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<H1><FONT COLOR="#000077">- Appendix D -<BR>
Glossary</FONT></H1>
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<H2><FONT COLOR="#000077">A</FONT></H2>
<P><B>aggregate functions</B>--Functions that operate on the collection of values
in a certain column. These operations include such things as <TT>SUM</TT>, <TT>COUNT</TT>,
<TT>AVG</TT>, <TT>MAX</TT>, and so on.</P>
<P><B>asynchronous I/O (AIO)</B>--Asynchronous I/O allows a process to submit an
I/O and not have to wait for the response. Later, when the I/O is completed, an interrupt
occurs or the process can check to see whether the I/O has completed. By using asynchronous
I/Os, the DBWR can manage multiple writes at once so that it is not starved waiting
for I/Os to complete.
<H2><FONT COLOR="#000077">B</FONT></H2>
<P><B>bandwidth</B>--A term often associated with networks or computer buses. The
bandwidth is the throughput capacity. The bandwidth of the bus is the maximum rate
at which data can be transferred across the bus.</P>
<P><B>batch processing system</B>--Used to perform large background jobs, usually
within a specified time window.</P>
<P><B>Binary Large Data</B>--<I>See</I> <B>BLOB</B>.</P>
<P><B>BLOB (Binary Large Data)</B>--A large amount of binary data stored within an
Oracle database. BLOB data can consist of audio, video, images, documents, and so
on; it is usually stored as <TT>LONG</TT> data.</P>
<P><B>block</B>--The smallest unit of storage in an Oracle database. The database
block contains header information concerning the block itself as well as the data.</P>
<P><B>buffer</B>--An amount of memory used to store data. A buffer stores data that
is about to be used or that has just been used. In many cases, buffers are in-memory
copies of data that is also on disk. Buffers can be used as a copy of data for quick
read access, they can be modified and written to disk, or they can be created in
memory as temporary storage.</P>
<P>In Oracle, the database buffers of the SGA store the most recently used blocks
of database data. The set of database block buffers is known as the <B>database buffer
cache</B>. The buffers used to temporarily store redo entries until they can be written
to disk are known as the <B>redo log buffers</B>.</P>
<P>A <B>clean buffer</B> is a buffer that has not been modified. Because this buffer
has not been changed, it is not necessary for the DBWR to write this buffer to disk.
A <B>dirty buffer</B> is a buffer that has been modified. It is the job of the DBWR
to eventually write all dirty block buffers out to disk.
<H2><FONT COLOR="#000077">C</FONT></H2>
<P><B>cache</B>--A storage area used to provide fast access to data. In hardware
terms, the cache is a small (relative to main RAM) amount of memory that is much
faster than main memory. This memory is used to reduce the time it takes to reload
frequently used data or instructions into the CPU. CPU chips themselves contain small
amounts of memory built in as a cache.</P>
<P>In Oracle, the block buffers and shared pool are considered caches because they
are used to store data and instructions for quick access. Caching is very effective
in reducing the time it takes to retrieve frequently used data.</P>
<P>Caching usually works using a <B>least recently used</B> <B>algorithm</B>. Data
that has not been used for a while is eventually released from the cache to make
room for new data. If data is requested and is in the cache (a phenomenon called
a <B>cache hit</B>), the data is retrieved from the cache, which means it does not
have to be retrieved from disk. After the data has been accessed again, it is marked
as recently used and put on the top of the cache list.</P>
<P><B>Cartesian products</B>--The result of a join with no join condition. Each row
in a table is matched with every row of another table.</P>
<P><B>checksum</B>--A number calculated from the contents of a storage unit such
as a file or data block. Using a mathematical formula, the checksum number is generated
from data. Because it is highly unlikely that data corruption can occur in such a
way that the checksum would remain the same, checksums are used to verify data integrity.
From Oracle version 7.2 onward, checksums can be enabled on data blocks and redo
blocks.</P>
<P><B>cluster (machine)</B>--A group of computers that together form a larger logical
machine. Oracle clusters computers with the Oracle Parallel Server option.</P>
<P><B>cluster (table)</B>--A set of independent tables with a common column stored
together. A cluster can improve performance by reducing I/Os and by preloading related
data into the SGA before it is needed.</P>
<P><B>cluster index</B>--The index on the cluster key. Each cluster key must have
an index before data can be entered into the cluster.</P>
<P><B>cluster key</B>--The common column in the set of tables built into a cluster.
The cluster key must be indexed.</P>
<P><B>cold data</B>--This term typically refers to infrequently used data. Cold data
is rarely in cache because it is infrequently accessed.</P>
<P><B>cold database</B>--This term typically refers to a database that is currently
closed and not mounted. No users can connect to the database and no datafiles can
be accessed.</P>
<P><B>collision</B>--Typically refers to a network collision. A network collision
occurs when two or more NICs try to use the network at the same time. When this happens,
all the NICs must resend their data.</P>
<P><B>Common Object Response Broker Architecture</B>--<I>See</I> <B>CORBA</B>.</P>
<P><B>complex statements</B>--A SQL statement that contains a subquery. A <B>subquery</B>
is a query within the SQL statement used to determine values in the main statement,
or <B>parent statement</B>.</P>
<P><B>compound query</B>--A query in which the set operators (<TT>UNION</TT>, <TT>UNION
ALL</TT>, <TT>INTERSECT</TT>, and <TT>MINUS</TT>) are used to join two or more simple
or complex statements. The individual statements in the compound query are referred
to as <B>component queries</B>.</P>
<P><B>concurrency</B>--The capability to perform many functions at the same time.
Oracle provides for concurrency by allowing many users to access the database simultaneously.</P>
<P><B>consistent mode</B>--In this mode, Oracle provides a consistent view of data
from a certain point in time for the duration of the transaction. Until the transaction
is complete, the data cannot change.</P>
<P><B>consistent read</B>--A data access that causes a read from the rollback segment,
thus keeping data consistent.</P>
<P><B>constraint</B>--The mechanism that ensures that certain conditions relating
columns and tables are maintained.</P>
<P><B>contention</B>--A term usually used to describe a condition that occurs when
two or more processes or threads attempt to obtain the same resource. The results
of contention can vary depending on the resource in question.</P>
<P><B>CORBA</B>--CORBA stands for the Common Object Request Broker Architecture,
and is a specification for object communications.</P>
<P><B>cost-based optimizer</B>--The Oracle optimizer that chooses an execution plan
based on information and statistics that it has for tables, indexes, and clusters.</P>
<P><B>current mode</B>--The mode in which Oracle provides a view as the data exists
at this moment. Queries typically use <B>consistent mode</B>.</P>
<P><B>current read</B>--A read in current mode; typically used for <TT>UPDATE</TT>,
<TT>INSERT</TT>, and <TT>DELETE</TT> statements.</P>
<P><B>cursor</B>--A handle to a specific private SQL area. Think of a cursor as a
pointer to or a name of a particular private SQL area.
<H2><FONT COLOR="#000077">D</FONT></H2>
<P><B>database administrator</B>--<I>See</I> <B>DBA</B>.</P>
<P><B>Data Definition Language commands</B>--<I>See</I> <B>DDL commands</B>.</P>
<P><B>data dictionary</B>--A set of tables Oracle uses to maintain information about
the database. The data dictionary contains information about tables, indexes, clusters,
and so on.</P>
<P><B>Data Manipulation Language commands</B>--<I>See</I> <B>DML commands</B>.</P>
<P><B>data warehouse</B>--An extremely large database consisting of data from many
sources to provide an information pool for business queries.</P>
<P><B>DBA</B>--Database Administrator. The person responsible for the operation and
configuration of the database. The DBA is responsible for the performance of the
database, and is charged with keeping the database operating smoothly, ensuring that
backups are performed on a regular basis (and that the backups work), and installing
new software. Other responsibilities might include planning for future expansion
and disk space needs, creating databases and tablespaces, adding users and maintaining
security, and monitoring the database and retuning it as necessary. Large installations
might have teams of DBAs to keep the system running smoothly; alternatively, the
tasks might be segmented among the DBAs.</P>
<P><B>DDL commands</B>--Data Definition Language commands. The commands used in the
creation and modification of schema objects. These commands include the ability to
create, alter, and drop objects; grant and revoke privileges and roles; establish
auditing options; and add comments to the data dictionary. These commands are related
to the management and administration of the Oracle database. Before and after each
DDL statement, Oracle implicitly commits the current transaction.</P>
<P><B>deadlock</B>--Deadlocks occur when two or more processes hold a resource that
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