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CHAPTER 9 REDO AND UNDO
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13 commit; 14 15 select v$mystat.value-l_start_redo 16 into l_redo 17 from v$mystat, v$statname 18 where v$mystat.statistic# = v$statname.statistic# 19 and v$statname.name = 'redo size'; 20 21 dbms_output.put_line 22 ( to_char(l_redo,'9,999,999') ||' bytes of redo generated for "' || 23 substr( replace( p_sql, chr(10), ' '), 1, 25 ) || '"...' ); 24 end; 25 / Procedure created. Then, I ran equivalent INSERTs, UPDATEs, and DELETEs against the PERM and TEMP tables: ops$tkyte@ORA10G> set serveroutput on format wrapped ops$tkyte@ORA10G> begin 2 do_sql( 'insert into perm 3 select 1,1,1 4 from all_objects 5 where rownum <= 500' ); 6 7 do_sql( 'insert into temp 8 select 1,1,1 9 from all_objects 10 where rownum <= 500' ); 11 dbms_output.new_line; 12 13 do_sql( 'update perm set x = 2' ); 14 do_sql( 'update temp set x = 2' ); 15 dbms_output.new_line; 16 17 do_sql( 'delete from perm' ); 18 do_sql( 'delete from temp' ); 19 end; 20 / 3,297,752 bytes of redo generated for "insert into perm 66,488 bytes of redo generated for "insert into temp 2,182,200 bytes of redo generated for "update perm set x = 2"... 1,100,252 bytes of redo generated for "update temp set x = 2"... 3,218,804 bytes of redo generated for "delete from perm"... 3,212,084 bytes of redo generated for "delete from temp"... PL/SQL procedure successfully completed.
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CHAPTER 9 REDO AND UNDO
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As you can see, The INSERT into the real table generated a lot of redo. Almost no redo was generated for the temporary table. This makes sense there is very little undo data generated for INSERTs and only undo data is logged for temporary tables. The UPDATE of the real table generated about twice the amount of redo as the temporary table. Again, this makes sense. About half of that UPDATE, the before image, had to be saved. The after image (redo) for the temporary table did not have to be saved. The DELETEs took about the same amount of redo space. This makes sense, as the undo for a DELETE is big, but the redo for the modified blocks is very small. Hence, a DELETE against a temporary table takes place very much in the same fashion as a DELETE against a permanent table.
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Note If you see the temporary table generating more redo than the permanent table with the INSERT
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statement, you are observing a product issue in the database that is fixed in at least Oracle 9.2.0.6 and 10.1.0.4 patch releases (the current shipping releases as of this writing).
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Therefore, the following generalizations may be made regarding DML activity on temporary tables: An INSERT will generate little to no undo/redo activity. A DELETE will generate the same amount of redo as a normal table. An UPDATE of a temporary table will generate about half the redo of an UPDATE of a normal table. There are notable exceptions to the last statement. For example, if I UPDATE a column that is entirely NULL with 2,000 bytes of data, there will be very little undo data generated. This UPDATE will behave like the INSERT. On the other hand, if I UPDATE a column with 2,000 bytes of data to be NULL, it will behave like the DELETE as far as redo generation is concerned. On average, you can expect an UPDATE against a temporary table to produce about 50 percent of the undo/redo you would experience with a real table. In general, common sense prevails on the amount of redo created. If the operation you perform causes undo data to be created, then determine how easy or hard it will be to reverse (undo) the effect of your operation. If you INSERT 2,000 bytes, the reverse of this is easy. You simply go back to no bytes. If you DELETE 2,000 bytes, the reverse is INSERTing 2,000 bytes. In this case, the redo is substantial. Armed with this knowledge, you will avoid deleting from temporary tables. You can use TRUNCATE (bearing in mind, of course, that TRUNCATE is DDL that will commit your transaction and, in Oracle9i and before, invalidate your cursors) or just let the temporary tables empty themselves automatically after a COMMIT or when your session terminated. All of these methods generate no undo and, therefore, no redo. You will try to avoid updating a temporary table unless you really have to for some reason. You will use temporary tables mostly as something to be INSERTed into and SELECTed from. In this fashion, you ll make optimum use of their
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