Databases Reference
In-Depth Information
Dedicated server
A server dedicated to running a single application.
Default constraint
Value entered when no value is provided during data entry.
Default database
Database context for running SQL commands when a database is
not specified.
Defining association
A means of expressing that the value of one particular attribute
is associated with a single, specific value of another attribute.
Denormalization
Taking pairs of related, third normal form tables and combining
them.
Determinant attribute
The attribute in a defining association whose value determines
the value of the attribute on the right side of the association.
Deterministic function
A function that always return the same result if you pass it
the same arguments.
Differential backup
Backup of all changes made since the most recent base (full)
backup.
Direct memory access (DMA)
Access channel that enables devices such as disk drives
and network adapters to directly access (write to and read from) the computer's main
memory.
Dirty page
A page that has been updated in memory, but not written to the hard disk.
Dirty read
A data read that reads data that has been modified, but not committed.
Disk duplexing
Fault tolerant storage method based on two hard disks with identical
data, with each hard disk connected to a different disk controller.
Disk mirroring
Fault tolerant storage method based on two hard disks with iden-
tical data.
Disk queue
Number of hard disk requests waiting to be resolved. With most operating
systems, this should average one or less.
Disk striping with parity
Fault tolerant storage method using three or more hard
disks with data and parity information striped across the physical disks.
Distributed data environment
Data configuration based on application data stored
on multiple database servers.
Distributed database servers
A multiple server configuration supporting related data.
Distributed join
Joins data from the different database.
Distributed model
Database deployment model characterized by multiple database
servers, typically deployed in different physical locations.
Distributed query
A query that retrieves data from multiple databases.
DML statement
Any statement used to enter or modify data.
DML trigger
Trigger that fires when data is entered, updated or modified.
Domain
In the context of Microsoft Active Directory, a security boundary.
Domain integrity
Relating to ensuring that the values entered in specified columns
are legal.
