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CHAPTER 9 REDO AND UNDO
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fault, but it is your problem since you hit it. It s the same as if you run out of temporary space during a query. You either configure sufficient temporary space for the system, or you rewrite the queries so they use a plan that does not require temporary space. To demonstrate this effect, we can set up a small, but somewhat artificial test. We ll create a very small undo tablespace with one session that will generate many small transactions, virtually assuring us that it will wrap around and reuse its allocated space many times regardless of the UNDO_RETENTION setting, since we are not permitting the undo tablespace to grow. The session that uses this undo segment will be modifying a table, T. It will use a full scan of T and read it from top to bottom. In another session, we will execute a query that will read the table T via an index. In this fashion, it will read the table somewhat randomly: it will read row 1, then row 1,000, then row 500, then row 20,001, and so on. In this way, we will tend to visit blocks very randomly and perhaps many times during the processing of our query. The odds of getting an ORA-01555 error in this case are virtually 100 percent. So, in one session we start with the following: ops$tkyte%ORA11GR2> create undo tablespace undo_small 2 datafile '/tmp/undo.dbf' size 2m 3 autoextend off 4 / Tablespace created. ops$tkyte%ORA11GR2> alter system set undo_tablespace = undo_small; System altered. Now, we ll set up the table T to query and modify. Note that we are ordering the data randomly in this table. The CREATE TABLE AS SELECT tends to put the rows in the blocks in the order it fetches them from the query. We ll just scramble the rows up so they are not artificially sorted in any order, randomizing their distribution: ops$tkyte%ORA11GR2> create table t 2 as 3 select * 4 from all_objects 5 order by dbms_random.random; Table created. ops$tkyte%ORA11GR2> alter table t add constraint t_pk primary key(object_id) 2 / Table altered. ops$tkyte%ORA11GR2> exec dbms_stats.gather_table_stats( user, 'T', cascade=> true ); PL/SQL procedure successfully completed. And now we are ready to do our modifications: ops$tkyte@ORA11GR2> begin 2 for x in ( select rowid rid from t ) 3 loop 4 update t set object_name = lower(object_name) where rowid = x.rid; 5 commit; 6 end loop; 7 end; 8 /
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CHAPTER 9 REDO AND UNDO
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Now, while that PL/SQL block of code is running, we will run a query in another session. That other query will read table T and process each record. It will spend about 1/100 of a second processing each record before fetching the next (simulated using DBMS_LOCK.SLEEP(0.01)). We will use the FIRST_ROWS hint in the query to have it use the index we created to read the rows out of the table via the index sorted by OBJECT_ID. Since the data was randomly inserted into the table, we would tend to query blocks in the table rather randomly. This block will only run for a couple of seconds before failing: ops$tkyte%ORA11GR2> declare 2 cursor c is 3 select /*+ first_rows */ object_name 4 from t 5 order by object_id; 6 7 l_object_name t.object_name%type; 8 l_rowcnt number := 0; 9 begin 10 open c; 11 loop 12 fetch c into l_object_name; 13 exit when c%notfound; 14 dbms_lock.sleep( 0.01 ); 15 l_rowcnt := l_rowcnt+1; 16 end loop; 17 close c; 18 exception 19 when others then 20 dbms_output.put_line( 'rows fetched = ' || l_rowcnt ); 21 raise; 22 end; 23 / rows fetched = 301 declare * ERROR at line 1: ORA-01555: snapshot too old: rollback segment number 46 with name "_SYSSMU46_2555494716$" too small ORA-06512: at line 21 As you can see, it got to process only 301 records before failing with the ORA-01555: snapshot too old error. To correct this, we want to make sure two things are done: UNDO_RETENTION is set in the database to be at least long enough for this read process to complete. That will allow the database to grow the undo tablespace to hold sufficient undo for us to complete. The undo tablespace is allowed to grow or you manually allocate more disk space to it.
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For this example, I have determined my long-running process takes about 720 seconds to complete (I have about 72,000 records in the table, so at 0.01 seconds per row we have 720 seconds). My UNDO_RETENTION is set to 900 (this is in seconds, so the undo retention is about 15 minutes). I altered the undo tablespace s data file to permit it to grow by 1MB at a time, up to 2GB in size:
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