Database Reference
In-Depth Information
Latency
Latency
is a time delay between the moment something is initiated and the moment effects
of the initiation become observable. The meaning of the word is highly context dependent.
The term is frequently used in the context of performance tuning. For example, unnecessary
round-trips between database server and client impose a performance penalty due to
network latency.
Lock
See
enqueue
.
Logical read
Reading an ORACLE block cached in the buffer cache within the System Global Area is called
a logical read.
Net service name
A
Net service name
is defined in the configuration file
tnsnames.ora
or a directory service
such as LDAP. Net service names are used in
connect strings
and with the command
tnsping
.
Paging
Writes of portions of virtual memory to disk by the operating system are called
paging
.
Paging occurs when free physical memory (RAM) becomes scarce or is exhausted. Paging
allows operating systems to execute programs that demand more virtual memory than RAM
installed in a system.
Parse
Syntactical and semantic analysis of SQL statements or PL/SQL blocks. Also includes verifi-
cation of applicable privileges. Each statement or block parsed is associated with a
cursor
.
Performance
Performance
of a computer or software system is the amount of useful work performed
compared to the time and resources used. Good performance implies short response times
and adequate resource usage, whereas bad performance is associated with unacceptable
response time
and an overly high demand on resources.
Physical read
Request to the operating system by some process of an ORACLE instance to read one or
more blocks from disk.
Physical write
Request to the operating system by some process of an ORACLE instance to write one or
more blocks to disk. Physical writes occur, for example, when the database writer process
(DBW
n
) writes modified blocks in the buffer cache to disk. Other examples are writes to sort
or LOB segments by dedicated or shared server processes.
