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CHAPTER 9 REDO AND UNDO
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Now, if we query the table again, we ll discover that it takes considerably more I/Os to read the table this time: ops$tkyte%ORA11GR2> select * from t; no rows selected ops$tkyte%ORA11GR2> set autotrace traceonly statistics ops$tkyte%ORA11GR2> select * from t; no rows selected Statistics ---------------------------------------------------------0 recursive calls 0 db block gets 1073 consistent gets 0 physical reads ...ORA11GR2> set autotrace off The blocks that our INSERT caused to be added under the table s high-water mark (HWM) are still there formatted, but empty. Our full scan had to read them to see if they contained any rows. Moreover, the first time we ran the query, we observed zero I/Os. That was due to the default mode of table creation in Oracle Database 11g Release 2 using deferred segment creation. When we issued that CREATE TABLE, no storage, not a single extent, was allocated. The segment creation was deferred until the INSERT took place, and when we rolled back, the segment persisted. You can see this easily with a smaller example, I ll explicitly request deferred segment creation this time although it is enabled by default in 11g Release 2: ops$tkyte%ORA11GR2> create table t ( x int ) 2 segment creation deferred; Table created. ops$tkyte%ORA11GR2> select extent_id, bytes, blocks 2 from user_extents 3 where segment_name = 'T' 4 order by extent_id; no rows selected ops$tkyte%ORA11GR2> insert into t(x) values (1); 1 row created. ops$tkyte%ORA11GR2> rollback; Rollback complete. ops$tkyte%ORA11GR2> select extent_id, bytes, blocks 2 from user_extents 3 where segment_name = 'T' 4 order by extent_id; EXTENT_ID BYTES BLOCKS ---------- ---------- ---------0 65536 8
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CHAPTER 9 REDO AND UNDO
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As you can see, after the table create there was no allocated storage no extents were used by this table. Upon performing an INSERT, followed immediately by ROLLBACK, we can see the INSERT allocated storage but the ROLLBACK does not release it. Those two things together that the segment was actually created by the INSERT but not uncreated by the ROLLBACK, and that the new formatted blocks created by the INSERT were scanned the second time around show that a rollback is a logical put the database back the way it was operation. The database will not be exactly the way it was, just logically the same.
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How Redo and Undo Work Together
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Now let s take a look at how redo and undo work together in various scenarios. We will discuss, for example, what happens during the processing of an INSERT with regard to redo and undo generation, and how Oracle uses this information in the event of failures at various points in time. An interesting point to note is that undo information, stored in undo tablespaces or undo segments, is protected by redo as well. In other words, undo data is treated just like table data or index data changes to undo generate some redo, which is logged. Why this is so will become clear in a moment when we discuss what happens when a system crashes. Undo data is added to the undo segment and is cached in the buffer cache, just like any other piece of data would be.
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Example INSERT-UPDATE-DELETE Scenario
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As an example, we will investigate what might happen with a set of statements like this: insert into t (x,y) values (1,1); update t set x = x+1 where x = 1; delete from t where x = 2; We will follow this transaction down different paths and discover the answers to the following questions: What happens if the system fails at various points in the processing of these statements What happens if we ROLLBACK at any point What happens if we succeed and COMMIT
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The INSERT
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The initial INSERT INTO T statement will generate both redo and undo. The undo generated will be enough information to make the INSERT go away. The redo generated by the INSERT INTO T will be enough information to make the INSERT happen again. After the INSERT has occurred, we have the scenario illustrated in Figure 9-1.
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