Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 11 ■ INDEXES 429 we ended up adding 4 bytes to every single index key entry and factoring out no repeating data. The OPT_CMPR_COUNT column in IDX_STATS is dead accurate at providing the best compression count to be used, and OPT_CMPR_PCTSAVE will tell you exactly how much savings to expect. Now, you do not get this compression for free. The compressed index structure is now more complex than it used to be. Oracle will spend more time processing the data in this structure, both while maintaining the index during modifications and when you search the index during a query. What we are doing here is trading off increased CPU time for reduced I/O time. With compression, our block buffer cache will be able to hold more index entries than before, our cache-hit ratio might go up, and our physical I/Os should go down, but it will take a little more CPU horsepower to process the index, and it will also increase the chance of block contention. Just as in our discussion of the hash cluster, where it might take more CPU to retrieve a million random rows but half the I/O, we must be aware of the tradeoff. If you are currently CPU bound, adding compressed key indexes may slow down your processing. On the other hand, if you are I/O bound, using them may speed up things. Reverse Key Indexes Another feature of a B*Tree index is the ability to “reverse” its keys. At first you might ask yourself, “Why would I want to do that?” B*Tree indexes were designed for a specific environment and for a specific issue. They were implemented to reduce contention for index leaf blocks in “right-hand-side” indexes, such as indexes on columns populated by a sequence value or a timestamp, in an Oracle RAC environment. ■Note We discussed RAC in Chapter 2. RAC is a configuration of Oracle in which multiple instances can mount and open the same database. If two instances need to modify the same block of data simultaneously, they will share the block by passing it back and forth over a hardware interconnect, a private network connection between the two (or more) machines. If you have a primary key index on a column populated from a sequence (a very popular implementation), everyone will be trying to modify the one block that is currently the left block on the right-hand side of the index structure as they insert new values (see Figure 11-1, which shows that higher values in the index go to the right, and lower values go to the left). Modifications to indexes on columns populated by sequences are focused on a small set of leaf blocks. Reversing the keys of the index allows insertions to be distributed across all the leaf keys in the index, though it could tend to make the index much less efficiently packed. ■Note You may also find reverse key indexes useful as a method to reduce contention, even in a single instance of Oracle. Again, you will mainly use them to alleviate buffer busy waits on the right-hand side of a busy index, as described in this section.
430 CHAPTER 11 ■ INDEXES Before we look at how to measure the impact of a reverse key index, let’s discuss what a reverse key index physically does. A reverse key index simply reverses the bytes of each column in an index key. If we consider the numbers 90101, 90102, and 90103, and look at their internal representation using the Oracle DUMP function, we will find they are represented as follows: ops$tkyte@ORA10GR1> select 90101, dump(90101,16) from dual 2 union all 3 select 90102, dump(90102,16) from dual 4 union all 5 select 90103, dump(90103,16) from dual 6 / 90101 DUMP(90101,16) ---------- --------------------- 90101 Typ=2 Len=4: c3,a,2,2 90102 Typ=2 Len=4: c3,a,2,3 90103 Typ=2 Len=4: c3,a,2,4 Each one is 4 bytes in length and only the last byte is different. These numbers would end up right next to each other in an index structure. If we reverse their bytes, however, Oracle will insert the following: ops$tkyte@ORA10GR1> select 90101, dump(reverse(90101),16) from dual 2 union all 3 select 90102, dump(reverse(90102),16) from dual 4 union all 5 select 90103, dump(reverse(90103),16) from dual 6 / 90101 DUMP(REVERSE(90101),1 ---------- --------------------- 90101 Typ=2 Len=4: 2,2,a,c3 90102 Typ=2 Len=4: 3,2,a,c3 90103 Typ=2 Len=4: 4,2,a,c3 ■Note REVERSE is an undocumented function and, as such, should be used carefully. I do not recommend using REVERSE in “real” code, as its undocumented nature implies that it is not supported. The numbers will end up “far away” from each other. This reduces the number of RAC instances going after the same block (the rightmost block) and reduces the number of block transfers between RAC instances. One of the drawbacks to a reverse key index is that you cannot use it in all cases where a regular index can be applied. For example, in answering the following predicate, a reverse key index on X would not be useful: where x > 5
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 13 ■ ■ ■ Partitioning
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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