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The scenario is this: you have a system where the transactions are small. As a result, you need very little undo segment space allocated. Say, for example, the following is true: Each transaction generates 8KB of undo on average. You do five of these transactions per second on average (40KB of undo per second, 2,400KB per minute). You have a transaction that generates 1MB of undo that occurs once per minute on average. In total, you generate about 3.5MB of undo per minute. You have 15MB of undo configured for the system.
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CHAPTER 9 REDO AND UNDO
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That is more than sufficient undo for this database when processing transactions. The undo segments will wrap around and reuse space about every three to four minutes or so, on average. If you sized undo segments based on your transactions that do modifications, you did all right. In this same environment, however, you have some reporting needs. Some of these queries take a really long time to run five minutes, perhaps. Here is where the problem comes in. If these queries take five minutes to execute and they need a view of the data as it existed when the query began, you have a very good probability of the ORA-01555 error occurring. Since your undo segments will wrap during this query execution, you know that some undo information generated since your query began is gone it has been overwritten. If you hit a block that was modified near the time you started your query, the undo information for this block will be missing, and you will receive the ORA-01555 error. Here s a small example. Let s say we have a table with blocks 1, 2, 3, . . . 1,000,000 in it. Table 9-2 shows a sequence of events that could occur. Table 9-2. Long-Running Query Timeline
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Time (Minutes:Seconds)
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Our query begins. Another session UPDATEs block 1,000,000. Undo information for this is recorded into some undo segment. This UPDATE session COMMITs. The undo data it generated is still there, but is now subject to being overwritten if we need the space. Our query is still chugging along. It is at block 200,000. Lots of activity going on. We have generated a little over 14MB of undo by now. Our query is still going strong. We are at block 600,000 or so by now. Our undo segments start to wrap around and reuse the space that was active when our query began at time 0:00. Specifically, we have just reused the undo segment space that the UPDATE to block 1,000,000 used back at time 0:01. Our query finally gets to block 1,000,000. It finds it has been modified since the query began. It goes to the undo segment and attempts to find the undo for that block to get a consistent read on it. At this point, it discovers the information it needs no longer exists. ORA-01555 is raised and the query fails.
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This is all it takes. If your undo segments are sized such that they have a good chance of being reused during the execution of your queries, and your queries access data that will probably be modified, you stand a very good chance of hitting the ORA-01555 error on a recurring basis. If this is the case, you must set your UNDO_RETENTION parameter higher and let Oracle take care of figuring out how much undo to retain (this is the suggested approach; it s much easier than trying to figure out the perfect undo size yourself) or resize your undo segments and make them larger (or have more of them). You need enough undo configured to last as long as your long-running queries. The system was sized for the transactions that modify data you forgot to size for the other components of the system.
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