free code 128 barcode font for crystal reports Reverse Key Indexes in Objective-C

Encoding Data Matrix in Objective-C Reverse Key Indexes

Reverse Key Indexes
Generating Data Matrix 2d Barcode In Objective-C
Using Barcode encoder for iPhone Control to generate, create DataMatrix image in iPhone applications.
www.OnBarcode.com
UPCA Creator In Objective-C
Using Barcode generation for iPhone Control to generate, create GTIN - 12 image in iPhone applications.
www.OnBarcode.com
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.
Barcode Printer In Objective-C
Using Barcode creation for iPhone Control to generate, create Barcode image in iPhone applications.
www.OnBarcode.com
QR-Code Encoder In Objective-C
Using Barcode generator for iPhone Control to generate, create QR-Code image in iPhone applications.
www.OnBarcode.com
Note We discussed RAC in 2 Architecture Overview.
Code 128 Code Set B Creator In Objective-C
Using Barcode generator for iPhone Control to generate, create Code 128 Code Set A image in iPhone applications.
www.OnBarcode.com
Barcode Creator In Objective-C
Using Barcode maker for iPhone Control to generate, create Barcode image in iPhone applications.
www.OnBarcode.com
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 blocks in the index, though it could tend to make the index much less efficiently packed.
Draw Code 39 In Objective-C
Using Barcode encoder for iPhone Control to generate, create Code-39 image in iPhone applications.
www.OnBarcode.com
Make EAN-8 In Objective-C
Using Barcode encoder for iPhone Control to generate, create UPC - 8 image in iPhone applications.
www.OnBarcode.com
Note You may also find reverse key indexes useful as a method to reduce contention, even in a single instance
DataMatrix Maker In None
Using Barcode creation for Software Control to generate, create Data Matrix 2d barcode image in Software applications.
www.OnBarcode.com
Encoding Data Matrix ECC200 In Java
Using Barcode encoder for BIRT Control to generate, create ECC200 image in BIRT applications.
www.OnBarcode.com
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.
DataMatrix Generation In VB.NET
Using Barcode drawer for VS .NET Control to generate, create Data Matrix image in .NET framework applications.
www.OnBarcode.com
Scanning Barcode In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
CHAPTER 11 INDEXES
Printing Barcode In Visual Studio .NET
Using Barcode creator for ASP.NET Control to generate, create Barcode image in ASP.NET applications.
www.OnBarcode.com
GS1 - 12 Generation In None
Using Barcode printer for Font Control to generate, create Universal Product Code version A image in Font applications.
www.OnBarcode.com
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@ORA11GR2> 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 ---------90101 90102 90103 DUMP(90101,16) --------------------Typ=2 Len=4: c3,a,2,2 Typ=2 Len=4: c3,a,2,3 Typ=2 Len=4: c3,a,2,4
Code-128 Creator In None
Using Barcode encoder for Excel Control to generate, create Code 128A image in Excel applications.
www.OnBarcode.com
Barcode Creation In Java
Using Barcode generation for BIRT Control to generate, create Barcode image in BIRT reports applications.
www.OnBarcode.com
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: 90101 reversed = 2,2,a,c3 90102 reversed = 3,2,a,c3 90103 reversed = 4,2,a,c3 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 The data in the index is not sorted by X before it is stored, but rather by REVERSE(X), hence the range scan for X > 5 will not be able to use the index. On the other hand, some range scans can be done on a reverse key index. If I have a concatenated index on (X, Y), the following predicate will be able to make use of the reverse key index and will range scan it: where x = 5 This is because the bytes for X are reversed, and then the bytes for Y are reversed. Oracle does not reverse the bytes of (X || Y), but rather stores (REVERSE(X) || REVERSE(Y)). This means all of the values for X = 5 will be stored together, so Oracle can range scan that index to find them all. Now, assuming you have a surrogate primary key on a table populated via a sequence, and you do not need to use range scanning on this index that is, you don t need to query for MAX(primary_key), MIN(primary_key), WHERE primary_key < 100, and so on then you could consider a reverse key index in high insert scenarios even in a single instance of Oracle. I set up two different tests, one in a pure PL/SQL environment and one using Pro*C to demonstrate the differences between inserting into a table with a reverse key index on the primary key and one with a conventional index. In both cases, the table used was created with the following DDL (we will avoid contention on table blocks by using ASSM so we can isolate the contention on the index blocks):
Make Data Matrix 2d Barcode In .NET
Using Barcode maker for VS .NET Control to generate, create DataMatrix image in .NET applications.
www.OnBarcode.com
EAN 13 Maker In Java
Using Barcode creator for Android Control to generate, create UPC - 13 image in Android applications.
www.OnBarcode.com
Code 3/9 Drawer In None
Using Barcode generator for Software Control to generate, create Code39 image in Software applications.
www.OnBarcode.com
Print QR Code 2d Barcode In .NET
Using Barcode creation for ASP.NET Control to generate, create QR Code JIS X 0510 image in ASP.NET applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.