With Hybrid Columnar Compression, the database stores the same column for a group of rows together. The data block does not store data in row-major format, but uses a combination of both row and columnar methods.

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Storing column data together, with the same data type and similar characteristics, dramatically increases the storage savings achieved from compression. The database compresses data manipulated by any SQL operation, although compression levels are higher for direct path loads. Database operations work transparently against compressed objects, so no application changes are required.

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Note

Hybrid Column Compression and In-Memory Column Store (IM column store) are closely related. The primary difference is that Hybrid Column Compression optimizes disk storage, whereas the IM column store optimizes memory storage.

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See also

“In-Memory Area” to learn more about the IM column store

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The ORACLE_SID environment variable is used to distinguish this instance from other Oracle Database instances that you may create later and run concurrently on the same host computer.

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1. Decide on a unique Oracle system identifier (SID) for your instance.
2. Open a command window.
3. Set the ORACLE_SID environment variable. Restrictions related to the valid characters in an ORACLE_SID are platform-specific. On some platforms, the SID is case-sensitive.

Step 2: Ensure That the Required Environment Variables Are Set

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Depending on your platform, before you can start SQL*Plus (as required in a later step), you may have to set environment variables, or at least verify that they are set properly.

• Set required environment variables.

For example, on most platforms, ORACLE_SID and ORACLE_HOME must be set. In addition, it is advisable to set the PATH variable to include the ORACLE_HOME/bin directory. On the UNIX and Linux platforms, you must set these environment variables manually.

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On the Windows platform, OUI automatically assigns values to ORACLE_HOME and ORACLE_SID in the Windows registry. If you did not create a database upon installation, OUI does not set ORACLE_SID in the registry, and you will have to set the ORACLE_SID environment variable when you create your database later.

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The database can use table compression to reduce the amount of storage required for the table.

Compression saves disk space, reduces memory use in the database buffer cache, and in some cases speeds query execution. Table compression is transparent to database applications.

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Basic Table Compression and Advanced Row Compression

Dictionary-based table compression provides good compression ratios for heap-organized tables.

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Oracle Database supports the following types of dictionary-based table compression:

• Basic table compressionThis type of compression is intended for bulk load operations. The database does not compress data modified using conventional DML. You must use direct path INSERT operations, ALTER TABLE . . . MOVE operations, or online table redefinition to achieve basic table compression.
• Advanced row compressionThis type of compression is intended for OLTP applications and compresses data manipulated by any SQL operation. The database achieves a competitive compression ratio while enabling the application to perform DML in approximately the same amount of time as DML on an uncompressed table.

For the preceding types of compression, the database stores compressed rows in row major format. All columns of one row are stored together, followed by all columns of the next row, and so on. The database replaces duplicate values with a short reference to a symbol table stored at the beginning of the block. Thus, information that the database needs to re-create the uncompressed data is stored in the data block itself.

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Compressed data blocks look much like normal data blocks. Most database features and functions that work on regular data blocks also work on compressed blocks.

You can declare compression at the tablespace, table, partition, or subpartition level. If specified at the tablespace level, then all tables created in the tablespace are compressed by default.

Example 2-4 Table-Level Compression

The following statement applies advanced row compression to the orders table:

CopyALTER TABLE oe.orders ROW STORE COMPRESS ADVANCED;

Example 2-5 Partition-Level Compression

The following example of a partial CREATE TABLE statement specifies advanced row compression for one partition and basic table compression for the other partition:

CopyCREATE TABLE sales (
    prod_id     NUMBER     NOT NULL,
    cust_id     NUMBER     NOT NULL, ... )
 PCTFREE 5 NOLOGGING NOCOMPRESS
 PARTITION BY RANGE (time_id)
 ( partition sales_2013 VALUES LESS THAN(TO_DATE(...)) ROW STORE COMPRESS BASIC,
   partition sales_2014 VALUES LESS THAN (MAXVALUE) ROW STORE COMPRESS ADVANCED );

See also

• “Row Format” to learn how values are stored in a row
• “Data Block Compression” to learn about the format of compressed data blocks
• “SQL*Loader” to learn about using SQL*Loader for direct path loads
• Oracle Database Administrator’s Guide and Oracle Database Performance Tuning Guide to learn about table compression

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When you use the CREATE DATABASE statement, you must complete additional actions before you have an operational database. These actions include building views on the data dictionary tables and installing standard PL/SQL packages. You perform these actions by running the supplied scripts.

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If you have existing scripts for creating your database, then consider editing those scripts to take advantage of new Oracle Database features.

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The instructions in this section apply to single-instance installations only. See the Oracle Real Application Clusters (Oracle RAC) installation guide for your platform for instructions for creating an Oracle RAC database.

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Using the CREATE DATABASE SQL statement is a more manual approach to creating a database than using Oracle Database Configuration Assistant (DBCA). One advantage of using this statement over using DBCA is that you can create databases from within scripts.

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• About Creating a Database with the CREATE DATABASE Statement
  When you use the CREATE DATABASE statement, you must complete additional actions before you have an operational database. These actions include building views on the data dictionary tables and installing standard PL/SQL packages. You perform these actions by running the supplied scripts.

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• Step 1: Specify an Instance Identifier (SID)
  The ORACLE_SID environment variable is used to distinguish this instance from other Oracle Database instances that you may create later and run concurrently on the same host computer.
• Step 2: Ensure That the Required Environment Variables Are Set
  Depending on your platform, before you can start SQL*Plus (as required in a later step), you may have to set environment variables, or at least verify that they are set properly.
• Step 3: Choose a Database Administrator Authentication Method
  You must be authenticated and granted appropriate system privileges in order to create a database.
• Step 4: Create the Initialization Parameter File
  When an Oracle instance starts, it reads an initialization parameter file. This file can be a text file, which can be created and modified with a text editor, or a binary file, which is created and dynamically modified by the database. The binary file, which is preferred, is called a server parameter file. In this step, you create a text initialization parameter file. In a later step, you create a server parameter file from the text file.

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• Step 5: (Windows Only) Create an Instance
  On the Windows platform, before you can connect to an instance, you must manually create it if it does not already exist. The ORADIM command creates an Oracle Database instance by creating a new Windows service.
• Step 6: Connect to the Instance
  Start SQL*Plus and connect to your Oracle Database instance with the SYSDBA administrative privilege.
• Step 7: Create a Server Parameter File
  The server parameter file enables you to change initialization parameters with the ALTER SYSTEM command and persist the changes across a database shutdown and startup. You create the server parameter file from your edited text initialization file.
• Step 8: Start the Instance
  Start an instance without mounting a database.
• Step 9: Issue the CREATE DATABASE Statement
  To create the new database, use the CREATE DATABASE statement.
• Step 10: Create Additional Tablespaces
  To make the database functional, you must create additional tablespaces for your application data.
• Step 11: Run Scripts to Build Data Dictionary Views
  Run the scripts necessary to build data dictionary views, synonyms, and PL/SQL packages, and to support proper functioning of SQL*Plus.
• Step 12: (Optional) Run Scripts to Install Additional Options
  You may want to run other scripts. The scripts that you run are determined by the features and options you choose to use or install.
• Step 13: Run Datapatch
  Run Datapatch to update information stored in the database to the correct patch level.
• Step 14: Back Up the Database
  Take a full backup of the database to ensure that you have a complete set of files from which to recover if a media failure occurs.
• Step 15: (Optional) Enable Automatic Instance Startup
  You might want to configure the Oracle instance to start automatically when its host computer restarts.

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A null is the absence of a value in a column. Nulls indicate missing, unknown, or inapplicable data.

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Nulls are stored in the database if they fall between columns with data values. In these cases, they require 1 byte to store the length of the column (zero). Trailing nulls in a row require no storage because a new row header signals that the remaining columns in the previous row are null. For example, if the last three columns of a table are null, then no data is stored for these columns.

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See also

Oracle Database SQL Language Reference to learn more about null values

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A rowid is effectively a 10-byte physical address of a row.

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Every row in a heap-organized table has a rowid unique to this table that corresponds to the physical address of a row piece. For table clusters, rows in different tables that are in the same data block can have the same rowid.

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Oracle Database uses rowids internally for the construction of indexes. For example, each key in a B-tree index is associated with a rowid that points to the address of the associated row for fast access. Physical rowids provide the fastest possible access to a table row, enabling the database to retrieve a row in as little as a single I/O.

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See also

• “Rowid Format” to learn about the structure of a rowid
• “Overview of B-Tree Indexes” to learn about the types and structure of B-tree indexes

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The database stores rows in data blocks. Each row of a table containing data for less than 256 columns is contained in one or more row pieces.

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If possible, Oracle Database stores each row as one row piece. However, if all of the row data cannot be inserted into a single data block, or if an update to an existing row causes the row to outgrow its data block, then the database stores the row using multiple row pieces.

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Rows in a table cluster contain the same information as rows in nonclustered tables. Additionally, rows in a table cluster contain information that references the cluster key to which they belong.

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See also

“Data Block Format” to learn about the components of a data block

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Oracle Database uses a data segment in a tablespace to hold table data.

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A segment contains extents made up of data blocks. The data segment for a table (or cluster data segment, for a table cluster) is located in either the default tablespace of the table owner or in a tablespace named in the CREATE TABLE statement.

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See also

“User Segments” to learn about the types of segments and how they are created

Table Organization

By default, a table is organized as a heap, which means that the database places rows where they fit best rather than in a user-specified order. Thus, a heap-organized table is an unordered collection of rows.

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Note

Index-organized tables use a different principle of organization.

As users add rows, the database places the rows in the first available free space in the data segment. Rows are not guaranteed to be retrieved in the order in which they were inserted.

The hr.departments table is a heap-organized table. It has columns for department ID, name, manager ID, and location ID. As rows are inserted, the database stores them wherever they fit. A data block in the table segment might contain the unordered rows shown in the following example:

Copy50,Shipping,121,1500
120,Treasury,,1700
70,Public Relations,204,2700
30,Purchasing,114,1700
130,Corporate Tax,,1700
10,Administration,200,1700
110,Accounting,205,1700

The column order is the same for all rows in a table. The database usually stores columns in the order in which they were listed in the CREATE TABLE statement, but this order is not guaranteed. For example, if a table has a column of type LONG, then Oracle Database always stores this column last in the row. Also, if you add a new column to a table, then the new column becomes the last column stored.

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A table can contain a virtual column, which unlike normal columns does not consume space on disk. The database derives the values in a virtual column on demand by computing a set of user-specified expressions or functions. You can index virtual columns, collect statistics on them, and create integrity constraints. Thus, virtual columns are much like nonvirtual columns.

An integrity constraint is a named rule that restrict the values for one or more columns in a table.

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Data integrity rules prevent invalid data entry into tables. Also, constraints can prevent the deletion of a table when certain dependencies exist.

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If a constraint is enabled, then the database checks data as it is entered or updated. Oracle Database prevents data that does not conform to the constraint from being entered. If a constraint is disabled, then Oracle Database allows data that does not conform to the constraint to enter the database.

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In Example 2-1, the CREATE TABLE statement specifies NOT NULL constraints for the last_name, email, hire_date, and job_id columns. The constraint clauses identify the columns and the conditions of the constraint. These constraints ensure that the specified columns contain no null values. For example, an attempt to insert a new employee without a job ID generates an error.

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You can create a constraint when or after you create a table. You can temporarily disable constraints if needed. The database stores constraints in the data dictionary.