Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 9 ■ REDO AND UNDO 323 Investigating Undo We’ve already discussed a lot of undo segment topics. We’ve seen how they are used during recovery, how they interact with the redo logs, and how they are used for consistent, nonblocking reads of data. In this section, we’ll look at the most frequently raised issues with undo segments. The bulk of our time will be spent on the infamous ORA-01555: snapshot too old error, as this single issue causes more confusion than any other topic in the entire set of database topics. Before we do this, however, we’ll investigate one other undo-related issue in the next section: the question of what type of DML operation generates the most and least undo (you might already be able to answer that yourself, given the preceding examples with temporary tables). What Generates the Most and Least Undo? This is a frequently asked but easily answered question. The presence of indexes (or the fact that a table is an index-organized table) may affect the amount of undo generated dramatically, as indexes are complex data structures and may generate copious amounts of undo information. That said, an INSERT will, in general, generate the least amount of undo, since all Oracle needs to record for this is a rowid to “delete.” An UPDATE is typically second in the race (in most cases). All that needs to be recorded are the changed bytes. It is most common that you UPDATE some small fraction of the entire row’s data. Therefore, a small fraction of the row must be remembered in the undo. Many of the previous examples run counter to this rule of thumb, but that is because they update large, fixed-sized rows, and they update the entire row. It is much more common to UPDATE a row and change a small percentage of the total row. A DELETE will, in general, generate the most undo. For a DELETE, Oracle must record the entire row’s before image into the undo segment. The previous temporary table example, with regard to redo generation, demonstrated that fact: the DELETE generated the most redo, and since the only logged element of the DML operation on a temporary table is the undo, we in fact observed that the DELETE generated the most undo. The INSERT generated very little undo that needed to be logged. The UPDATE generated an amount equal to the before image of the data that was changed, and the DELETE generated the entire set of data written into the undo segment. As previously mentioned, you must also take into consideration the work performed on an index. You will find that an update of an unindexed column not only executes much faster, but will tend to generate significantly less undo than an update of an indexed column. For example, we’ll create a table with two columns both containing the same information and index one of them: ops$tkyte@ORA10G> create table t 2 as 3 select object_name unindexed, 4 object_name indexed 5 from all_objects 6 / Table created.
324 CHAPTER 9 ■ REDO AND UNDO ops$tkyte@ORA10G> create index t_idx on t(indexed); Index created. ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats(user,'T'); PL/SQL procedure successfully completed. Now we’ll update the table, first updating the unindexed column and then the indexed column. We’ll need a new V$ query to measure the amount of undo we’ve generated in each case. The following query accomplishes this for us. It works by getting our session ID (SID) from V$MYSTAT, using that to find our record in the V$SESSION view, and retrieving the transaction address (TADDR). It uses the TADDR to pull up our V$TRANSACTION record (if any) and selects the USED_UBLK column—the number of used undo blocks. Since we currently are not in a transaction, the query is expected to return 0 rows right now: ops$tkyte@ORA10G> select used_ublk 2 from v$transaction 3 where addr = (select taddr 4 from v$session 5 where sid = (select sid 6 from v$mystat 7 where rownum = 1 8 ) 9 ) 10 / no rows selected This is the query I’ll use after each UPDATE, but I won’t repeat it in the text—I’ll just show the results. Now we are ready to perform the updates and measure the number of undo blocks used by each: ops$tkyte@ORA10G> update t set unindexed = lower(unindexed); 48771 rows updated. ops$tkyte@ORA10G> select used_ublk ... 10 / USED_UBLK ---------- 401 ops$tkyte@ORA10G> commit; Commit complete. That UPDATE used 401 blocks to store its undo. The commit would “free” that up, or release it, so if we rerun the query against V$TRANSACTION it would once again show us no rows selected. When we update the same data—only indexed this time—we’ll observe the following:
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 13 ■ ■ ■ Partitioning
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