Oracle Unleashed

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It reduces the number of processes running and the amount of memory used on the machine and can improve system performance. The multi-threaded server introduces two new types of system processes that support this part of the architecture. Using one of the shared server processes that comes as part of the multi-threaded server configuration is not appropriate when a user process is making many database requests such as an export backup of the database ; for that process, you could use a dedicated server.

A mixture of both configurations can coexist. You must configure at least one dispatcher for each network protocol that is used to route requests to the instance. The number of dispatchers configured does not increase if the system load increases because the dispatchers are only providing the routing. The actual work is done by the shared servers. The shared servers provide the same functionality as the dedicated server processes and contain the Oracle server code that performs the work for the client. They can service requests from many different user processes. The actual shared server used might differ from one call to another so that no user process can monopolize any one particular shared server process.

Oracle uses an area in the SGA for messaging between the different processes involved. The number of shared server processes is automatically increased or decreased to an initial number defined by the database administrator according to the system activity. In this part, I discuss how Oracle uses the machine's memory.

Generally, the greater the real memory available to Oracle, the quicker the system runs. The system global area, sometimes known as the shared global area, is for data and control structures in memory that can be shared by all the Oracle background and user processes running on that instance. The SGA memory area is allocated when the instance is started, and it's flushed and deallocated when the instance is shut down. The contents of the SGA are divided into three main areas: the database buffer cache, the shared pool area, and the redo cache. The size of each of these areas is controlled by parameters in the INIT.

ORA file. The bigger you can make the SGA and the more of it that can fit into the machine's real memory as opposed to virtual memory , the quicker your instance will run.

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The database buffer cache of the SGA holds Oracle blocks that have been read in from the database files. When one process reads the blocks for a table into memory, all the processes for that instance can access those blocks.

If a process needs to access some data, Oracle checks to see if the block is already in this cache thereby avoiding a disk read. If the Oracle block is not in the buffer, it must be read from the database files into the buffer cache. The buffer cache must have a free block available before the data block can be read from the database files. The Oracle blocks in the database buffer cache in memory are arranged with the most recently used at one end and the least recently used at the other. This list is constantly changing as the database is used. If data must be read from the database files into memory, the blocks at the least recently used end are written back to the database files first if they've been modified.

The DBWR process is the only process that writes the blocks from the database buffer cache to the database files.

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The online redo log files record all the changes made to user objects and system objects. Before the changes are written out to the redo logs, Oracle stores them in the redo cache memory area. For example, the entries in the redo log cache are written down to the online redo logs when the cache becomes full or when a transaction issues a commit.

The entries for more than one transaction can be included together in the same disk write to the redo log files. The LGWR background process is the only process that writes out entries from this redo cache to the online redo log files. You can alter the size of these two components only by changing the size of the entire shared pool area. A SQL statement sent for execution to the database server must be parsed before it can execute. Because the shared pool area is a fixed size, you might not see the entire set of statements that have been executed since the instance first came up; Oracle might have flushed out some statements to make room for others.

If another user executes exactly the same statement on the same objects, Oracle doesn't need to reparse the second statement because the parse tree and execution plan is already in the SQL area. This part of the architecture saves on reparsing overhead. The dictionary cache in the shared pool area holds entries retrieved from the Oracle system tables, otherwise known as the Oracle data dictionary. The cache itself holds a subset of the data from the data dictionary.

It is loaded with an initial set of entries when the instance is first started and then populated from the database data dictionary as further information is required. The cache holds information about all the users, the tables and other objects, the structure, security, storage, and so on. The data dictionary cache grows to occupy a larger proportion of memory within the shared pool area as needed, but the size of the shared pool area remains fixed. The process global area, sometimes called the program global area or PGA, contains data and control structures for one user or server process.

There is one PGA for each user process connection to the database. The actual contents of the PGA depend on whether the multi-threaded server configuration is implemented, but it typically contains memory to hold the session's variables, arrays, some rows results, and other information. If you're using the multi-threaded server, some of the information that is usually held in the PGA is instead held in the common SGA.

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  7. The size of the PGA depends on the operating system used to run the Oracle instance, and once allocated, it remains the same. Memory used in the PGA does not increase according to the amount of processing performed in the user process.

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    This Kernel code, also known as reentrant code, saves memory because it requires that only one copy of the code be loaded into memory. In this section, I discuss the organization of the database files themselves.

    Toad for Oracle Unleashed

    You have already learned that the database files are binary, fixed-size files on disk. I do not discuss the control and redo log files in this section. For management, security, and performance reasons, the database is logically divided into one or more tablespaces that each comprise one or more database files. A database file is always associated with only one tablespace. Typically, you create many tablespaces to partition the different parts of the database.

    For example, you might have one tablespace for tables, another to hold indexes, and so on, and each of these tablespaces would have one or more database files associated to them. When you create objects that use storage in the database such as tables , you should specify the tablespace location of the object as part of the CREATE statement for the object. Objects such as synonyms and views do not take up storage within the database other than the storage in the data dictionary table for their definitions, along with the definitions for all other types of objects.

    Tablespaces can be added, dropped, taken offline and online, and associated with additional database files. By adding another file to a tablespace, you increase the size of the tablespace and therefore the database itself. A segment is a generic name given to any object that occupies storage in the database files. Some examples of segments are table segments data segments , index segments, rollback segments, temporary segments, and the cache bootstrap segment. A segment uses a number of Oracle blocks that are in the same tablespace although the blocks themselves can be in different files that make up the tablespace. The storage for any object on the database is allocated in a number of blocks that must be contiguous in the database files. These contiguous blocks are known as extents.

    For example, when a table is first created using default settings, five Oracle blocks are allocated to the table for the very first extent otherwise known as the initial extent. As rows are inserted and updated into the table, the five blocks fill with data.

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    When the last block has filled and new rows are inserted, the database automatically allocates another set of blocks five blocks for the table, and the new rows are inserted into the new set of blocks. This allocating of additional storage additional extents continues until there is no more free space in the tablespace. The table starts with the one initial extent and is then allocated other secondary or next extents. The blocks for an extent must be contiguous within the database files. Once an extent is allocated to a segment table , those blocks cannot be used by another other database object, even if all the rows in the table are deleted.

    The table must be dropped or truncated to release the storage allocated to the table. The exception to this is rollback segments, which can dynamically release storage that was allocated to them.