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5 without validation 6 UPDATE GLOBAL INDEXES 7 / Table altered. then we would have discovered the indexes to be perfectly valid and usable both during and after the operation: ops$tkyte@ORA10G> select index_name, status from user_indexes; INDEX_NAME -----------------------------FY_2006_IDX FY_2004_IDX PARTITIONED_IDX_GLOBAL PARTITIONED_IDX_LOCAL 6 rows selected. ops$tkyte@ORA10G> set autotrace on explain ops$tkyte@ORA10G> select count(*) 2 from partitioned 3 where timestamp between sysdate-50 and sysdate; COUNT(*) ---------6750 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=9 Card=1 Bytes=9) 1 0 SORT (AGGREGATE) 2 1 FILTER 3 2 INDEX (RANGE SCAN) OF 'PARTITIONED_IDX_GLOBAL' (INDEX) (Cost=9... But there is a tradeoff: we are performing the logical equivalent of INSERT and DELETE operations on the global index structures. When we drop a partition, we have to delete all of the global index entries that might be pointing to that partition. When we did the exchange of a table with a partition, we had to delete all of the global index entries pointing to the original data and then insert all of the new ones that we just slid in there. So the amount of work performed by the ALTER commands was significantly increased. In fact, using runstats and a slightly modified version of the preceding example, we can measure the amount of extra work performed to maintain the global indexes during the partition operation. We ll slide out FY_2004 and slide in FY_2006 as before, adding in the requisite index rebuild. This will be the sliding window implementation that causes the data to become unavailable due to the need to rebuild the global indexes. We ll also then slide out FY_2005 and slide in FY_2007 using the UPDATE GLOBAL INDEXES clause, to emulate the sliding window STATUS -------VALID VALID VALID N/A
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implementation that provides for full data availability, even during the partition operations. In that manner, we can measure the same operations using the different techniques and compare them. Our expected outcome will be that the first approach will consume fewer database resources and, therefore, perform faster but will incur a measurable period of downtime. The second approach, while consuming more resources and perhaps taking longer overall, will not incur downtime. As far as the end users are concerned, their ability to work never ceased. They might have been processing a bit slower (since we were competing with them for resources), but they were still processing, and they never stopped. So, if we take the earlier example, but also create an empty FY_2005 table like FY_2004, and a full FY_2007 table like FY_2006, we can measure the differences between the index rebuild approaches, starting first with the less available approach : exec runStats_pkg.rs_start; alter table partitioned exchange partition fy_2004 with table fy_2004 including indexes without validation; alter table partitioned drop partition fy_2004; alter table partitioned add partition fy_2006 values less than ( to_date('01-jan-2007','dd-mon-yyyy') ); alter table partitioned exchange partition fy_2006 with table fy_2006 including indexes without validation; alter index partitioned_idx_global rebuild; exec runStats_pkg.rs_middle; Here is the highly available UPDATE GLOBAL INDEXES approach: alter table partitioned exchange partition fy_2005 with table fy_2005 including indexes without validation update global indexes; alter table partitioned drop partition fy_2005 update global indexes; alter table partitioned add partition fy_2007 values less than ( to_date('01-jan-2008','dd-mon-yyyy') ); alter table partitioned exchange partition fy_2007 with table fy_2007 including indexes without validation update global indexes; exec runStats_pkg.rs_stop;
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We might observe the following: ops$tkyte@ORA10G> exec runStats_pkg.rs_stop; Run1 ran in 81 hsecs Run2 ran in 94 hsecs run 1 ran in 86.17% of the time Name ... STAT...CPU used when call star ... STAT...redo entries STAT...db block gets STAT...session logical reads ... LATCH.cache buffers chains ... STAT...table scan rows gotten STAT...undo change vector size STAT...redo size Run1 39 938 1,348 2,178 5,675 97,711 35,100 2,694,172 Run2 59 3,340 5,441 6,455 27,695 131,427 3,404,056 6,197,988 Diff 20 2,402 4,093 4,277 22,020 33,716 3,368,956 3,503,816
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The index rebuild approach did run faster, both as observed by the elapsed time and the CPU time. This fact has caused many a DBA to pause and say, Hey, I don t want to use UPDATE GLOBAL INDEXES it s slower. That is too simplistic of a view, however. What you need to remember is that while the operations overall took longer, processing on your system was not necessarily interrupted. Sure, you as the DBA might be looking at your screen for a longer period of time, but the really important work that takes place on your system was still taking place. What you need to do is see if this tradeoff makes sense for you. If you have an eighthour maintenance window overnight in which to load new data, then by all means, use the rebuild approach if that makes sense. However, if you have a mandate to be available continuously, then the ability to maintain the global indexes will be crucial. Looking at the redo generated by each approach, we can see that the UPDATE GLOBAL INDEXES generated considerably more over 230 percent more and we would expect that to only go up as we add more and more global indexes to the table. The redo generated by the UPDATE GLOBAL INDEXES is unavoidable and cannot be turned off via NOLOGGING, since the maintenance of the global indexes is not a complete rebuild of their structure but more of an incremental maintenance. Additionally, since we are maintaining the live index structure, we must generate undo for that in the event the partition operation fails, we must be prepared to put the index back the way it was. And remember, undo is protected by redo itself, so some of the redo you see generated is from the index updates and some is from the rollback. Add another global index or two and you would reasonably expect these numbers to increase. So, UPDATE GLOBAL INDEXES is an option that allows you to trade off availability for resource consumption. If you have the need to provide continuous availability, it will be the option for you. But you will have to understand the ramifications and size other components of your system appropriately. Specifically, many data warehouses have been crafted over time to use bulk, direct path operations, bypassing undo generation and, when permitted, redo generation as well. Using UPDATE GLOBAL INDEXES cannot bypass either of those two elements. You ll need to examine the rules you used to size these components before using this feature,
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