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CHAPTER 9 REDO AND UNDO
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behave in general just like normal tables, temporary tables must generate undo. Since undo data must be logged, temporary tables will generate some redo log for the undo they generate. This is not nearly as ominous as it seems. The primary SQL statements used against temporary tables are INSERTs and SELECTs. Fortunately, INSERTs generate very little undo (you need to restore the block to nothing, and it doesn t take very much room to store nothing ), and SELECTs generate no undo. Hence, if you use temporary tables for INSERTs and SELECTs exclusively, this section means nothing to you. It is only if you UPDATE or DELETE that you might be concerned about this. I set up a small test to demonstrate the amount of redo generated while working with temporary tables, an indication therefore of the amount of undo generated for temporary tables, since only the undo is logged for them. To demonstrate, I ll take identically configured permanent and temporary tables, and then perform the same operations on each, measuring the amount of redo generated each time. The tables I ll use are as follows: ops$tkyte%ORA11GR2> create table perm 2 ( x char(2000) , 3 y char(2000) , 4 z char(2000) ) 5 / Table created. ops$tkyte%ORA11GR2> create global temporary table temp 2 ( x char(2000) , 3 y char(2000) , 4 z char(2000) ) 5 on commit preserve rows 6 / Table created. I set up a small stored procedure to allow me to perform arbitrary SQL and report the amount of redo generated by that SQL. I ll use this routine to perform INSERTs, UPDATEs, and DELETEs against both the temporary and permanent tables: ops$tkyte%ORA11GR2> create or replace procedure do_sql( p_sql in varchar2 ) 2 as 3 l_start_redo number; 4 l_redo number; 5 begin 6 l_start_redo := get_stat_val( 'redo size' ); 7 8 execute immediate p_sql; 9 commit; 10 11 l_redo := get_stat_val( 'redo size' ) - l_start_redo; 12 13 dbms_output.put_line 14 ( to_char(l_redo,'99,999,999') ||' bytes of redo generated for "' || 15 substr( replace( p_sql, chr(10), ' '), 1, 25 ) || '"...' ); 16 end; 17 / Procedure created.
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CHAPTER 9 REDO AND UNDO
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Then I ran equivalent INSERTs, UPDATEs, and DELETEs against both the PERM and TEMP tables: ops$tkyte%ORA11GR2> set serveroutput on format wrapped ops$tkyte%ORA11GR2> 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 / 3,330,636 bytes of redo generated for "insert into perm 68,492 bytes of redo generated for "insert into temp 2,519,848 bytes of redo generated for "update perm set x = 2"... 1,106,960 bytes of redo generated for "update temp set x = 2"... 3,258,620 bytes of redo generated for "delete from perm"... 3,245,180 bytes of redo generated for "delete from temp"... As you can see, The INSERT into the real table generated a lot of redo, while 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 each 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 statement,
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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 and above.
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