free barcode generator in asp.net c# THE ANATOMY OF A DATABASE SYSTEM in Font

Creator QR Code JIS X 0510 in Font THE ANATOMY OF A DATABASE SYSTEM

CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
QR Code JIS X 0510 Generator In None
Using Barcode creation for Font Control to generate, create Quick Response Code image in Font applications.
www.OnBarcode.com
Print GS1 - 13 In None
Using Barcode creation for Font Control to generate, create EAN-13 image in Font applications.
www.OnBarcode.com
Figure 2-2. Query processing steps The first step in this process is to translate the logical query from SQL into a query tree in relational algebra. This step is done by the parser and usually involves breaking the SQL statement into parts and then building the query tree from there. The next step is to translate the query tree in logical algebra into a physical plan. There are generally a large number of plans that could implement the query tree. The process of finding the best execution plan is called query optimization. That is, for some query execution performance measure (e.g., execution time), we want to find the plan with the best execution performance. The goal is that the plan be optimal or near optimal within the search space of the optimizer. The optimizer starts by copying the relational algebra query tree into its search space. The optimizer then expands the search space by forming alternative execution plans (to a finite iteration) and then searches for the best plan (the one that executes fastest). At this level of generality, the optimizer can be viewed as the code generation part of a query compiler for the SQL language. In fact, in some database systems the compilation step translates the query into an executable program. However, most database systems translate the query into a form that can be executed using the internal library of execution steps. The code compilation in this case produces code to be interpreted by the query execution engine, except that the optimizer s emphasis is on producing very efficient code. For example, the optimizer uses the database system s catalog to get information (e.g., the number of tuples) about the stored relations referenced by the query, something traditional programming language compilers normally do not do. Finally, the optimizer copies the optimal physical plan out of its memory structure and sends it to the query execution engine. The query execution engine executes the plan using the relations in the stored database as input, and produces the table of rows that match the query criteria. All of this activity requires additional processing time and places a greater burden on the process by forcing database implementers to consider the performance of the query optimizer and execution engine as a factor in their overall efficiency. This optimization is costly because of the number of alternative execution plans that use different access methods (ways of reading the data) and different execution orders. Thus it is possible to generate an infinite number of plans for a single query. However, database systems typically bound the problem to a few known best practices.
PDF 417 Creator In None
Using Barcode creation for Font Control to generate, create PDF-417 2d barcode image in Font applications.
www.OnBarcode.com
UPC-A Supplement 5 Creation In None
Using Barcode drawer for Font Control to generate, create UPC-A Supplement 5 image in Font applications.
www.OnBarcode.com
CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
Barcode Creation In None
Using Barcode creator for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Create Barcode In None
Using Barcode maker for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
One of the primary reasons for the large number of query plans is that optimization will be required for many different values of important runtime parameters whose actual values are unknown at optimization time. Database systems make certain assumptions about the database contents (e.g., value distribution in relation attributes), the physical schema (e.g., index types), the values of the system parameters (e.g., the number of available buffers), and the values of the query constants.
Quick Response Code Maker In None
Using Barcode encoder for Font Control to generate, create Quick Response Code image in Font applications.
www.OnBarcode.com
Paint Ames Code In None
Using Barcode printer for Font Control to generate, create Codabar image in Font applications.
www.OnBarcode.com
Query Optimizer
QR-Code Reader In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
Paint QR-Code In None
Using Barcode drawer for Online Control to generate, create QR Code ISO/IEC18004 image in Online applications.
www.OnBarcode.com
Some mistakenly believe that the query optimizer performs all of the steps outlined in the query execution phases. As you will see, query optimization is just one of the steps that the query takes on the way to be executed. The following paragraphs describe the query optimizer in detail and illustrate the role of the optimizer in the course of the query execution. Query optimization is the part of the query compilation process that translates a data manipulation statement in a high-level, nonprocedural language, such as SQL, into a more detailed, procedural sequence of operators, called a query plan. Query optimizers usually select a plan by estimating the cost of many alternative plans and then choosing the least expensive among them (the one that executes fastest). Database systems that use a plan-based approach to query optimization assume that many plans can be used to produce any given query. Although this is true, not all plans are equivalent in the number of resources (or cost) needed to execute the query, nor are all plans executed in the same amount of time. The goal then is to discover the plan that has the least cost and/or runs in the least amount of time. The distinction of either resource usage or cost usage is a trade-off often encountered when designing systems for embedded integration or running on a small platform (with low resource availability) versus the need for higher throughput (or time). Figure 2-3 depicts a plan-based query processing strategy where the query follows the path of the arrows. The SQL command is passed to the query parser, where it is parsed and validated and then translated into an internal representation, usually based on a relational algebra expression or a query tree as described earlier. The query is then passed to the query optimizer, which examines all of the algebraic expressions that are equivalent, generating a different plan for each combination. The optimizer then chooses the plan with the least cost and passes the query to the code generator, which translates the query into an executable form, either as directly executable or as interpretative code. The query processor then executes the query and returns a single row in the result set at a time. This is a common implementation scheme and is typical of most database systems. However, the machines that the database system runs on have improved over time. It is no longer the case that the query plans have diverse execution costs. In fact, most query plans have been shown to execute with approximately the same cost. This realization has led some database system implementers to adopt a query optimizer that focuses on optimizing the query using some well-known good rules (called heuristics) or practices for query optimization. Some database systems use hybrids of optimization techniques that are based on one form while maintaining aspects of other techniques during execution.
Read DataMatrix In VS .NET
Using Barcode scanner for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Paint UPC-A Supplement 2 In None
Using Barcode generator for Office Word Control to generate, create UPC-A Supplement 2 image in Office Word applications.
www.OnBarcode.com
Drawing QR Code In None
Using Barcode printer for Software Control to generate, create QR Code ISO/IEC18004 image in Software applications.
www.OnBarcode.com
GS1-128 Encoder In Java
Using Barcode generator for Java Control to generate, create EAN / UCC - 14 image in Java applications.
www.OnBarcode.com
UPC Code Drawer In C#.NET
Using Barcode creator for VS .NET Control to generate, create GS1 - 12 image in VS .NET applications.
www.OnBarcode.com
Barcode Scanner In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
Read Code 128 Code Set A In Visual Basic .NET
Using Barcode decoder for Visual Studio .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
EAN / UCC - 13 Creator In Java
Using Barcode creator for Android Control to generate, create UCC - 12 image in Android applications.
www.OnBarcode.com
Barcode Generator In Java
Using Barcode drawer for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
Scan Code 128 Code Set B In None
Using Barcode decoder for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.