.NET/Java PDF, Tiff, Barcode SDK Library

Candidate keys are those attributes that can uniquely identify a row in a table, and a table can have more than one candidate key. For example, it s fairly common for an employee table to have both a uniquely generated sequence number as well as another identifier, like an employee number (or social security number). (Of course, any whole row, itself, could serve as a candidate key, because by definition a relational model can t have any duplicate tuples. However, a whole row is rarely used as the key, since the point of a key is to easily access the row.) The primary key is the candidate key that s chosen to uniquely identify each row in a table. You should always strive to select a key based on a single attribute rather than on multiple attributes, for simplicity and efficiency. Keys are vital when you come to the point of physically building the entity-relationship models. A natural primary key is one that consists of data items or entity attributes. Almost all modern relational databases, including Oracle databases, also offer simple system numbers or sequenced numbers that are generated and maintained by the RDBMS as an alternative to a natural primary key (such as a sequence number to identify orders). Such keys are often referred to as surrogate or artificial primary keys. Whatever method you choose a natural key or a surrogate key certain rules apply: The primary key value must be unique. The primary key can t be null (blank). The primary key can t be changed (it must remain stable over the life of the entity). The primary key must be as concise as possible.

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Later in this chapter, I provide some guidelines about selecting keys (primary keys in particular).

Large transactions can sometimes run out of temporary space. Large sort jobs, especially those involving tables with many partitions, lead to heavy use of the temporary tablespaces, thus potentially leading to a performance hit. Oracle Database 10g introduces the concept of a temporary

character for all strings on UNIX and Linux systems, and normally is not visible. When all the special characters were escaped in the previous sed commands, this trailing \$ became visible at the end of the string. It wasn t part of the original encrypted password, so the last two characters (\$) need to be removed from the string. This use of sed is discussed in more detail in 24.

tablespace group, which allows a user to utilize multiple temporary tablespaces simultaneously in different sessions. Here are some of the main characteristics of a temporary tablespace group: A temporary tablespace group must consist of at least one tablespace. There is no explicit maximum number of tablespaces. If you delete all members from a temporary tablespace group, the group is automatically deleted as well. A temporary tablespace group has the same namespace as the temporary tablespaces that are part of the group. The name of a temporary tablespace cannot be the same as the name of any tablespace group. When you assign a temporary tablespace to a user, you can use the temporary tablespace group name instead of the actual temporary tablespace name. You can also use the temporary tablespace group name when you assign the default temporary tablespace for the database.

Using a temporary tablespace group, rather than the usual single temporary tablespace, provides several benefits: SQL queries are less likely to run out of sort space because the query can now simultaneously use several temporary tablespaces for sorting. You can specify multiple default temporary tablespaces at the database level. Parallel execution servers in a parallel operation will efficiently utilize multiple temporary tablespaces. A single user can simultaneously use multiple temporary tablespaces in different sessions.

// Console::WriteLine("Resolution of desktop: {0} x {1}", pixelsX , pixelsY); ldstr "Resolution of desktop: {0} x {1}" ldloc.1 box [mscorlib]System.Int32 ldloc.0 box [mscorlib]System.Int32 call void [mscorlib]System.Console::WriteLine(string, object, object) // return 0; ldc.i4.0 ret } Due to the .locals section at the beginning of main, the runtime will allocate 24 bytes on the stack for the three variables: 4 bytes for each of the two integers pixelsX and pixelsY, and 16 bytes for the variable rect. To pass the address of the rect variable, the ldloca instruction (load a local variable s address) is used: call ldloca call valuetype HWND__* GetDesktopWindow() rect int32 GetWindowRect(valuetype HWND__*, valuetype tagRECT*)

When you assign the first temporary tablespace to a tablespace group, you automatically create the temporary tablespace group. To create a tablespace group, simply specify the TABLESPACE GROUP clause in the CREATE TABLESPACE statement, as shown here: SQL> CREATE TEMPORARY TABLESPACE temp01 TEMPFILE '/u01/oracle/oradata/temp01_01.dbf' SIZE 500M TABLESPACE GROUP tmpgrp1; The preceding SQL statement will create a new temporary tablespace, temp01, along with the new tablespace group named tmpgrp1. Oracle creates the new tablespace group because the key clause TABLESPACE GROUP was used while creating the new temporary tablespace. You can also create a temporary tablespace group by specifying the same TABLESPACE GROUP clause in an ALTER TABLESPACE command, as shown here: SQL> ALTER TABLESPACE temp02 TABLESPACE GROUP tmpgrp1 Tablespace altered. SQL>

The preceding statement will cause Oracle to create a new group named tmpgrp1, since there wasn t a prior temporary tablespace group with that name. If you specify a pair of quotes ('') for the tablespace group name, you are implicitly telling Oracle not to allocate that temporary tablespace to a tablespace group. Here s an example: SQL> CREATE TEMPORARY TABLESPACE temp02 TEMPFILE '/u01/oracle/oradata/temp02_01.dbf' TABLESPACE GROUP ''; SIZE 500M

old_pass_word=`echo $old_pass_word | sed 's/\(.*\)\(.\)\(.\)$/\1/'` cur_pass_word=`echo $cur_pass_word | sed 's/\(.*\)\(.\)\(.\)$/\1/'`