You will see a lot of Oracle examples on the internet using the tables from the SCOTT schema. For options 1 -3, see the appropriate information for that option. Zero is less than 'something' isn't it? Data Definition Language (DDL) commands perform an implicit commit, which also confirms all outstanding DML changes in the current session. 18c |
From what I understand, TRUNCATE TABLE resets the High Water Mark and according to some discussions and information on the web creates a small amount of undo. We are in archivelog mode.Is it possible using logminer ? This Undo information cannot be used by the user to ROLLBACK a TABLE TRUNCATE, which also applies to other DDL statements, but can be required by the instance to perform database crash recovery. Restore and recover the primary database to a point in time before the drop. only for educational reasons, how that can be verified is my interest currently. As far as I understand, that's a fundamental difference between DML and DDL. P.S. The TRUNCATE TABLE statement can remove the storage associated with the table, or leave it to be reused later. The following example deletes all the rows from the EMPLOYEES table, then issues a ROLLBACK to cancel the deletion. This option should ONLY be used if the RMAN duplicate command cannot. Our standby was reset a year ago So no more standby. 2. You won't see it very often, but you can also delete via inline views.
Software in Silicon (Sample Code & Resources).
What is correct? This is the RECOMMENDED method for recovering a table (or data) as it has very little affect on the rest of the objects within the database. . SQL> select name, value from V$mystat ms, v$statname sn where sn.statistic# = ms.statistic# and sn.name like '%undo%' and ms.value != 0; NAME VALUE, ---------------------------------------------------------------- ----------, undo change vector size 192, undo change vector size 31272. As a result, you can't test for failure to delete rows using the NO_DATA_FOUND exception in PL/SQL, as it doesn't get raised. No - I didn't miss it. At the end of the recovery process, uncommitted transactions are rolled back, probably cleaning some information and updating some data headers. This is because of the timestamp check. Several statements can be grouped together to form a single transaction. We used it for disaster manual recovery, becuase we only have 5 table to export and import to DR and. But it seems very clear that table truncate creates no undo that could be used for a user managed rollback. 21c |
All Data Manipulation Language (DML) changes are done as part of a transaction. The responses I have marked as helpful are those responses that I think provided good explanations or added very interesting new aspects or information to the topic. Is there a simply way to query or compare the undo table before and after a truncate table operation to see the difference? Sorry our standby was reset. See: RMAN RECOVER TABLE Feature New to Oracle Database 12c . The WHERE clause allows you to limit the rows to be deleted. I believe it does, but at the moment i cannot prove that to myself. 19c |
The undo that would have been generated by a delete would have been in the order of a hundred and twenty megabytes - but you'll have to take my word for that. Once the duplicate is up and running just datapump out the data you want and load back in to production. I think the only course of action open is to restore the live database somewhere else somehow and then extract the table from there. Note that you'd need to do a FLASHBACK DATABASE, not just a FLASHBACK TABLE in order to restore a table to a point prior to DDL. Maybe data wasn't changed on standby. How will the recovery process recognize that a undo of a DDL was necessary? Starting with Oracle 11gR2 (22.214.171.124), a TRUNCATE statement can also specify the DROP ALL STORAGE clause to release the space currently allocated for a table to the containing tablespace. Since TABLE TRUNCATE modifies metadata in the Data Dictionary and resets the High Water Mark of the object, it produces much less Undo than DML statements like DELETE. Articles |
You can find the original table definitions in the "$ORACLE_HOME/rdbms/admin/utlsampl.sql" script. The DELETE statement is used to remove rows from the table. See: Flashback Table feature in Oracle Database 10g. There would be a need to roll back to the point of the first commit. It's possible to delete from the base table associated with a view. If recovery finds a SCN in a datafile for which there is no corresponding record in the redo log, the information is cleaned or rolled back using the appropriate undo data. Bottom line - people often say that "truncate" generates no undo, what they really mean is that it generates no undo for the data although it still generates relatively tiny amount of undo for the meta-data. Scripts |
How do I check If flashback is opened in our Prod db? Looks like there is no real easy way to verify what exactly happens. It is a DDL statement and cannot be rolled back. Some tools and programming languages default to auto-commit, so they automatically issue a COMMIT statement after every DML statement they process. This is the standard tablespace point in time recovery (TSPITR) which will require an auxiliary database, but will bring all the data of the tablespace back in time in the target database. Quote: A TRUNCATE statement does not generate any undo information and it commits immediately. This is a better option, but again, it takes the entire tablespace's data back in time. Do changes to REDO get written into UNDO? In Oracle10g and higher, you can use the Recyclebin to recover a DROP table assuming it has not been explicitly disabled. There is NOTHING in that article that says TRUNCATE generates ANY undo at all. Thanks Satyaki. Note, in 12c RMAN has an option for 'recover table'. Comparing just the size does not. PL/SQL |
because I don't want to restore 1Tb of our PROD database to recovery server. tried to trace events 100054 and 10055 but i cannot recognize undo activities (don't know what to look for). 11g |
So when there is recovery, the information from the redo log is recovered. About, About Tim Hall
No command is "replayed" during recovery. Yes, the TRUNCATE has not changed the table structure(ie, the columns or their datatypes), but the system time has changed since the last snapshot was taken. Once committed, the only way you can revert a change is to issue a new DML statement to alter the data. Linux. This document describes how to recover from a DROP/TRUNCATE/DELETE or any type of data mistake you need to recover using RMAN. These tables are a variant of the EMP and DEPT tables from the SCOTT schema. The auxiliary database can be a subset of the tablespaces so only necessary tablespaces are restored and recovered. 13c |
Instead, you must manually test for the number of rows deleted using SQL%ROWCOUNT. Restore and recover a subset of the database as a DUMMY database to export the table data and import it into the primary database. You can perform all these queries online for free using SQL Fiddle.
This tiny amount of undo would have been related to a few space management blocks being cleared, the segment header being updated, and possibly some updates to the data dictionary (your tab_stats$ changes). However, the Oracle 11gR2 documentation at Truncating Tables and Clusters clearly outlines that it does not generate any undo. WebLogic |
SQL> select flashback_on from v$database; Flashback query will not work as it cannot back to before the truncate operation happened. Copyright & Disclaimer, SQL for Beginners (Part 10) : The DELETE and TRUNCATE TABLE Statements, Database SQL Language Reference : TRUNCATE TABLE. Using the TRUNCATE statement provides a fast, efficient method for deleting all rows from a table or cluster. Use RMAN duplicate to create a subset of the database as a clone to the point in time before the drop. The need is to roll back to the point of the first commit, which simply means undoing the small changes Jonathan listed. If the Recyclebin has been disabled or Database is running Oracle9i, there are several options available: 1. My Oracle Support provides customers with access to over a million knowledge articles and a vibrant support community of peers and Oracle experts. Since DDL's create an entry in the redo before being executed and have an implicit commit, why would there be a need for rollback, and instead not simply be a replay of the DDL? A TRUNCATE statement does not affect any structures associated with the table being truncated (constraints and triggers) or authorizations. This can be used to control the number of rows deleted, rather than using a filter in the WHERE clause of the DELETE statement itself. FLASHBACK is in 10G so if i am using prior DB version it is not possible ? Videos |
Flashback and Flachback Query are different things. AskTom mentions that "if the machine crashed in the middle of doing DDL, we need to be able to undo that operation when we recover.".
No, you need to restore from backup or use the FLASHBACK feature depending on your db version. Flashback query example, you can change "minute" part. If you decide you don't want to keep some uncommitted changes, you can throw them away using using the ROLLBACK statement. This can prove rather confusing to beginners. My simulation illustrate what Justin has already said. I think question is, does truncate generate any undo from anything. As far as I know, during recovery, everything that has been committed is recovered using redo. Before anyone points out that there are always special cases - truncate of a table in a cluster is implemented as a delete. They are not permanent until they are confirmed using the COMMIT statement.
edit: i'm sure undo is not generated from that truncated table. However there are other examples: http://stackoverflow.com/questions/2701879/drop-truncate-and-rollback-segment, https://asktom.oracle.com/pls/asktom/f?p=100:11:0%3a%3a%3a%3aP11_QUESTION_ID:2860243091362. You can mkae use of this syntax to skip all the tablespaces you dont need and only restore the tablespace containing the table you want, DUPLICATE TARGET DATABASE TO dupdb SKIP TABLESPACE tools. Without a WHERE clause, all rows in the table are deleted by a single statement. Well, from what I understand, the recovery process uses redo logs to roll the database forward, which in turn also rebuilds undo. The examples in this article require the following tables to be present. A TRUNCATE statement does not generate any undo information and it commits immediately. See for details. To prove that some of the undo related to the space management blocks for the table (and its indexes) you'd probably have to switch log files just before the truncate, then truncate, then dump the current log file to get a minimal log file dump that you could review to see which blocks had been affected by redo and which bits of the redo had matching (or, rather, reversing) entries created in the undo. But if I truncate a table, how come I can get it . 2. 8i |
In the following example, we check the number of rows in the table, issue the TRUNCATE TABLE statement, immediately ROLLBACK and check the number of rows in the table again. 12c |
And if I'm not mistaken, any DML or DDL statement needs to create an entry in the redo log first, before being executed. You can always use Flachback Query regarding of whether flashback is open or not. I mean it was aborted due to constant error which caused by so many big transaction archivelogs which our network bandwidth can not handle. A deletion of zero rows is a valid deletion, and does not result in an error. It is not. According to http://docs.oracle.com/cd/B19306_01/server.102/b14220/transact.htm. In this article we take a look at the DELETE and TRUNCATE TABLE Statements. Software in Silicon (Sample Code & Resources), http://docs.oracle.com/cd/B19306_01/server.102/b14220/transact.htm. If you happen to find this question interesting and wish to learn more about the redo and undo process in regard to DDL statements, some of the responses I received where very resourceful and provided information that you many not know or won't find in the usual documentation. That requires UNDO, not REDO - except for any REDO needed to recreate the UNDO. If a crash happens in the middle of table truncate, the table is either there or empty depending on what can be recovered from the redo log anyway. Does TRUNCATE really create no UNDO and the implicit COMMIT is perhaps just only a safeguard, or what is the real story here? This is an extreme measure for one table as the entire database data goes back in time. If you want to remove all the rows from a table, the TRUNCATE TABLE statement is a lot more efficient than the DELETE statement. I can only mark one answer as correct and simply took the first response that provided an answer to my initial question. A TRUNCATE statement also specifies whether space currently allocated for the table is returned to the containing tablespace after truncation.In previous release 11gR1 when a truncate was executed Oracle keeps the segment associated with a table. The answer, put simply, is that TABLE TRUNCATE produces Undo. Thanks to everyone for your time and contributions! Oracle 11gR1 does provide the DROP STORAGE and REUSE STORAGE clauses but theDROP STORAGE clause only drops extents beyond the minimum extents. Thisdocument will deal with the fourth option which should ONLY be used when the first three are not possible. I tried a couple of tests, playing with the employee_demo table, but I have not been able so far to find out what changed in the UNDO tablespace. As SamB has already mentioned: restore it from your latest backup, if possible. I cannot see why there should be any undo of a DDL necessary. Many of the examples in this article will issue ROLLBACK statements after the test, to revert the data to its original state. You'd then be able to see exactly what had happened to the table (and index) blocks, and how much of that action had been protected by undo. How do I recover small table which was accidentally truncated 3 hours ago? I recommend you use this method as soon as possible on standby because undo data will be overwriiten. We have standby but it was already opened resetlogs for testing. 3. SQL |
If for some reason you are unable to use RMAN duplicate, there is a forth option: 4. Personally i would do an rman duplicate of the live system to another box (to a point in time earlier than the truncate). Restore and recover a subset of the database: d. Restore of the necessary tablespaces, RESTORE TABLESPACE: g. RECOVER DATABASE SKIP FOREVER TABLESPACE ; h. ALTER DATABASE RENAME FILE all Online REDO log files: 3: Open auxiliary database with RESETLOGS. It is a DDL statement and cannot be rolled back. General overview of procedure to recover from a DROP or TRUNCATE table by using RMAN. But maybe that's a type and should read "redo". Software in Silicon (Sample Code & Resources). Unfortunately I'm not a big expert on PL/SQL and don't use it very much. In the following example, we create a simple view on the EMPLOYEES table, then delete via it. Certification |
For example, http://www.orafaq.com. Home |
The caller who said "TRUNCATE" isn't connected anymore, so how could Oracle notify him that the command finally succeeded? TRUNCATE does apparently do a COMMIT before and after it executes, which would explain why there is no ROLLBACK. However, it would probably not be Oracle database if everything was quite that simple. This is the tenth part of a series of articles showing the basics of SQL. DDLs have two implicit commits, one before and one after. Once RMAN creates the clone and opens the database, the table can be exported from the auxiliary database and imported back into production. The TRUNCATE TABLE statement is a DDL command, so it includes an implicit COMMIT, so there is no way to issue a ROLLBACK if you decide you didn't want to remove the rows. My Oracle Support provides customers with access to over a million knowledge articles and a vibrant support community of peers and Oracle experts. There are some restrictions associated with this, but they are a little out of scope for a beginner level article. we can manage to reroute our PROD there using export/import of the 5 master tables. Hovever, DDL statements have an implicit commit and require an entry in the redo log before being executed. Thanks for the response. even if changed, you can use flashback query on your standby to recover your data. This is all automated in 12.1 for you but that doesnt help you here. Don't let this fool you into thinking this is default behaviour. 9i |
You will see from the output, the ROLLBACK does not cancel the TRUNCATE TABLE statement. Restore and recover the tablespace to a point in time before the drop. With 11.2's new feature of targetless duplicate, the RMAN duplicate command can be used in most cases. RAC |
Here is the quote from the link in case you missed it: TRUCATE is faster and doesn't use as much undo space as a DELETE. I found out that after truncate there is happening updates and deletes, WAIT #127983856: nam='SQL*Net message to client'.