RE: Function-Based Index not working

Even when the high-water mark thing isn’t a problem, it’s sometimes more efficient to read every row in a table through an index than via a full-table scan.

 

If you’re curious, try this. Create a table with two columns, “key” and “value”, and insert one row with key=1, value=’x’. Create an index on “key”. Then…

 

alter session set events ‘10046 trace name context forever, level 8’;

select * from onerow;  /* just to make sure it’s cached */

select * from onerow;

select * from onerow where key=1;  /* just to make sure it’s cached */

select * from onerow where key=1;

exit;

 

Now look at your trace data. You’ll find that the full-table scan of this table is both cheaper and faster through the index.

 

The age-old advice from many SQL tuning “experts” is badly wrong when they tell you never to index small tables. For applications that execute a lot of small-table queries, the performance impact really adds up.

 

-----Original Message-----
From: root@fatcity.com [mailto:root@fatcity.com] On Behalf Of Fink, Dan
Sent: Thursday, September 05, 2002 12:19 PM
To: Multiple recipients of list ORACLE-L
Subject: RE: Function-Based Index not working

 

-----Original Message-----
From: Yechiel Adar [mailto:adar76@inter.net.il]
Sent: Thursday, September 05, 2002 10:19 AM
To: Multiple recipients of list ORACLE-L
Subject: Re: Function-Based Index not working

Hello

 

I think that the amount of records you read is also taken into account.

If you run a query that selects ALL the records in the tables

it is ALWAYS more efficient to do full table scan then to access

by index.

 

Yechiel Adar
Mehish

----- Original Message -----

From: Marul Mehta

To: Multiple recipients of list ORACLE-L

Sent: Saturday, August 31, 2002 4:23 PM

Subject: Re: Function-Based Index not working

 

Hi All,

 

Thanks a lot to you all. At last I got the function-based index working properly.

This is what I noticed :-

Have to alter session/system for :-

+ alter session set QUERY_REWRITE_ENABLED=TRUE;
+ alter session set QUERY_REWRITE_INTEGRITY=TRUSTED;
+ alter session set optimizer_mode=FIRST_ROWS;

 

And

+ can't use IS NULL & IS NOT NULL clause.

+ can't use Like operator.

 

Regards,

Marul.

 

 

 

----- Original Message -----

From: Marul Mehta

To: Multiple recipients of list ORACLE-L

Sent: Saturday, August 31, 2002 6:33 PM

Subject: Re: Function-Based Index not working

 

Hi Naveen,

Thanks a lot for the efforts you are putting in for me for such a simple problem, but unfortunately, for me all the tips and tricks are not solving the problem.

Now these are my current statistics :-

 

+ alter session set QUERY_REWRITE_ENABLED=TRUE;
+ alter session set QUERY_REWRITE_INTEGRITY=TRUSTED;
+ alter session set optimizer_mode=FIRST_ROWS;
+ alter session set DB_FILE_MULTIBLOCK_READ_COUNT=1;

 

This procedure writes 180,000 records in employeees table

+ execute bulk_insert

 

Analyzing table and rebuilding index (though its not necessary)

+ analyze table employees compute statistics;

+ alter index upper_ix rebuild;

Making autotrace on

+ set autotrace traceonly explain

 

Fired the query:

SELECT last_name FROM employees WHERE UPPER(last_name) IS NOT NULL  ORDER BY UPPER(last_name);
Elapsed: 00:00:00.00

 

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=57 Card=4001 Bytes=2
          0005)

 

   1    0   SORT (ORDER BY) (Cost=57 Card=4001 Bytes=20005)
   2    1     TABLE ACCESS (FULL) OF 'EMPLOYEES' (Cost=38 Card=4001 By
          tes=20005)

 

Any clues what is happening? Should I insert more records in the table.

 

TIA,

Marul.

 

 

 

 

 

 

----- Original Message -----

From: Naveen Nahata

To: Multiple recipients of list ORACLE-L

Sent: Saturday, August 31, 2002 4:58 PM

Subject: RE: Function-Based Index not working

 

See the table's size is very small. Till it atleast 2 times the value of DB_BLOCK_SIZE * DB_FILE_MULTIBLOCK_READ_COUNT it will not use index.

 

Set the value of DB_FILE_MULTIBLOCK_READ_COUNT to one.

 

Insert lots of values in the table. You can make a procedure to insert random characters into the table, and then put it in a big loop. Analyze table and thn run the same query.

 

It should work 

 

naveen

 -----Original Message-----
From: Marul Mehta [mailto:marul@zycus.com]
Sent: Saturday, August 31, 2002 4:03 PM
To: Multiple recipients of list ORACLE-L
Subject: Re: Function-Based Index not working

Thanks a lot Naveen,

 

Even after executing the following the execution plan shows full table scan :-

 

+ alter session set QUERY_REWRITE_ENABLED=TRUE;
+ alter session set QUERY_REWRITE_INTEGRITY=TRUSTED;
+ alter session set optimizer_mode=FIRST_ROWS;

+ Insert into employees values('A');

+ Insert into employees values('B');

+ analyze table employees compute statistics;

+

    select last_name
   FROM employees WHERE UPPER(last_name) IS NOT NULL
   ORDER BY UPPER(last_name);  2    3
Elapsed: 00:00:00.00

 

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=FIRST_ROWS (Cost=3 Card=2 Bytes=2
          )

 

   1    0   SORT (ORDER BY) (Cost=3 Card=2 Bytes=2)
   2    1     TABLE ACCESS (FULL) OF 'EMPLOYEES' (Cost=1 Card=2 Bytes=
          2)

 

Even after using the hint no change in the plan :-

+ select /* INDEX employees(upper_ix) */ last_name FROM employees WHERE UPPER(last_name) IS NOT NULL;

 

Please tell me what else should I do to make this query use the index which is created.

 

 

TIA,

Marul.

 

----- Original Message -----

From: Naveen Nahata

To: Multiple recipients of list ORACLE-L

Sent: Saturday, August 31, 2002 3:03 PM

Subject: RE: Function-Based Index not working

 

Marul,

 

1. you don't have table analyzed in which case Rule based optimizer will be used. CBO is used if atleast one of the tables in the query is ANALYZED

2. There is no data in your table. Optimizer goes for a full tablescan if it thinks that it will be moer advisable to do a full table scan. e.g. You will not use the INDEX if your book has only one page.

 

The decision of going for a full tablescan is based on DB_BLOCK_SIZE * DB_FILE_MULTI_BLOCK_READCOUNT, which tells how much data Oracle fetches at one time. If your entire table can be fetched in atleast 2 fetches, full table scan will be done instead of INDEX scan, to avoid doubling of work.

 

 

Naveen

-----Original Message-----
From: Marul Mehta [mailto:marul@zycus.com]
Sent: Saturday, August 31, 2002 2:18 PM
To: Multiple recipients of list ORACLE-L
Subject: Function-Based Index not working

Hi,

 

Can you please help me out in solving this weird problem of funcation-based index not being used when I query the table.

This is the comand I fired and the result it returned me.

 

1. SQL> create table employees  (last_name varchar2(20));

    Table created.

 

2. SQL> CREATE INDEX upper_ix ON employees (UPPER(last_name));

    Index created.

 

Made the autotrace on and than:-

 

3. SELECT last_name FROM employees WHERE UPPER(last_name) IS NOT NULL  ORDER BY UPPER(last_name);

    no rows selected.

 

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (ORDER BY)
   2    1     TABLE ACCESS (FULL) OF 'EMPLOYEES'

 

 

I fired without order by clause also but no use.

 

Now can any body please let tell me why this Oracle is having a full scan of the employee table.

 

TIA,

Marul.

 

   

 

 

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