4 on a Solaris 10 machine.
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