Why "full scans are not evil, indexes are not good"

in this article inside asktom page
http://asktom.oracle.com/pls/ask/f?p=4950:8:::::F4950_P8_DISPLAYID:9422487749968
tell about "full scans are not evil, indexes are not good"
someone tell me why for this select mentioned index are not good?

...and if you read next post, nothing is black or white, it is all shades of grey .
This is not because your query will use index that it would be faster, and the example which Tom gave is really explicit :
With index usage 60127 consistent gets, and without index usage 92 consistent gets
Nicolas.
Message was edited by:
N. Gasparotto

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Documents are marked for indexing but indexing is not happening

Hi All,
I have completed the configuration of Trex 7.10 ; and also I have tested the settings by executing the report "RCF_CHECK_SEARCH_SETTINGSu201D in SE38 and found no errors, but the problem here is documents are marked for indexing but not getting indexed.
I need some help to resolve this issue
Regards
Siva

Hi Siva,
This should help http://wiki.sdn.sap.com/wiki/x/gQ8oCw

INDEX UNIQUE SCAN instead of   INDEX FULL SCAN or TABLE ACCESS FULL

I have calculated statistics in all tables and indexes
I have a table and a view and when I put it
SELECT *
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX UNIQUE SCAN (FIELD_A_TABLE_A_PK)
Itâs OK. I NEED IT (INDEX UNIQUE SCAN)
But If I put
SELECT A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID
WHERE (B.FK_ID_XXX = 1)
In table A make a TABLE ACCESS FULL.
Then If I put:
SELECT /*+ INDEX(A FIELD_A_TABLE_A_PK) */ A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX UNIQUE SCAN (FIELD_A_TABLE_A_PK)
Itâs OK. I NEED IT (INDEX UNIQUE SCAN)
Finally, If I put other tables and views in the query (I NEED IT)
For example:
SELECT /*+ INDEX(A FIELD_A_TABLE_A_PK) */ A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID
INNER JOIN TABLE_Câ¦.
LEFT JOIN VIEW_Dâ¦.
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX FULL SCAN (FIELD_A_TABLE_A_PK)
I need INDEX UNIQUE SCAN instead of INDEX FULL SCAN or TABLE ACCESS FULL.
How can obtain it?
What happens???
Thanks!

Notice the difference in cardinality between your two select statements:
SELECT STATEMENT, GOAL = ALL_ROWS Cost=5 Cardinality=1
SELECT STATEMENT, GOAL = ALL_ROWS Cost=10450 Cardinality=472161Apparently since the optimizer believed the first statement was going to return one row, it used an index. But in the second statement it believed it was going to return nearly the whole table (didn't you say it had around 500k rows?). Hence full table scan.

An index can not being used and still afect a query performance?

Hi i have a query with a high cost so i created two indexes, A and B, to improve its performance.
After the creation of the indexes when i reviewed the execution plan of the query the cost had been reduced, but i noticed that the index B is not being used,
and if i try to force the query to use index B with a HINT the cost increases, so i decided to drop the index B.
Once i droped the index B i checked the execution plan again and then i noticed that the cost of the query increased, if i recreate the index B the explain plan
shows a lower cost even though its not being used by the execution plan.
Does anyone know why is this happening?
An index can, not being used by the execution plan and still affect a query performance?

user11173393 wrote:
Hi i have a query with a high cost so i created two indexes, A and B, to improve its performance.
After the creation of the indexes when i reviewed the execution plan of the query the cost had been reduced, but i noticed that the index B is not being used,
and if i try to force the query to use index B with a HINT the cost increases, so i decided to drop the index B.
Once i droped the index B i checked the execution plan again and then i noticed that the cost of the query increased, if i recreate the index B the explain plan
shows a lower cost even though its not being used by the execution plan.
Does anyone know why is this happening?
An index can, not being used by the execution plan and still affect a query performance?You said that is what is happening, & I believe you.

"Tables are not using indexes"-Please help

We have a new database , version
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for IBM/AIX RISC System/6000: Version 10.2.0.1.0 - Productio
NLSRTL Version 10.2.0.1.0 - Production
We have created Indexes for the tables and analysed the table. Problem is , non of the tables are using indexes. Explain paln for the sql queries are showing full table scan.
Kindly let me know any parameter I have to change or is there any setttings needed on database level?
Thanks in advance.
Regards,
Chotu

What is your optimizer mode?
Another question why you want to use indexes? Use of indexes depends,
Here is a simple example
test@>ed
Wrote file afiedt.buf
1 explain plan for
2 select c.cname from mytable c, mytable1 m
3* where c.cname=m.cname
test@>/
Explained.
test@>SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
PLAN_TABLE_OUTPUT
Plan hash value: 2460944079
| Id | Operation         | Name         | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |              |     8 |    64 |     1   (0)| 00:00:01 |
|   1 | NESTED LOOPS     |              |     8 |    64 |     1   (0)| 00:00:01 |
|   2 |   INDEX FULL SCAN | C_MYTAB_IDX |     8 |    32 |     1   (0)| 00:00:01 |
|* 3 |   INDEX RANGE SCAN| C_MYTAB1_IDX |     1 |     4 |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("C"."CNAME"="M"."CNAME")
15 rows selected.
test@>explain plan for
2 select * from mytable;
Explained.
test@>SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
PLAN_TABLE_OUTPUT
Plan hash value: 1229213413
| Id | Operation         | Name    | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |         |     8 |    32 |     3   (0)| 00:00:01 |
|   1 | TABLE ACCESS FULL| MYTABLE |     8 |    32 |     3   (0)| 00:00:01 |
8 rows selected.As you see MYTABLE has an index but it not always uses that index it depends on the need.
sql queries are showing full table scan this is a not a curse. So unless you are having some problem. You should not change queries only because to avoid full table scans, there is no rule that indexes should always be used in every case.
Adith

Why full index scan is faster than full table scan?

Hi friends,
In the where clause of a query,if we give a column that contains index on it,then oracle uses index to search data rather than a TABLE ACCESS FULL Operation.
Why index searching is faster?

Sometimes it is faster to use index and sometimes it is faster to use full table scan. If your statistics are up to date Oracle is far more likely to get it right. If the query can be satisfied entirely from the index, then an index scan will almost always be faster as there are fewer blocks to read in the index than there would be if the table itself were scanned. However if the query must extract data from the table when that data is not in te index, then the index scan will be faster only if a small percentage of the rows are to be returned. Consiter the case of an index where 40% of the rows are returned. Assume the index values are distributed evenly among the data blocks. Assume 10 rows will fit in each data block thus 4 of the 10 rows will match the condition. Then the average datablock will be fetched 4 times since most of the time adjacent index entries will not be in the same block. The number of single datablock fetches will be about 4 times the number of datablocks. Compare this to a full table scan that does multiblock reads. Far fewer reads are required to read the entire table. Though it depends on the number of rows per block, a general rule is any query returning more than about 10% of a table is faster NOT using an index.

In the attached VI I dont understand why data from the inner case structures are not being transmitted to the array.

In the attached VI I don't understand why data from the inner case structures are not being transmitted to the array.
Thank you.
Solved!
Go to Solution.
Attachments:
TEMP.vi ‏25 KB

It took me some time to figure what you are trying to do but I think I have it now.
The SR is still required.
What is happening is your "Bundle By Name" is replacing all of the fields of the cluster, not just the value coming out of the case structure. So to maintain the cluster stuff you do in earlier iterations you can either...
1) Put the "Index array, Bundle by name, and Replace array" inside the case structure and ONLY bundle the value you are setting in that iteration. (as you will see Tim post shortly)
OR
2) Move the Index array before the Case, unbundle all of the fields and feed teh case structure and run them across the case so that ALL of your output tunnels come from the coresponding input tunnel EXCEPT for the filed you are trying to set.
Ben
Message Edited by Ben on 05-05-2010 01:31 PM
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction

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Why "full scans are not evil, indexes are not good"

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