Samir,
To disable an index, use the hint FULL in the SQL.
select /*+ full(e) */ ename, phone, address
from emp e
where ename = 'Smith'
This will cause a full table scan on the emp table (instead of
using an index). Note you must give the table an alias in the
FROM clause, and you must use that alias in the hint. Also,
the hint must have no errors. If it contains errors you won't
get an error message. So after putting in a hint, be sure to
run an explain plan to see if your hint worked. For the FULL
hint, obviously, your explain plan should show a full table scan and
should not show use of an index.
The explain plan you sent is very interesting. The words
"MERGE JOIN (CARTESIAN)" are very interesting. Normally a
Cartesian join is a mistake, because the programmer forgot to
include a join between two tables in the WHERE clause. In your
case, however, the tables appear to be all properly joined.
Oracle enhanced the optimizer for data warehouses, where there
is usually one big table an many smaller ones. The smaller
tables can't be joined to each other, only to the large table.
However, the most efficient solution is for Oracle to first join all
the small tables, and since they can't be linked with primary and
foreign keys, Oracle just does a Cartesian join. This produces
lots of results, but not too many because the tables are all
small. Then, for the last step, Oracle finally joins the
results from these small tables to the big table.
The idea is to put off dealing with the big table until the
end, and then only deal with it once. This is faster than
joining each small table to the big table, one at a time.
Although this was meant for data warehouses, sometimes Oracle
will create a plan like this simply because it sees that one table
is very big and the others are small. I think that is what is
happening with your SQL and explain plan.
I am sorry to report that even with the proper indentation, I
cannot fully understand your explain plan. It is simply
different from what I am used to seeing and I don't understand how a
couple of the steps work together.
I can, however, offer some ideas for things to try:
(1) Use timed_statistics=true and then run a trace and
TKPROF. Then look in the TKPROF report at the very left of the
explain plan. There is a column that says ROWS. Right
now it contains all zero's! This column has very useful
information -- it tells how many rows were processed by each step in
the plan. Often you can use this information to isolate the
step where things are going wrong because you see too many rows
being processed. Often this is an indication that an index
needs to have a column added, or some other change needs to be
made. At least you see where lots of work is being done, so
you can focus your efforts on that step.
(2) It will be interesting to try your idea of using the
FULL hint instead of indexes. I hope you are able to turn on
timed_statistics, if only for your session, so you can see complete
TKPROF reports when you try various hints, so you can see all the
things that change.
(3) The major thing that jumps out at me is the last line
of the explain plan, where there is a full table scan on
valid_pricing_unit. I'm reasonably sure that this step is the
inner part of the nested loops step that appears near the top of the
plan. This means that Oracle is looping through the
valid_pricing_unit table again and again, and each time it's a full
table scan! If you had timed_statistics=true then you could
see if this is causing trouble, because each full scan means all the
rows are processed, so if you are looping through that table
thousands of times, the ROWS column would show a very large
number. But even without that information, this seems
suspicious, unless the table is very small. Is there an index
on this table? What columns? Is it on the columns used
in your WHERE clause? Does it also include any columns from
this table in the SELECT list? If it includes all the columns
in the SQL, Oracle can use the index alone and not use the table,
and this might be the best solution of all, unless this table is
very small.
That's all from me. Have fun!
----- Original Message -----
Sent: Wednesday, November 07,
2001 3:40 AM
Subject: RE: SQL query tuning
problem
Hi
Greg,
Thanks for
replying.......I have reformatted the Explain Plan statement which
is
appended below
:
Rows Execution
Plan
----------
----------------------------------------------------------------------------------------------------------------------------------------------
0
INSERT STATEMENT GOAL:
CHOOSE
0 FILTER
0
SORT ( GROUP BY )
0
NESTED
LOOPS
0
MERGE JOIN
(CARTESIAN)
0 &
nbsp; TABLE ACCESS GOAL: ANALYZED (BY INDEX ROWID)
OF
'VALID_SEGMENT’
0
INDEX (RANGE
SCAN) OF ‘IX_VALID_SEGMENT’
(UNIQUE)
0
SORT
(JOIN)
0
TABLE ACCESS GOAL: ANALYZED (BY INDEX ROWID)
OF
‘VALID_PRICING_UNIT’
0
INDEX (RANGE SCAN) OF ‘IX_VALID_PR_UNIT’
(UNIQUE)
0
INDEX (UNIQUE SCAN) OF ‘IX_VALID_TRANS’
(UNIQUE)
0
SORT (AGGREGATE)
0
TABLE ACCESS GOAL: ANALYZED (FULL) OF
‘VALID_PRICING_UNIT’
Also, if I
want to disable the indexes on this query, could u please tell me
how do I provide a hint here ??
Sorry to ask this
but I am not very conversant with Application Query
Tuning.
Any help would be greatly
appreciated.
Samir Sarkar
Oracle DBA -
Lennon Team
SchlumbergerSema
Email :
[EMAIL PROTECTED]
s
[EMAIL PROTECTED]
Phone : +44 (0) 115 - 95
76217
EPABX : +44 (0) 115 - 957 6418 Ext. 76217
Fax : +44 (0)
115 - 957
6018
Samir,
(1) The TKPROF report is incomplete because your
init.ora parameter TIMED_STATISTICS is not set to true.
This is why your report shows elapsed time = 0, etc.
Issue
alter system set timed_statistics=true
This allows you to use TKPROF and see the complete TKPROF
report. If you don't do this the TKPROF reports don't show
much useful information, so you might as well just be using
explain plan.
(2) Your explain plan is difficult to read because in
your email the indentation is messed up. Can you run an
explain plan on this statement and then send it using a
font in your email that will preserve the spacing, such as
courier font?
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