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CHAPTER 9 REDO AND UNDO
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If we insert 10,000 rows and only commit when all 10,000 are inserted, we get results similar to the following: begin /* commit size = 10000 */ commit; end; .... Elapsed times include waiting on following events: Event waited on Times ---------------------------------------Waited log file sync 1 SQL*Net message to client 1 SQL*Net message from client 1
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Max. Wait ---------0.00 0.00 0.00
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Total Waited -----------0.00 0.00 0.00
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When we committed after every INSERT, we waited almost every time and if you wait a little bit of time but you wait often, then it all adds up. Fully two seconds of our runtime was spent waiting for a COMMIT to complete in other words, waiting for LGWR to write the redo to disk. In stark contrast, when we committed once, we didn t wait very long (not a measurable amount of time actually). This proves that a COMMIT is a fast operation; we expect the response time to be more or less flat, not a function of the amount of work we ve done. So, why is a COMMIT s response time fairly flat, regardless of the transaction size Before we even go to COMMIT in the database, we ve already done the really hard work. We ve already modified the data in the database, so we ve already done 99.9 percent of the work. For example, operations such as the following have already taken place: Undo blocks have been generated in the SGA. Modified data blocks have been generated in the SGA. Buffered redo for the preceding two items has been generated in the SGA. Depending on the size of the preceding three items, and the amount of time spent, some combination of the previous data may be flushed onto disk already. All locks have been acquired. When we COMMIT, all that is left to happen is the following: An SCN is generated for our transaction. In case you are not familiar with it, the SCN is a simple timing mechanism Oracle uses to guarantee the ordering of transactions and to enable recovery from failure. It is also used to guarantee read-consistency and checkpointing in the database. Think of the SCN as a ticker; every time someone COMMITs, the SCN is incremented by one. LGWR writes all of our remaining buffered redo log entries to disk and records the SCN in the online redo log files as well. This step is actually the COMMIT. If this step occurs, we have committed. Our transaction entry is removed from V$TRANSACTION this shows that we have committed. All locks recorded in V$LOCK held by our session are released, and everyone who was enqueued waiting on locks we held will be woken up and allowed to proceed with their work.
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CHAPTER 9 REDO AND UNDO
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Some of the blocks our transaction modified will be visited and cleaned out in a fast mode if they are still in the buffer cache. Block cleanout refers to the lock-related information we store in the database block header. Basically, we are cleaning out our transaction information on the block, so the next person who visits the block won t have to. We are doing this in a way that need not generate redo log information, saving considerable work later (this is discussed more fully in the upcoming Block Cleanout section). As you can see, there is very little to do to process a COMMIT. The lengthiest operation is, and always will be, the activity performed by LGWR, as this is physical disk I/O. The amount of time spent by LGWR here will be greatly reduced by the fact that it has already been flushing the contents of the redo log buffer on a recurring basis. LGWR will not buffer all of the work you do for as long as you do it; rather, it will incrementally flush the contents of the redo log buffer in the background as you are going along. This is to avoid having a COMMIT wait for a very long time in order to flush all of your redo at once. So, even if we have a long-running transaction, much of the buffered redo log it generates would have been flushed to disk, prior to committing. On the flip side of this is the fact that when we COMMIT, we must wait until all buffered redo that has not been written yet is safely on disk. That is, our call to LGWR is a synchronous one. While LGWR may use asynchronous I/O to write in parallel to our log files, our transaction will wait for LGWR to complete all writes and receive confirmation that the data exists on disk before returning. Now, earlier I mentioned that we were using a Java program and not PL/SQL for a reason and that reason is a PL/SQL commit-time optimization. I said that our call to LGWR is a synchronous one, and that we wait for it to complete its write. That is true in Oracle 10g Release 1 and before for every programmatic language except PL/SQL. The PL/SQL engine, realizing that the client does not know whether or not a COMMIT has happened in the PL/SQL routine until the PL/SQL routine is completed, does an asynchronous commit. It does not wait for LGWR to complete; rather, it returns from the COMMIT call immediately. However, when the PL/SQL routine is completed, when we return from the database to the client, the PL/SQL routine will wait for LGWR to complete any of the outstanding COMMITs. So, if you commit 100 times in PL/SQL and then return to the client, you will likely find you waited for LGWR once not 100 times due to this optimization. Does this imply that committing frequently in PL/SQL is a good or OK idea No, not at all just that it is not as bad an idea as it is in other languages. The guiding rule is to commit when your logical unit of work is complete not before.
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