Database schema Most Oracle database installations traditionally came with a default schema called SCOTT. After the installation process has set up the sample tables, the user can log into the database with the username scott and the password tiger. The name of the SCOTT schema originated with Bruce Scott, one of the first employees at Oracle (then Software Development Laboratories), who had a cat named Tiger.[10] Oracle Corporation has de-emphasized the use of the SCOTT schema, as it uses few of the features of the more recent releases of Oracle. Most recent examples supplied by Oracle Corporation reference the default HR or OE schemas. Other default schemas[11][12] include: SYS (essential core database structures and utilities) SYSTEM (additional core database structures and utilities, and privileged account) OUTLN (utilized to store metadata for stored outlines for stable query-optimizer execution plans.[13]) BI, IX, HR, OE, PM, and SH (expanded sample schemas[14] containing more data and structures than the older SCOTT schema). System Global Area Main article: System Global Area Each Oracle instance uses a System Global Area or SGA—a shared-memory area—to store its data and control-information.[15] Each Oracle instance allocates itself an SGA when it starts and de-allocates it at shut-down time. The information in the SGA consists of the following elements, each of which has a fixed size, established at instance startup: Datafiles Every Oracle database has one or more physical datafiles, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database. Datafiles have the following characteristics: One or more datafiles form a logical unit of database storage called a tablespace. A datafile can be associated with only one tablespace. Datafiles can be defined to extend automatically when they are full. Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle Database. For example, if a user wants to access some data in a table of a database, and if the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory. Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once the redo log buffer: this stores redo entries—a log of changes made to the database. The instance writes redo log buffers to the redo log as quickly and efficiently as possible. The redo log aids in instance recovery in the event of a system failure. the shared pool: this area of the SGA stores shared-memory structures such as shared SQL areas in the library cache and internal information in the data dictionary. An insufficient amount of memory allocated to the shared pool can cause performance degradation. the Large pool Optional area that provides large memory allocations for certain large processes, such as Oracle backup and recovery operations, and I/O server processes Database buffer cache: Caches blocks of data retrieved from the database KEEP buffer pool: A specialized type of database buffer cache that is tuned to retain blocks of data in memory for long periods of time RECYCLE buffer pool: A specialized type of database buffer cache that is tuned to recycle or remove block from memory quickly nK buffer cache: One of several specialized database buffer caches designed to hold block sizes different than the default database block size Java pool:Used for all session-specific Java code and data in the Java Virtual Machine (JVM) Streams pool: Used by Oracle Streams to store information required by capture and apply When you start the instance by using Enterprise Manager or SQL*Plus, the amount of memory allocated for the SGA is displayed.[16]
Posted on: Fri, 04 Oct 2013 22:28:59 +0000
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