Question

How to delete duplicate rows from a big table

Asked by: stamese

Hello, happy to join the club.
I'm trying to delete 100000 duplicate rows from one Oracle table.
For each row it appears twice, identical to itself.

I tested a small SQL script as the following :
DELETE FROM TABLEDUMMY target WHERE ROWID > (
 SELECT min(rowid) FROM TABLEDUMMY source
  WHERE target.OJTCID = source.OJTCID);

I'ts working fine with a dummy table of 20 records.
But in real time, with my 200000 records, It never finish the task.
How can I do ? Thank you in advance.
Serge

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Asked On
2003-05-04 at 05:38:11ID20605063
Tags

duplicate

,

oracle

,

delete

,

table

,

rows

Topic

Oracle Database

Participating Experts
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Answers

 

by: andrewstPosted on 2003-05-04 at 07:05:32ID: 8455080

You will want an index on OJTCID, is there one?

Since ALL your data is duplicated, it may be easier to do this:

CREATE TABLE new_tabledummy AS
SELECT DISTINCT .....
FROM tabledummy;

Then drop the old table and rename the new one.

And for goodness sake, put a primary key constraint on OJTCID this time!

 

by: ttokerPosted on 2003-05-04 at 07:07:57ID: 8455092

if you do not need partial data in the table you can just
truncate table table_name drop storage;

this will wipe out all the data in the table without using rollbacks.

if you have to remove 200 000 records and concerned about rollbacks you can write a loop:
LOOP
  DELETE FROM table_name WHERE condition
  AND ROWNUM <= 10000;
  COMMIT;
  EXIT WHEN SQL%rowcount < 10000;
END LOOP;

or
DELETE FROM table WHERE condition
 AND ROWNUM <= 10000;
commit;
DELETE FROM table WHERE condition
 AND ROWNUM <= 10000;
commit;

and keep going until all are deleted

thanks

 

by: ttokerPosted on 2003-05-04 at 07:10:08ID: 8455103

oh wait, i think i misunderstood, andrewst is right

 

by: steveyamPosted on 2003-05-04 at 12:08:41ID: 8456212

It is a self-table join with out index. The operation is n * n order. You have 20,000 rows. Then the database need to scan full these 20,000 rows for each row to delete.

1. Create non-unique index on OTCID should solve your case. And there is no harm to your existing logical DB structure.

2. Never consider to create new table. It may screw up the referencing and triggers among tables.

3. If you are not allowed to create non-unique index, try

delete dummytable where (OTCID, rowid) not in
(select OTCID, min(rowid) from dummytable group by OTCID);

This is 2 * n operation.

Good luck,
Steve

 

by: andrewstPosted on 2003-05-04 at 15:49:55ID: 8457217

> 2. Never consider to create new table. It may screw up the referencing and triggers among tables.

It already is screwed up!  And there can't be any child tables referencing this one, since there is no unique/primary key to reference against.

 

by: asimkovskyPosted on 2003-05-04 at 16:18:44ID: 8457394

I agree with andrewst.  A simple CTAS is the best and fastest way to go.  You can run it with NOLOGGING and in parallel to speed it up.




Andrew

 

by: vanmeerendonkPosted on 2003-05-05 at 00:12:37ID: 8459613

Create an unique constraint on the column and use the "exceptions into".
Full power to Thomas Kyte:

http://asktom.oracle.com/pls/ask/f?p=4950:8:125789827938754463::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:4274289639999,

 

by: schwertnerPosted on 2003-05-05 at 02:52:47ID: 8460553

I think that the delay is caused by the MIN grouping function you use.
In the following script no grouping functions are used, so may be it will run faster.

You can use the following script to remove duplicate rows from a table:
delete from &&table_name t1
where t1.&&column_name in (select &&column_name
from &&table_name t2
where t1.rowid > t2.rowid
and t1.&&column_name = t2.&&column_name
)
/
undefine table_name
undefine column_name
The script will prompt for a table name and a column name from which duplicate values are to be removed. It will then remove all rows with ROWID values higher than the lowest ROWID for this particular column.
When using more than 1 column to define uniqueness for the table, augment the script to contain cseveral "&&column_name" variables (of course using
unique variable names).

20120131-EE-VQP-002

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