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CHAPTER 9 REDO AND UNDO
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Therefore, the following generalizations can 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 with a permanent table. An UPDATE will generate about half the redo as with a permanent table.
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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 d experience with a permanent table. In addition, you must consider any indexes in place on your temporary tables. Index modifications will also generate undo which in turn generates redo. If you rerun the above example with these two indexes in place: ops$tkyte%ORA11GR2> create index perm_idx on perm(x); Index created. ops$tkyte%ORA11GR2> create index temp_idx on temp(x); Index created. You will find redo generated in the order of (for brevity, all of the code from the previous example is not repeated here): 19 end; 20 / 12,107,704 bytes of redo generated for "insert into perm 3,369,336 bytes of redo generated for "insert into temp 9,694,752 bytes of redo generated for "update perm set x = 2"... 5,415,832 bytes of redo generated for "update temp set x = 2"... 4,412,408 bytes of redo generated for "delete from perm"... 4,389,828 bytes of redo generated for "delete from temp"... PL/SQL procedure successfully completed. The numbers hold true from what we saw before but you can see that the index definitely added to the redo generated. The INSERT into the global temporary table went from generating almost no redo to generating 3.3MB of redo. All of this additional redo was related to the undo produced for the index maintenance. "... "...
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Note This is an exaggerated example. The index in question was on a CHAR(2000) column; the index key is
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much larger than you ll normally see in real life. Don t expect this much additional redo typically.
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CHAPTER 9 REDO AND UNDO
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In general, common sense prevails in estimating 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 terminates. All of these methods generate no undo and, therefore, no redo. You should try to avoid updating a temporary table unless you really have to for some reason. You should use temporary tables mostly as something to be INSERTed into and SELECTed from. In this fashion, you ll make optimum use of their unique ability to not generate redo.
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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, non-blocking 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: 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).
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