- Tablespace & Datafile
- Database Blocks, Extents, Segments
- Database Schema Objects
- Database logs
Total four parts,
Database Blocks, Extents, Segments,
Database Schema Objectsand
Oracle Database stores logically in
tablespacesand physically in
datadfilesassociated with the corresponding tablespace.
select tablespace_name,file_id,block_id,bytes,blocks from dba_free_space;
Enlarge a database in three ways:
-- add new datafile
-- create new tablespace and using disk space auto mamagement
-- allow already exisit datafile autoextend
From logical side to divide,
Blocks < Extents < Segments < Tablespaces
Oracle Database manages the storage space in the datafiles of a database in units called
data blocks. A data block is the smallest unit of data used by a database. In contrast, at the physical, operating system level, all data is stored in bytes. Each operating system has a
block size. Oracle Database requests data in multiples of Oracle Database data blocks, not operating system blocks.
The standard block size is specified by the
DB_BLOCK_SIZEinitialization parameter, default value is
extentis a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a
segment. When the existing space in a segment is completely used, Oracle Database allocates
a new extentfor the
1 Related system tables: `DBA_EXTENTS`, `USER_EXTENTS`.
segmentis a set of extents that contains all the data for a specific logical storage structure within a tablespace. For example, for each table, Oracle Database allocates one or more extents to form that table’s data segment, and for each index, Oracle Database allocates one or more extents to form its index segment.
Inculde the specific data objects:
Cluster Table, and
undo segmentsand all the types
1 Related system tables: `DBA_SEGMENTS`, `USER_SEGMENTS`.
A database is divided into one or more logical storage units called
tablespaces. Tablespaces are divided into logical units of storage called
segments, which are further divided into
extents. Extents are a collection of contiguous
When create default tablespace, contains
1 Related views: `V$TABLESPACE`, `V$DATAFILE`, `V$TEMPFILE`.
schemais a collection of logical structures of data, or schema objects. A
schemais owned by a database user and has the same name as that user. Each user owns a single schema. Schema objects can be created and manipulated with SQL and include the following types of objects:
Tables is very basic and simple. You know how to operate table
CURD, and tables row, column, etc. Certainly, Tables has a lot of others properties, and divide
Temporary Tables. etc.
-- create table
It’s a virtual table that doesn’t physically exist, rather it’s created by a query joining one or more other tables.
-- create view
Materialized viewsare schema objects that can be used to summarize, compute, replicate, and distribute data. They are suitable in various computing environments such as data warehousing, decision support, and distributed or mobile computing:
-- Creating Materialized Aggregate Views
Indexesare optional structures associated with
clusters. You can create many
tableas long as the combination of
columnsdiffers for each
index. You can create more than one
indexusing the same
columnsif you specify distinctly different combinations of the
Oracle Database provides several indexing schemes,
Bitmap, etc., By default, Oracle creates
Sequencesare database objects from which multiple users can generate unique integers. The sequence generator generates sequential numbers, which can help to generate
unique primary keysautomatically, and to coordinate keys across
Special attention, Uses and Restrictions of
# cann't be used in these places:
A view query or materialized view query
A SELECT statement with the DISTINCT operator
A SELECT statement with a GROUP BY or ORDER BY clause
A SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, or MINUS set operator
The WHERE clause of a SELECT statement
DEFAULT value of a column in a CREATE TABLE or ALTER TABLE statement
The condition of a CHECK constraint
-- create syntax - alter same as create, replace CREATE to ALTER
synonymis an alias for a schema object. Synonyms can provide a level of security by masking the name and owner of an object and by providing location transparency for remote objects of a distributed database. Also, they are convenient to use and reduce the complexity of SQL statements for database users.
You can create both
public synonymis owned by the special user group named
PUBLICand is accessible to every user in a database. A
private synonymis contained in the schema of a specific user and available only to the user and to grantees for the underlying object.
-- I think the sql statement is very clear to express
PRIMARY KEY CONSTRAINT,
FOREIGN KEY CONSTRAINT,
NOT NULL CONSTRAINTand
CHECK CONSTRAINT, etc.
primarykey constraint designates a column as the primary key of a table or view. A composite primary key designates a combination of columns as the
primarykey. When you define a primary key constraint inline, you need only the
PRIMARY KEYkeywords. When you define a primary key constraint out of line, you must also specify one or more columns. You must define a composite primary key out of line.
A primary key constraint combines a
UNIQUEconstraint in one declaration.
uniqueconstraint designates a column as a unique key. A composite unique key designates a combination of columns as the
uniquekey. When you define a unique constraint inline, you need only the
UNIQUEkeyword. When you define a unique constraint out of line, you must also specify one or more columns. You must define a composite unique key out of line.
To satisfy a unique constraint, no two rows in the table can have the same value for the unique key. However, the unique key made up of a single column can contain nulls. To satisfy a composite unique key, no two rows in the table or view can have the same combination of values in the key columns. Any row that contains nulls in all key columns automatically satisfies the constraint. However, two rows that contain nulls for one or more key columns and the same combination of values for the other key columns violate the constraint.
-- syntax it's same as primary key
foreign keyconstraint (also called a referential integrity constraint) designates a column as the foreign key and establishes a relationship between that foreign key and a specified primary or unique key, called the referenced key. A composite foreign key designates a combination of columns as the foreign key.
You can define a foreign key constraint on a single key column either inline or out of line. You must specify a composite foreign key and a foreign key on an attribute out of line.
To satisfy a composite foreign key constraint, the composite foreign key must refer to a composite unique key or a composite primary key in the parent table or view, or the value of at least one of the columns of the foreign key must be null.
Specifc Attention ON DELETE Clause
The ON DELETE clause lets you determine how Oracle Database automatically maintains referential integrity if you remove a referenced primary or unique key value. If you omit this clause, then Oracle does not allow you to delete referenced key values in the parent table that have dependent rows in the child table.
CASCADEif you want Oracle to remove dependent foreign key values.
SET NULLif you want Oracle to convert dependent foreign key values to
NULL. You cannot specify this clause for a virtual column, because the values in a virtual column cannot be updated directly. Rather, the values from which the virtual column are derived must be updated.
NOT NULLconstraint prohibits a column from containing nulls. The
NULLkeyword by itself does not actually define an integrity constraint, but you can specify it to explicitly permit a column to contain nulls. You must define
NULLusing inline specification. If you specify neither
NULL, then the default is
NOT NULLconstraints are the only constraints you can specify inline on
columns. To satisfy aNOT NULL` constraint, every row in the table must contain a value for the column.
- You cannot specify
NOT NULLin a view constraint.
- You cannot specify
NOT NULLfor an attribute of an object. Instead, use a
CHECKconstraint with the
IS [NOT] NULLcondition.
checkconstraint lets you specify a condition that each row in the table must satisfy. To satisfy the constraint, each row in the table must make the condition either
TRUEor unknown (due to a null). When Oracle evaluates a check constraint condition for a particular row, any column names in the condition refer to the column values in that row.
The syntax for inline and out-of-line specification of check constraints is the same. However, inline specification can refer only to the column (or the attributes of the column if it is an object column) currently being defined, whereas out-of-line specification can refer to multiple columns or attributes.
Restrictions on Check Constraints
- You cannot specify a
check constraintfor a view. However, you can define the view using the
WITH CHECK OPTIONclause, which is equivalent to specifying a check constraint for the view.
- The condition of a check constraint can refer to any column in the table, but it cannot refer to columns of other tables.
triggersare procedures that are stored in the database and activated (“fired“) when specific conditions occur, such as adding a row to a table. You can use triggers to supplement the standard capabilities of the database to provide a highly customized database management system.
clusterprovides an optional method of storing table data. A
clusteris made up of a group of tables that share the same data blocks. The tables are grouped together because they share common columns and are often used together.
- Disk I/O is reduced and access time improves for joins of clustered tables.
cluster keyis the column, or group of columns, that the clustered tables have in common. You specify the columns of the cluster key when creating the cluster. You subsequently specify the same columns when creating every table added to the cluster. Each cluster key value is stored only once each in the cluster and the cluster index, no matter how many rows of different tables contain the value.
database linkis a schema object in one database that enables you to access objects on another database. The other database need not be an Oracle Database system. However, to access non-Oracle systems you must use Oracle Heterogeneous Services.
By the type, It’s contain
Redo logscontain a record of changes that were made to datafiles. Redo logs are stored in redo log groups, and you must have at least two redo log groups for your database. After the redo log files in a group have filled up, the log writer process (
LGWR) switches the writing of redo records to a new redo log group.
And specific point:
- Record of how to reproduce a change
- Used for Rolling forward database changes
- Stored in redo log files
- Protect against data loss.
To find sessions generating lots of redo, you can use either of the following methods. Both methods examine the amount of undo generated. When a transaction generates undo, it will automatically generate redo as well.
Oracle Database can automatically save the inactive group of redo log files to one or more offline destinations, known collectively as the
archived redo log(also called the archive log). The process of turning redo log files into archived redo log files is called archiving.
sqlplus / as sysdba
# first shutdown database
Undo tablespacesare special tablespaces used solely for storing undo information.
You cannot create any other
segment types(for example,
tables or indexes) in
undo tablespaces. Each database contains zero or more undo tablespaces. In automatic undo management mode, each Oracle instance is assigned one (and only one) undo tablespace.
Undo data is managed within an undo tablespace using undo segments that are automatically created and maintained by Oracle.
And undo records are used to：
- Roll back transactions when a ROLLBACK statement is issued
- Recover the database
- Provide read consistency
- Analyze data as of an earlier point in time by using Oracle Flashback Query
- Recover from logical corruptions using Oracle Flashback features
but, It’s different with redo:
- Record of how to undo a change.
- Used for Rollback, read-consistency
- Stored in Undo Segments
- Protect against inconsistent reads in multiuser systems
-- Displays all redo log groups for the database and indicates which need to be archived.
-- list all directories in the database.
License: CC BY-NC-ND 4.0