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Undo is conceptually the opposite of redo. Undo information is generated by the database as you make modifications to data to put it back the way it was before the modifications, in the event the transaction or statement you are executing fails for any reason or if you request it with a ROLLBACK statement. Whereas redo is used to replay a transaction in the event of failure to recover the transaction undo is used to reverse the effects of a statement or set of statements. Undo, unlike redo, is stored internally in the database in a special set of segments known as undo segments.
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CHAPTER 9 REDO AND UNDO
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Note Rollback segment and undo segment are considered synonymous terms. Using manual undo
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management, the DBA will create rollback segments. Using automatic undo management, the system will automatically create and destroy undo segments as necessary. These terms should be considered the same for all intents and purposes in this discussion.
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It is a common misconception that undo is used to restore the database physically to the way it was before the statement or transaction executed, but this is not so. The database is logically restored to the way it was any changes are logically undone but the data structures, the database blocks themselves, may well be different after a rollback. The reason for this lies in the fact that, in any multiuser system, there will be tens or hundreds or thousands of concurrent transactions. One of the primary functions of a database is to mediate concurrent access to its data. The blocks that our transaction modifies are, in general, being modified by many other transactions as well. Therefore, we cannot just put a block back exactly the way it was at the start of our transaction that could undo someone else s work! For example, suppose our transaction executed an INSERT statement that caused the allocation of a new extent (i.e., it caused the table to grow). Our INSERT would cause us to get a new block, format it for use, and put some data on it. At that point, some other transaction might come along and insert data into this block. If we were to roll back our transaction, obviously we cannot unformat and unallocate this block. Therefore, when Oracle rolls back, it is really doing the logical equivalent of the opposite of what we did in the first place. For every INSERT, Oracle will do a DELETE. For every DELETE, Oracle will do an INSERT. For every UPDATE, Oracle will do an anti-UPDATE, or an UPDATE that puts the row back the way it was prior to our modification.
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Note This undo generation is not true for direct path operations, which have the ability to bypass undo
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generation on the table. We ll discuss these in more detail shortly.
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How can we see this in action Perhaps the easiest way is to follow these steps: 1. Create an empty table. 2. Full scan the table and observe the amount of I/O performed to read it. 3. Fill the table with many rows (no commit). 4. Roll back that work and undo it. 5. Full scan the table a second time and observe the amount of I/O performed.
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CHAPTER 9 REDO AND UNDO
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First, we ll create an empty table: ops$tkyte@ORA10G> create table t 2 as 3 select * 4 from all_objects 5 where 1=0; Table created. And then we ll query it, with AUTOTRACE enabled in SQL*Plus to measure the I/O.
Note In this example, we will full scan the tables twice each time. The goal is to only measure the I/O
performed the second time in each case. This avoids counting additional I/Os performed by the optimizer during any parsing and optimization that may occur.
The query initially takes three I/Os to full scan the table: ops$tkyte@ORA10G> select * from t; no rows selected ops$tkyte@ORA10G> set autotrace traceonly statistics ops$tkyte@ORA10G> select * from t; no rows selected Statistics ---------------------------------------------------------0 recursive calls 0 db block gets 3 consistent gets ... ops$tkyte@ORA10G> set autotrace off Next, we ll add lots of data to the table. We ll make it grow but then roll it all back: ops$tkyte@ORA10G> insert into t select * from all_objects; 48350 rows created. ops$tkyte@ORA10G> rollback; Rollback complete. Now, if we query the table again, we ll discover that it takes considerably more I/Os to read the table this time: ops$tkyte@ORA10G> select * from t; no rows selected ops$tkyte@ORA10G> set autotrace traceonly statistics
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