As a newcomer to SQL, you probably will not be exposed to database tuning unlessyou are a new DBA or a DBA moving into a relational database environment. Whetheryou will be managing a database or using SQL in applications or programming, youwill benefit by knowing something about the database-tuning process. The key to thesuccess of any database is for all parties to work together. Some general tips fortuning a database follow.<UL>	<LI>Minimize the overall size required for the database.</UL><DL>	<DD>It's good to allow room for growth when designing a database, but don't go overboard.	Don't tie up resources that you may need to accommodate database growth.</DL><UL>	<LI>Experiment with the user process's time-slice variable.</UL><DL>	<DD>This variable controls the amount of time the database server's scheduler allocates	to each user's process.</DL><UL>	<LI>Optimize the network packet size used by applications.</UL><DL>	<DD>The larger the amount of data sent over the network, the larger the network packet	size should be. Consult your database and network documentation for more details.</DL><UL>	<LI>Store transaction logs on separate hard disks.</UL><DL>	<DD>For each transaction that takes place, the server must write the changes to the	transaction logs. If you store these log files on the same disk as you store data,	you could create a performance bottleneck. (See Figure 15.3.)</DL><UL>	<LI>Stripe extremely large tables across multiple disks.</UL><DL>	<DD>If concurrent users are accessing a large table that is spread over multiple	disks, there is much less chance of having to wait for system resources. (See Figure	15.3.)</DL><UL>	<LI>Store database sort area, system catalog area, and rollback areas on separate	hard disks.</UL><DL>	<DD>These are all areas in the database that most users access frequently. By spreading	these areas over multiple disk drives, you are maximizing the use of system resources.	(See Figure 15.3.)</DL><UL>	<LI>Add CPUs.</UL><DL>	<DD>This system administrator function can drastically improve database performance.	Adding CPUs can speed up data processing for obvious reasons. If you have multiple	CPUs on a machine, then you may be able to implement parallel processing strategies.	See your database documentation for more information on parallel processing, if it	is available with your implementation.</DL><UL>	<LI>Add memory.</UL><DL>	<DD>Generally, the more the better.</DL><UL>	<LI>Store tables and indexes on separate hard disks.</UL><DL>	<DD>You should store indexes and their related tables on separate disk drives when-	ever possible. This arrangement enables the table to be read at the same time the	index is being referenced on another disk. The capability to store objects on multiple	disks may depend on how many disks are connected to a controller. (See Figure 15.3.)</DL><P>Figure 15.3 shows a simple example of how you might segregate the major areasof your database.</P><P><A NAME="03"></A><A HREF="03.htm"><B>Figure 15.3.</B></A><B><BR></B><I>Using available disks to enhance performance.</I></P><P>The scenario in Figure 15.3 uses four devices: disk01 through disk04. The objectivewhen spreading your heavy database areas and objects is to keep areas of high useaway from each another.<UL>	<LI>Disk01-- The system catalog stores information about tables, indexes, users,	statistics, database files, sizing, growth information, and other pertinent data	that is often accessed by a high percentage of transactions.<BR>	<BR>		<LI>Disk02--Transaction logs are updated every time a change is made to a table (insert,	update, or delete). Transaction logs are a grand factor in an online transactional	database. They are not of great concern in a read-only environment, such as a data	warehouse or DSS.<BR>	<BR>		<LI>Disk03--Rollback segments are also significant in a transactional environment.	However, if there is little transactional activity (insert, update, delete), rollback	segments will not be heavily used.<BR>	<BR>		<LI>Disk04-- The database's sort area, on the other hand, is used as a temporary	area for SQL statement processing when sorting data, as in a <TT>GROUP BY</TT> or	<TT>ORDER BY</TT> clause. Sort areas are typically an issue in a data warehouse or	DSS. However, the use of sort areas should also be considered in a transactional	environment.</UL><BLOCKQUOTE>	<P><HR><FONT COLOR="#000077"><B>TIP:</B></FONT><B> </B>Also note how the application tables	and indexes have been placed on each disk. Tables and indexes should be spread as	much as possible. <HR></BLOCKQUOTE><P>Notice that in Figure 15.3 the tables and indexes are stored on different devices.You can also see how a &quot;Big Table&quot; or index may be <I>striped</I> acrosstwo or more devices. This technique splits the table into smaller segments that canbe accessed simultaneously. Striping a table or index across multiple devices isa way to control fragmentation. In this scenario, tables may be read while theircorresponding indexes are being referenced, which increases the speed of overalldata access.</P><P>This example is really quite simple. Depending on the function, size, and system-relatedissues of your database, you may find a similar method for optimizing system resourcesthat works better. In a perfect world where money is no obstacle, the best configurationis to have a separate disk for each major database entity, including large tablesand indexes.<BLOCKQUOTE>	<P><HR><FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>The DBA and system administrator	should work together to balance database space allocation and optimize the memory	that is available on the server. <HR></BLOCKQUOTE><P>Tuning a database very much depends on the specific database system you are using.Obviously, tuning a database entails much more than just preparing queries and lettingthem fly. On the other hand, you won't get much reward for tuning a database whenthe application SQL is not fine-tuned itself. Professionals who tune databases fora living often specialize on one database product and learn as much as they possiblycan about its features and idiosyncrasies. Although database tuning is often lookedupon as a painful task, it can provide very lucrative employment for the people whotruly understand it.<H2><FONT COLOR="#000077"><B>Performance Obstacles</B></FONT></H2><P>We have already mentioned some of the countless possible pitfalls that can hinderthe general performance of a database. These are typically general bottlenecks thatinvolve system-level maintenance, database maintenance, and management of SQL statementprocessing.</P><P>This section summarizes the most common obstacles in system performance and databaseresponse time.<UL>	<LI>Not making use of available devices on the server--A company purchases multiple	disk drives for a reason. If you do not use them accordingly by spreading apart the	vital database components, you are limiting the performance capabilities. Maximizing	the use of system resources is just as important as maximizing the use of the database	server capabilities.<BR>	<BR>		<LI>Not performing frequent <TT>COMMIT</TT>s--Failing to use periodic <TT>COMMIT</TT>s	or <TT>ROLLBACK</TT>s during heavy batch loads will ultimately result in database	bottlenecks.<BR>	<BR>		<LI>Allowing batch loads to interfere with daily processing--Running batch loads	during times when the database is ex亚洲欧美第一页_禁久久精品乱码_粉嫩av一区二区三区免费野_久草精品视频



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