1. Database SecureFiles and Large Objects Developer's Guide
  2. Getting Started
  3. Using Oracle LOB Storage

3 Using Oracle LOB Storage

Oracle LOB storage has two types, SecureFiles LOB storage and BasicFiles LOB storage, which are used with different types of tablespaces.

You design, create, and modify tables with LOB column types.

Topics:

LOB Storage

Earlier Oracle database releases supported only one type of LOB storage. In Oracle Database 11g, SecureFiles LOB storage was introduced; the original storage type was given the name BasicFiles LOB storage and became the default.

LOBs created using BasicFiles LOB storage became known as BasicFiles LOBs and LOBs created using SecureFiles LOB storage were named SecureFiles LOBs. The CREATE TABLE statement added new keywords to indicate the differences: BASICFILE specifies BasicFiles LOB storage and SECUREFILE specifies SecureFiles LOB storage.

Beginning with Oracle Database 12c, SecureFiles LOB storage became the default in the CREATE TABLE statement. If no storage type is explicitly specified, new LOB columns use SecureFiles LOB storage.

The term LOB can represent LOBs of either storage type unless the storage type is explicitly indicated, by name or by reference to archiving or linking (can only apply to the SecureFiles LOB storage type).

See Also:

Initialization, Compatibility, and Upgrading for more information about Initialization and compatibility.

The following sections discuss the two storage types in detail:

BasicFiles LOB Storage

You must use BasicFiles LOB storage for LOB storage in tablespaces that are not managed with Automatic Segment Space Management (ASSM).

SecureFiles LOB Storage

SecureFiles LOBs can only be created in tablespaces managed with Automatic Segment Space Management (ASSM), unlike BasicFiles LOB storage.

SecureFiles LOB storage is designed to provide much better performance and scalability compared to BasicFiles LOBs and to meet or exceed the performance capabilities of traditional network file systems.

SecureFiles LOB storage supports three features that are not available with the BasicFiles LOB storage option: compression, deduplication, and encryption.

Oracle recommends that you enable compression, deduplication, and encryption through the CREATE TABLE statement. If you enable these features through the ALTER TABLE statement, all SecureFiles LOB data in the table is read, modified, and written; this can cause the database to lock the table during a potentially lengthy operation, though there are online capabilities in the ALTER TABLE statement which can help you avoid this issue.

Topics:

About Advanced LOB Compression

Advanced LOB Compression transparently analyzes and compresses SecureFiles LOB data to save disk space and improve performance.

License Requirement: You must have a license for the Oracle Advanced Compression Option to implement Advanced LOB Compression.

See Also:

About Advanced LOB Deduplication

Advanced LOB Deduplication enables Oracle Database to automatically detect duplicate LOB data within a LOB column or partition, and conserve space by storing only one copy of the data.

License Requirement: You must have a license for the Oracle Advanced Compression Option to implement Advanced LOB Deduplication.

See Also:

About SecureFiles Encryption

SecureFiles Encryption introduces a new encryption facility for LOBs. The data is encrypted using Transparent Data Encryption (TDE), which allows the data to be stored securely, and still allows for random read and write access.

License Requirement: You must have a license for the Oracle Advanced Security Option to implement SecureFiles Encryption.

See Also:

CREATE TABLE with LOB Storage

The CREATE TABLE statement works with LOB storage using parameters that are specific to SecureFiles or BasicFiles LOB storage, or both.

Example 3-1 provides the syntax for CREATE TABLE in Backus Naur (BNF) notation, with LOB-specific parameters in bold.

The SHRINK option is not supported for SecureFiles LOBs.

See Also:

Example 3-1 BNF for CREATE TABLE

CREATE [ GLOBAL TEMPORARY ] TABLE
   [ schema.]table OF
   [ schema.]object_type
   [ ( relational_properties ) ]
   [ ON COMMIT { DELETE | PRESERVE } ROWS ]
   [ OID_clause ]
   [ OID_index_clause ]
   [ physical_properties ]
   [ table_properties ] ;
<relational_properties> ::= 
{ column_definition
| { out_of_line_constraint
  | out_of_line_ref_constraint
  | supplemental_logging_props
  }
}
  [, { column_definition
     | { out_of_line_constraint
       | out_of_line_ref_constraint
       | supplemental_logging_props
       }
  ]...
 <column_definition> ::= 
column data_type [ SORT ]
      [ DEFAULT expr ]
      [ ENCRYPT encryption_spec ]
      [ ( inline_constraint [ inline_constraint ] ... )
      | inline_ref_constraint 
      ]
<data_type> ::=
{ Oracle_built_in_datatypes
| ANSI_supported_datatypes
| user_defined_types
| Oracle_supplied_types
}

<Oracle_built_in_datatypes> ::=
{ character_datatypes
| number_datatypes
| long_and_raw_datatypes
| datetime_datatypes
| large_object_datatypes
| rowid_datatypes
}
<large_object_datatypes> ::= 
{ BLOB | CLOB | NCLOB| BFILE }
<table_properties> ::=
  [ column_properties ]
  [ table_partitioning_clauses ]
  [ CACHE | NOCACHE ]
  [ parallel_clause ]
  [ ROWDEPENDENCIES | NOROWDEPENDENCIES ]
  [ enable_disable_clause ]
  [ enable_disable_clause ]...
  [ row_movement_clause ]
  [ AS subquery ]
<column_properties> ::=
  { object_type_col_properties
  | nested_table_col_properties
  | { varray_col_properties | LOB_storage_clause }
    [ (LOB_partition_storage
        [, LOB_partition_storage ]...
      )
    ]
  | XMLType_column_properties
  }
  [ { object_type_col_properties
    | nested_table_col_properties
    | { varray_col_properties | LOB_storage_clause }
      [ ( LOB_partition_storage
          [, LOB_partition_storage ]...
        )
      ]
    | XMLType_column_properties
    }
  ]...
<LOB_partition_storage> ::=
  PARTITION partition
  { LOB_storage_clause | varray_col_properties }
    [ LOB_storage_clause | varray_col_properties ]...
  [ ( SUBPARTITION subpartition
     { LOB_storage_clause | varray_col_properties }
       [ LOB_storage_clause
       | varray_col_properties
       ]...
    )
  ]
<LOB_storage_clause> ::=
  LOB
  { (LOB_item [, LOB_item ]...)
      STORE AS [ SECUREFILE | BASICFILE ] (LOB_storage_parameters)
  | (LOB_item)
      STORE AS [ SECUREFILE | BASICFILE ]
        { LOB_segname (LOB_storage_parameters)
        | LOB_segname
        | (LOB_storage_parameters)
        }
  }
<LOB_storage_parameters> ::=
  { TABLESPACE tablespace
  | { LOB_parameters [ storage_clause ]
    }
  | storage_clause
  }
    [ TABLESPACE tablespace
    | { LOB_parameters [ storage_clause ]
      }
    ]...
<LOB_parameters> ::=
  [ { ENABLE | DISABLE } STORAGE IN ROW
  | CHUNK integer
  | PCTVERSION integer
  | RETENTION [ { MAX | MIN integer | AUTO | NONE } ]
  | FREEPOOLS integer
  | LOB_deduplicate_clause
  | LOB_compression_clause
  | LOB_encryption_clause
  | { CACHE |  NOCACHE | CACHE READS } [ logging_clause ] } }
  ]
<logging_clause> ::=
  { LOGGING | NOLOGGING | FILESYSTEM_LIKE_LOGGING }
<storage_clause> ::=
  STORAGE
  ({ INITIAL integer [ K | M ]
   | NEXT integer [ K | M ]
   | MINEXTENTS integer
   | MAXEXTENTS { integer | UNLIMITED }
   | PCTINCREASE integer
   | FREELISTS integer
   | FREELIST GROUPS integer
   | OPTIMAL [ integer [ K | M ]
             | NULL
             ]
   | BUFFER_POOL { KEEP | RECYCLE | DEFAULT }
   }
     [ INITIAL integer [ K | M ]
     | NEXT integer [ K | M ]
     | MINEXTENTS integer
     | MAXEXTENTS { integer | UNLIMITED }
     | MAXSIZE { { integer { K | M | G | T | P } } | UNLIMITED }
     | PCTINCREASE integer
     | FREELISTS integer
     | FREELIST GROUPS integer
     | OPTIMAL [ integer [ K | M ]
               | NULL
               ]
     | BUFFER_POOL { KEEP | RECYCLE | DEFAULT }
     ]...
  )
<LOB_deduplicate_clause> ::=
  { DEDUPLICATE 
  | KEEP_DUPLICATES
  }
<LOB_compression_clause> ::=
  { COMPRESS [ HIGH | MEDIUM | LOW ]
  | NOCOMPRESS 
  }
<LOB_encryption_clause> ::=
  { ENCRYPT [ USING 'encrypt_algorithm' ] 
    [ IDENTIFIED BY password ]
  | DECRYPT 
  }
<XMLType_column_properties> ::= 
XMLTYPE [ COLUMN ] column
   [ XMLType_storage ]
   [ XMLSchema_spec ]
<XMLType_storage> ::=
       STORE AS
   { OBJECT RELATIONAL
   | [ SECUREFILE | BASICFILE ] { CLOB | BINARY XML }
       [ { LOB_segname [ (LOB_parameters) ]
         | LOB_parameters
         }
         ]
<varray_col_properties> ::=

        VARRAY varray_item 
   { [ substitutable_column_clause ]
     STORE AS [ SECUREFILE | BASICFILE ] LOB
        { [ LOB_segname ] (LOB_parameters)
        | LOB_segname 
        }
   | substitutable_column_clause
   }

CREATE TABLE LOB Storage Parameters

The CREATE TABLE statement uses parameters relating to LOB storage, and more specifically to either BasicFiles LOB or SecureFiles LOB.

Table 3-1 summarizes the parameters of the CREATE TABLE statement that relate to LOB storage, where necessary noting whether a parameter is specific to BasicFiles LOB or SecureFiles LOB storage.

Table 3-1 Parameters of CREATE TABLE Statement Related to LOBs

Parameter Description

BASICFILE

Specifies BasicFiles LOB storage, the original architecture for LOBs.

If you set the compatibility mode to Oracle Database 11g, then BASICFILE functionality is enabled by default and specified for completeness.

Starting with Oracle Database 12c, you must explicitly specify the parameter BASICFILE to use the BasicFiles LOB storage type. Otherwise, the CREATE TABLE statement uses SecureFiles LOB, the current default.

For BasicFiles LOBs, specifying any of the SecureFiles LOB options results in an error.

See Also:

Initialization, Compatibility, and Upgrading

SECUREFILE

Specifies SecureFiles LOBs storage.

Starting with Oracle Database 12c, the SecureFiles LOB storage type, specified by the parameter SECUREFILE, is the default.

A SecureFiles LOB can only be created in a tablespace managed with Automatic Segment Space Management (ASSM).

CHUNK

For BasicFiles LOBs, specifies the chunk size when creating a table that stores LOBs.

CHUNK is one or more Oracle blocks and corresponds to the data size used by Oracle Database when accessing or modifying the LOB.

For SecureFiles LOBs, it is an advisory size provided for backward compatibility.

RETENTION

Configures the LOB column to store old versions of LOB data in a specified manner.

In Oracle Database Release 12c, this parameter specifies the retention policy.

RETENTION has these possible values:

  • MAX specifies that the system keep old versions of LOB data blocks until the space used by the segment has reached the size specified in the MAXSIZE parameter. If MAXSIZE is not specified, MAX behaves like AUTO.

  • MIN specifies that the system keep old versions of LOB data blocks for the specified number of seconds.

  • NONE specifies that there is no retention period and space can be reused in any way deemed necessary.

  • AUTO specifies that the system manage the space as efficiently as possible weighing both time and space needs.

See Also:

RETENTION Parameter for BasicFiles LOBs for more information about RETENTION parameter used with BasicFiles LOBs.

MAXSIZE

Specifies the upper limit of storage space that a LOB may use.

If this amount of space is consumed, new LOB data blocks are taken from the pool of old versions of LOB data blocks as needed, regardless of time requirements.

FREEPOOLS

Specifies the number of FREELIST groups for BasicFiles LOBs, if the database is in automatic undo mode. Under Release 12c compatibility, this parameter is ignored when SecureFiles LOBs are created.

LOGGING, NOLOGGING, or FILESYSTEM_LIKE_LOGGING

Specifies logging options:

  • LOGGING specifies logging the creation of the LOB and subsequent inserts into the LOB, in the redo log file. LOGGING is the default.

  • NOLOGGING specifies no logging.

  • FILESYSTEM_LIKE_LOGGING specifies that the system only logs the metadata. This is similar to metadata journaling of file systems, which reduces mean time to recovery from failures. FILESYSTEM_LIKE_LOGGING ensures that data is completely recoverable (an instance recovery) after a server failure.

    This option is invalid for BasicFiles LOBs.

For SecureFiles LOBs, the following applies:

  • The NOLOGGING setting is converted internally to FILESYSTEM_LIKE_LOGGING.

  • The LOGGING setting is similar to the data journaling of file systems.

  • Both the LOGGING and FILESYSTEM_LIKE_LOGGING settings provide a complete transactional file system.

For a non-partitioned object, the value specified for this clause is the actual physical attribute of the segment associated with the object.

For partitioned objects, the value specified for this clause is the default physical attribute of the segments associated with all partitions specified in the CREATE statement (and in subsequent ALTER ... ADD PARTITION statements), unless you specify the logging attribute in the PARTITION description.

CAUTION:

For LOB segments with NOLOGGING or FILESYSTEM_LIKE_LOGGING settings, it is possible that data can change on the disk during a backup operation. This results in read inconsistency. To avoid this situation, ensure that changes to LOB segments are saved in the redo log file by setting LOGGING for LOB storage.

NOLOGGING and FILESYSTEM_LIKE_LOGGING SecureFiles are recoverable after an instance failure, but not after a media failure. LOGGING SecureFiles are recoverable after both instance and media failures.

See Also:

FREELISTS or FREELIST GROUPS

Specifies the number of process freelists or freelist groups, respectively, allocated to the segment; NULL for partitioned tables. Under Release 12c compatibility, these parameters are ignored when SecureFiles LOBs are created.

PCTVERSION

Specifies the percentage of used BasicFiles LOB data space that may be occupied by old versions of the LOB data pages.

Under Release 12c compatibility, this parameter is ignored when SecureFiles LOBs are created.

COMPRESS or NOCOMPRESS

The COMPRESS option turns on Advanced LOB Compression, and NOCOMPRESS turns it off.

Note that setting table or index compression does not affect Advanced LOB Compression.

DEDUPLICATE or KEEP_DUPLICATES

The DEDUPLICATE option enables Advanced LOB Deduplication; it specifies that SecureFiles LOB data that is identical in two or more rows in a LOB column, partition or subpartition must share the same data blocks. The database combines SecureFiles LOBs with identical content into a single copy, reducing storage and simplifying storage management. The opposite of this option is KEEP_DUPLICATES.

ENCRYPT or DECRYPT

The ENCRYPT option turns on SecureFiles Encryption, and encrypts all SecureFiles LOB data using Oracle Transparent Data Encryption (TDE). The DECRYPT options turns off SecureFiles Encryption.

CREATE TABLE and SecureFiles LOB Features

Note usage notes and examples for SecureFiles LOBs used with theCREATE TABLE.

This section provides usage notes and examples for features specific to SecureFiles LOBs used with CREATE TABLE.

Note:

Clauses in example discussions refer to the Backus Naur (BNF) notation Example 3-1.

See Also:

CREATE TABLE LOB Storage Parameters for more information about parameters

Topics:

CREATE TABLE with Advanced LOB Compression

You can use Advanced LOB Compression with the CREATE TABLE statement under certain circumstances.

Topics:

Usage Notes for Advanced LOB Compression

Consider these issues when using the CREATE TABLE statement and Advanced LOB Compression.

  • Advanced LOB Compression is performed on the server and enables random reads and writes to LOB data. Compression utilities on the client, like utl_compress, cannot provide random access.

  • Advanced LOB Compression does not enable table or index compression. Conversely, table and index compression do not enable Advanced LOB Compression.

  • The LOW, MEDIUM, and HIGH options provide varying degrees of compression. The higher the compression, the higher the latency incurred. The HIGH setting incurs more work, but compresses the data better. The default is MEDIUM.

    The LOW compression option uses an extremely lightweight compression algorithm that removes the majority of the CPU cost that is typical with file compression. Compressed SecureFiles LOBs at the LOW level provide a very efficient choice for SecureFiles LOB storage. SecureFiles LOBs compressed at LOW generally consume less CPU time and less storage than BasicFiles LOBs, and typically help the application run faster because of a reduction in disk I/O.

  • Compression can be specified at the partition level. The CREATE TABLE lob_storage_clause enables specification of compression for partitioned tables on a per-partition basis.

  • The DBMS_LOB.SETOPTIONS procedure can enable and disable compression on individual SecureFiles LOBs.

See Also:

Oracle Database PL/SQL Packages and Types Reference for more information about DBMS_LOB.SETOPTIONS procedure

Examples of CREATE TABLE and Advanced LOB Compression

These examples demonstrate how to issue CREATE TABLE statements for specific compression scenarios.

Example 3-2 Creating a SecureFiles LOB Column with LOW Compression 

CREATE TABLE t1 (a CLOB)
    LOB(a) STORE AS SECUREFILE(
    COMPRESS LOW
    CACHE
    NOLOGGING
  );

Example 3-3 Creating a SecureFiles LOB Column with MEDIUM (default) Compression

CREATE TABLE t1 ( a CLOB)
    LOB(a) STORE AS SECUREFILE (
         COMPRESS
         CACHE
         NOLOGGING
    );

Example 3-4 Creating a SecureFiles LOB Column with HIGH Compression

CREATE TABLE t1 ( a CLOB)
    LOB(a) STORE AS SECUREFILE (
         COMPRESS HIGH
         CACHE
    );

Example 3-5 Creating a SecureFiles LOB Column with Disabled Compression

CREATE TABLE t1 ( a CLOB)
    LOB(a) STORE AS SECUREFILE (
         NOCOMPRESS
         CACHE
    );

Example 3-6 Creating a SecureFiles LOB Column with Compression on One Partition

CREATE TABLE t1 ( REGION VARCHAR2(20), a BLOB) 
     LOB(a) STORE AS SECUREFILE (
        CACHE
     )
     PARTITION BY LIST (REGION) (
          PARTITION p1 VALUES ('x', 'y')
               LOB(a) STORE AS SECUREFILE (
                   COMPRESS
                ),
          PARTITION p2 VALUES (DEFAULT)
     );
CREATE TABLE with Advanced LOB Deduplication

You can use Advanced LOB Deduplication with the CREATE TABLE statement.

Topics:

Usage Notes for Advanced LOB Deduplication

Consider these issues when using CREATE TABLE and Advanced LOB Deduplication.

  • Identical LOBs are good candidates for deduplication. Copy operations can avoid data duplication by enabling deduplication.

  • Duplicate detection happens within a LOB segment. Duplicate detection does not span partitions or subpartitions for partitioned and subpartitioned LOB columns.

  • Deduplication can be specified at a partition level. The CREATE TABLE lob_storage_clause enables specification for partitioned tables on a per-partition basis.

  • The DBMS_LOB.SETOPTIONS procedure can enable or disable deduplication on individual LOBs.

Examples of CREATE TABLE and Advanced LOB Deduplication

These examples demonstrate how to issue CREATE TABLE statements for specific deduplication scenarios.

Example 3-7 Creating a SecureFiles LOB Column with Deduplication 

CREATE TABLE t1 ( a CLOB)
    LOB(a) STORE AS SECUREFILE (
        DEDUPLICATE
        CACHE
    );

Example 3-8 Creating a SecureFiles LOB Column with Disabled Deduplication

CREATE TABLE t1 ( a CLOB)
    LOB(a) STORE AS SECUREFILE (
         KEEP_DUPLICATES
         CACHE
    );

Example 3-9 Creating a SecureFiles LOB Column with Deduplication on One Partition

CREATE TABLE t1 ( REGION VARCHAR2(20), a BLOB) 
     LOB(a) STORE AS SECUREFILE (
           CACHE
)
PARTITION BY LIST (REGION) (
     PARTITION p1 VALUES ('x', 'y')
          LOB(a) STORE AS SECUREFILE (
               DEDUPLICATE
          ),
     PARTITION p2 VALUES (DEFAULT)
);

Example 3-10 Creating a SecureFiles LOB column with Deduplication Disabled on One Partition

CREATE TABLE t1 ( REGION VARCHAR2(20), ID NUMBER, a BLOB)
     LOB(a) STORE AS SECUREFILE (
           DEDUPLICATE
           CACHE
)
PARTITION BY RANGE (REGION)
      SUBPARTITION BY HASH(ID) SUBPARTITIONS 2 (
        PARTITION p1 VALUES LESS THAN (51)
           lob(a) STORE AS a_t2_p1
           (SUBPARTITION t2_p1_s1 lob(a) STORE AS a_t2_p1_s1,
            SUBPARTITION t2_p1_s2 lob(a) STORE AS a_t2_p1_s2),
        PARTITION p2 VALUES LESS THAN (MAXVALUE)
           lob(a) STORE AS a_t2_p2 ( KEEP_DUPLICATES ) 
           (SUBPARTITION t2_p2_s1 lob(a) STORE AS a_t2_p2_s1,
            SUBPARTITION t2_p2_s2 lob(a) STORE AS a_t2_p2_s2)
      );
CREATE TABLE with SecureFiles Encryption

You can use SecureFiles Encryption with the CREATE TABLE statement.

Topics:

Usage Notes for SecureFiles Encryption

Consider these issues when using CREATE TABLE and SecureFiles Encryptions

  • Transparent Data Encryption (TDE) supports encryption of LOB data types.

  • Encryption is performed at the block level.

  • The encrypt_algorithm indicates the name of the encryption algorithm. Valid algorithms are: AES192 (default), 3DES168, AES128, and AES256.

  • The column encryption key is derived from PASSWORD, if specified.

  • The default for LOB encryption is SALT. NO SALT is not supported.

  • All LOBs in the LOB column are encrypted.

  • DECRYPT keeps the LOBs in clear text.

  • LOBs can be encrypted only on a per-column basis, similar to TDE. All partitions within a LOB column are encrypted.

  • Key management controls the ability to encrypt or decrypt.

  • TDE is not supported by the traditional import and export utilities or by transportable-tablespace-based export. Use the Data Pump expdb and impdb utilities with encrypted columns instead.

    See Also:

    "Oracle Database Advanced Security Guide for information about using the ADMINISTER KEY MANAGEMENT statement to create TDE keystores

Examples of CREATE TABLE and SecureFiles Encryption

These examples demonstrate how to issue CREATE TABLE statements for specific encryption scenarios.

Example 3-11 Creating a SecureFiles LOB Column with a Specific Encryption Algorithm

CREATE TABLE t1 ( a CLOB ENCRYPT USING 'AES128')
    LOB(a) STORE AS SECUREFILE (
         CACHE
    );

Example 3-12 Creating a SecureFiles LOB column with encryption for all partitions 

CREATE TABLE t1 ( REGION VARCHAR2(20), a BLOB)
  LOB(a) STORE AS SECUREFILE (
  ENCRYPT USING 'AES128'
  NOCACHE
  FILESYSTEM_LIKE_LOGGING
)
PARTITION BY LIST (REGION) (
PARTITION p1 VALUES ('x', 'y'),
PARTITION p2 VALUES (DEFAULT)
);

Example 3-13 Creating a SecureFiles LOB Column with Encryption Based on a Password Key

CREATE TABLE t1 ( a CLOB ENCRYPT IDENTIFIED BY foo)
    LOB(a) STORE AS SECUREFILE (
        CACHE
    );

The following example has the same result because the encryption option can be set in the LOB_deduplicate_clause section of the statement: 

CREATE TABLE t1 (a CLOB)
    LOB(a) STORE AS SECUREFILE (
        CACHE
        ENCRYPT
        IDENTIFIED BY foo
    );

Example 3-14 Creating a SecureFiles LOB Column with Disabled Encryption

CREATE TABLE t1 ( a CLOB )
    LOB(a) STORE AS SECUREFILE (
        CACHE DECRYPT
    );

ALTER TABLE with LOB Storage

You can modify LOB storage with an ALTER TABLE statement and specific LOB-related parameters.

Topics:

About ALTER TABLE and LOB Storage

You can use ALTER TABLE to enable compression, deduplication, or encryption features for a LOB column.

The ALTER TABLE statement supports online operations and Oracle Database supports parallel operations on SecureFiles LOBs columns, making this a resource-efficient approach.

As an alternative to ALTER TABLE, you can use online redefinition to enable one or more of these features. As with ALTER TABLE, online redefinition of SecureFiles LOB columns can be executed in parallel.

Note that the SHRINK option is not supported for SecureFiles LOBs.

See Also:

BNF for the ALTER TABLE Statement

This Backus Naur (BNF) notation provides the syntax for ALTER TABLE with LOB-specific parameters in bold.

See Also:

ALTER TABLE [ schema.]table
[ alter_table_properties
| column_clauses
| constraint_clauses
| alter_table_partitioning
| alter_external_table_clauses
| move_table_clause
]
[ enable_disable_clause
| { ENABLE | DISABLE }
{ TABLE LOCK | ALL TRIGGERS }
[ enable_disable_clause
| { ENABLE | DISABLE }
{ TABLE LOCK | ALL TRIGGERS }
]...
] ;
<column_clauses> ::=
{ { add_column_clause
| modify_column_clause
| drop_column_clause
}
[ add_column_clause
| modify_column_clause
| drop_column_clause
]...
| rename_column_clause
| modify_collection_retrieval
[ modify_collection_retrieval ]...
| modify_LOB_storage_clause
[ modify_LOB_storage_clause ] ...
| alter_varray_col_properties
[ alter_varray_col_properties ]
}
<modify_LOB_storage_clause> ::=
MODIFY LOB (LOB_item) ( modify_LOB_parameters )
<modify_LOB_parameters> ::=
{ storage_clause
| PCTVERSION integer
| FREEPOOLS integer
| REBUILD FREEPOOLS
| LOB_retention_clause
| LOB_deduplicate_clause
| LOB_compression_clause
| { ENCRYPT encryption_spec | DECRYPT }
| { CACHE
| { NOCACHE | CACHE READS } [ logging_clause ]
}
| allocate_extent_clause
| shrink_clause
| deallocate_unused_clause
} ...

ALTER TABLE LOB Storage Parameters

You must use specific parameters of the ALTER TABLE statement that relate to LOB storage.

Parameters may be specific to BasicFiles LOB or SecureFiles LOB storage, as indicated.

Table 3-2 Parameters of ALTER TABLE Statement Related to LOBs

Parameter Description

RETENTION

Configures the LOB column to store old versions of LOB data in a specified manner. Altering RETENTION only affects space created after the ALTER TABLE statement runs.

COMPRESS or NOCOMPRESS

Enables or disables Advanced LOB Compression. All LOBs in the LOB segment are altered with the new setting.

DEDUPLICATE or KEEP_DUPLICATES

Enables or disables Advanced LOB Deduplication.

The option DEDUPLICATE enables you to specify that LOB data that is identical in two or more rows in a LOB column share the same data blocks. The database combines LOBs with identical content into a single copy, reducing storage and simplifying storage management. The opposite of this option is KEEP_DUPLICATES.

ENCRYPT or DECRYPT

Enables or disables SecureFiles LOB encryption. Alters all LOBs in the LOB segment with the new setting. A LOB segment can be only altered to enable or disable LOB encryption. That is, ALTER cannot be used to update the encryption algorithm or the encryption key. Update the encryption algorithm or encryption key using the ALTER TABLE REKEY syntax.

ALTER TABLE SecureFiles LOB Features

Certain features specific to SecureFiles LOBs work with the ALTER TABLEstatement.

These SecureFiles LOBs features work with ALTER TABLE as described in the usage notes and examples.

Note:

Clauses in example discussions refer to the Backus Naur (BNF) notation "BNF for the ALTER TABLE Statement".

Parameters are described in "ALTER TABLE LOB Storage Parameters".

Topics:

ALTER TABLE with Advanced LOB Compression

Advanced LOB Compression works with the ALTER TABLE statement.

Topics:

Usage Notes for Advanced LOB Compression

Consider these issues when using ALTER TABLE and Advanced LOB Compression.

  • This syntax alters the compression mode of the LOB column.

  • The DBMS_LOB.SETOPTIONS procedure can enable or disable compression on individual LOBs.

  • Compression may be specified either at the table level or the partition level.

  • The LOW, MEDIUM, and HIGH options provide varying degrees of compression. The higher the compression, the higher the latency incurred. The HIGH setting incurs more work, but compresses the data better. The default is MEDIUM.

See Also:

CREATE TABLE with Advanced LOB Compression

Examples of ALTER TABLE and Advanced LOB Compression

These examples demonstrate how to issue ALTER TABLE statements for specific compression scenarios.

Example 3-15 Altering a SecureFiles LOB Column to Enable LOW Compression

ALTER TABLE t1 MODIFY
   LOB(a) (
           COMPRESS LOW
   );

Example 3-16 Altering a SecureFiles LOB Column to Disable Compression

ALTER TABLE t1 MODIFY 
    LOB(a) (
         NOCOMPRESS
    );

Example 3-17 Altering a SecureFiles LOB Column to Enable HIGH Compression

ALTER TABLE t1 MODIFY 
    LOB(a) (
         COMPRESS HIGH
    );

Example 3-18 Altering a SecureFiles LOB Column to Enable Compression on One partition

ALTER TABLE t1 MODIFY PARTITION p1
    LOB(a) (
         COMPRESS HIGH
    );
ALTER TABLE with Advanced LOB Deduplication

Advanced LOB Deduplication works with the ALTER TABLE statement.

Topics:

Usage Notes for Advanced LOB Deduplication

Consider these issues when using ALTER TABLE and Advanced LOB Deduplication.

  • The ALTER TABLE syntax can enable or disable LOB-level deduplication.

  • This syntax alters the deduplication mode of the LOB column.

  • The DBMS_LOB.SETOPTIONS procedure can enable or disable deduplication on individual LOBs.

  • Deduplication can be specified at a table level or partition level. Deduplication does not span across partitioned LOBs.

Examples of ALTER TABLE and Advanced LOB Deduplication

These examples demonstrate how to issue ALTER TABLE statements for specific deduplication scenarios.

Example 3-19 Altering a SecureFiles LOB Column to Disable Deduplication

ALTER TABLE t1 MODIFY 
     LOB(a) (
         KEEP_DUPLICATES 
    );

Example 3-20 Altering a SecureFiles LOB Column to Enable Deduplication

ALTER TABLE t1 MODIFY 
    LOB(a) (
         DEDUPLICATE
    );

Example 3-21 Altering a SecureFiles LOB Column to Enable Deduplication on One Partition

ALTER TABLE t1 MODIFY PARTITION p1
    LOB(a) (
         DEDUPLICATE
    );
ALTER TABLE with SecureFiles Encryption

SecureFiles Encryption works with the ALTER TABLE statement.

Topics:

Usage Notes for SecureFiles Encryption

Consider these issues when using ALTER TABLE and SecureFiles Encryption.

  • ALTER TABLE enables and disables SecureFiles Encryption. This syntax also allows the user to re-key LOB columns with a new key or algorithm.

  • ENCRYPT and DECRYPT options enable or disable encryption on all LOBs in the specified SecureFiles LOB column.

  • The default for LOB encryption is SALT. NO SALT is not supported.

  • The DECRYPT option converts encrypted columns to clear text form.

  • Key management controls the ability to encrypt or decrypt.

  • LOBs can be encrypted only on a per-column basis. A partitioned LOB has either all partitions encrypted or not encrypted.

Examples of ALTER TABLE and SecureFiles Encryption

These examples demonstrate how to issue ALTER TABLE statements for specific encryption scenarios.

Example 3-22 Altering a SecureFiles LOB Column by Encrypting Based on a Specific Algorithm

Enable LOB encryption using 3DES168. 

ALTER TABLE t1 MODIFY 
      ( a CLOB ENCRYPT USING '3DES168');

This is another example of enabling LOB encryption using 3DES168. 

ALTER TABLE t1 MODIFY LOB(a)
      (ENCRYPT USING '3DES168');

Example 3-23 Altering a SecureFiles LOB Column by Encrypting Based on a Password Key

Enable encryption on a SecureFiles LOB column and build the encryption key using a password.

ALTER TABLE t1 MODIFY 
    ( a CLOB ENCRYPT IDENTIFIED BY foo);

Example 3-24 Altering a SecureFiles LOB Column by Re-keying the Encryption

To re-encrypt the LOB column with a new key, re-key the table.

ALTER TABLE t1 REKEY USING '3DES168';

Initialization, Compatibility, and Upgrading

You must perform LOB initialization using appropriate compatibility parameters.

Topics:

Compatibility and Upgrading

All features described in this document are enabled with compatibility set to 11.2.0.0.0 or higher. There is no downgrade capability after 11.2.0.0.0 is set.

If you want to upgrade BasicFiles LOBs to SecureFiles LOBs, you must use typical methods for upgrading data (CTAS/ITAS, online redefinition, export/import, column to column copy, or using a view and a new column). Most of these solutions require twice the disk space used by the data in the input LOB column. However, partitioning and taking these actions on a partition-by-partition basis lowers the disk space requirements.

Initialization Parameter for SecureFiles LOBs

You, as database administrator, using the DB_SECUREFILE initialization parameter, can modify the initial settings that the COMPATIBILITY parameter sets as default.

By changing the intial settings, you change the circumstances under which SecureFiles LOBs or BasicFiles LOBs are created or allowed. The DB_SECUREFILE parameter is typically set in the file init.ora.

See Also:

The DB_SECUREFILE initialization parameter is dynamic and can be modified with the ALTER SYSTEM statement. Example 3-25 shows the format for changing the parameter value:

The valid values for DB_SECUREFILE are:

  • NEVER prevents SecureFiles LOBs from being created. If NEVER is specified, any LOBs that are specified as SecureFiles LOBs are created as BasicFiles LOBs. If storage options are not specified, the BasicFiles LOB defaults are used. All SecureFiles LOB-specific storage options and features such as compress, encrypt, or deduplicate throw an exception.

  • IGNORE disallows SecureFiles LOBs and ignores any errors that forcing BasicFiles LOBs with SecureFiles LOBs options might cause. If IGNORE is specified, the SECUREFILE keyword and all SecureFiles LOB options are ignored.

  • PERMITTED allows SecureFiles LOBs to be created, if specified by users. Otherwise, BasicFiles LOBs are created.

  • PERFERRED attempts to create a SecureFiles LOB unless BasicFiles LOB is explicitly specified for the LOB or the parent LOB (if the LOB is in a partition or sub-partition). PREFERRED is the default value starting with Oracle Database 12c.

  • ALWAYS attempts to create SecureFiles LOBs but creates any LOBs not in ASSM tablespaces as BasicFiles LOBs, unless the SECUREFILE parameter is explicitly specified. Any BasicFiles LOB storage options specified are ignored, and the SecureFiles LOB defaults are used for all storage options not specified.

  • FORCE attempts to create all LOBs as SecureFiles LOBs even if users specify BASICFILE. This option is not recommended. Instead, PREFERRED or ALWAYS should be used.

Example 3-25 Setting DB_SECUREFILE parameter through ALTER SYSTEM

ALTER SYSTEM SET DB_SECUREFILE = 'ALWAYS';

Migrating Columns from BasicFiles LOBs to SecureFiles LOBs

You can use several methods of migrating LOBs columns.

Topics:

Preventing Generation of REDO Data When Migrating to SecureFiles LOBs

Migrating BasicFiles LOB columns generates redo data, which can cause performance problems.

Redo changes for the table are logged during the migration process if the CREATE TABLE statement had the LOGGING clause set.

Redo changes for a column being converted from BasicFiles LOB to SecureFiles LOB are logged if LOGGING is the storage setting for the SecureFiles LOB column. The logging setting (LOGGING or NOLOGGING) for the LOB column is inherited from the tablespace in which the LOB is created.

You can prevent redo space generation during migration to SecureFiles LOB.

  • Specify the NOLOGGING storage parameter for any new SecureFiles LOB columns.

    You may turn LOGGING on when the migration is complete.

Online Redefinition for BasicFiles LOBs

Online redefinition is the recommended method for migration of BasicFiles LOBs to SecureFiles LOBs.

You can perform online redefinition at the table or partition level.

Online Redefinition Advantage

  • No requirement to take the table or partition offline

  • Can be done in parallel

Online Redefinition Disadvantages

  • Additional storage equal to the entire table or partition required and all LOB segments must be available

  • Global indexes must be rebuilt

Online Redefinition Example for Migrating Tables with BasicFiles LOBs

You can migrate a table using Online Redefinition.

Online Redefinition has the advantage of not requiring the table to be off line, but it requires additional free space equal to or even slightly greater than the space used by the table. Example 3-26 demonstrates how to migrate a table using Online Redefinition.

Example 3-26 Example of Online Redefinition

REM Grant privileges required for online redefinition.
GRANT EXECUTE ON DBMS_REDEFINITION TO pm;
GRANT ALTER ANY TABLE TO pm;
GRANT DROP ANY TABLE TO pm;
GRANT LOCK ANY TABLE TO pm;
GRANT CREATE ANY TABLE TO pm;
GRANT SELECT ANY TABLE TO pm;
REM Privileges required to perform cloning of dependent objects.
GRANT CREATE ANY TRIGGER TO pm;
GRANT CREATE ANY INDEX TO pm;
CONNECT pm
//  ALTER SESSION FORCE parallel dml;
DROP TABLE cust;
CREATE TABLE cust(c_id NUMBER PRIMARY KEY,
    c_zip NUMBER,
    c_name VARCHAR(30) DEFAULT NULL,
    c_lob CLOB
);
INSERT INTO cust VALUES(1, 94065, 'hhh', 'ttt');
-- Creating Interim Table
-- There is no requirement to specify constraints because they are
-- copied over from the original table.
CREATE TABLE cust_int(c_id NUMBER NOT NULL,
    c_zip NUMBER,
    c_name VARCHAR(30) DEFAULT NULL,
    c_lob CLOB
) LOB(c_lob) STORE AS SECUREFILE (NOCACHE FILESYSTEM_LIKE_LOGGING);
DECLARE
    col_mapping VARCHAR2(1000);
BEGIN
-- map all the columns in the interim table to the original table
    col_mapping :=
    'c_id c_id , '||
    'c_zip c_zip , '||
    'c_name c_name, '||
    'c_lob c_lob';
DBMS_REDEFINITION.START_REDEF_TABLE('pm', 'cust', 'cust_int', col_mapping);
END;
/
DECLARE
    error_count pls_integer := 0;
BEGIN
    DBMS_REDEFINITION.COPY_TABLE_DEPENDENTS('pm', 'cust', 'cust_int',
      1, TRUE,TRUE,TRUE,FALSE, error_count);
    DBMS_OUTPUT.PUT_LINE('errors := ' || TO_CHAR(error_count));
END;
/
EXEC DBMS_REDEFINITION.FINISH_REDEF_TABLE('pm', 'cust', 'cust_int');
-- Drop the interim table
DROP TABLE cust_int;
DESC cust;
-- The following insert statement fails. This illustrates
-- that the primary key constraint on the c_id column is
-- preserved after migration.
INSERT INTO cust VALUES(1, 94065, 'hhh', 'ttt');
SELECT * FROM cust;

Redefining a SecureFiles LOB in Parallel

You can redefine a SecureFiles LOB column in parallel, if the system has sufficient resources for parallel execution.

To set up parallel execution of online redefinition, run ALTER SESSION.

  • Add the following statement after the connect statementExample 3-26 in the last section: 

    ALTER SESSION FORCE PARALLEL DML;

PL/SQL Packages for LOBs and DBFS

There are PL/SQL packages that can be used with BasicFiles LOBs and SecureFiles LOBs.

Changes made to accommodate SecureFiles LOBs and DBFS are emphasized.

Topics:

The DBMS_LOB Package Used with SecureFiles LOBs and DBFS

The DBMS_LOB package provides subprograms to operate on, or access and manipulate specific parts of a LOB or complete LOBs.

The DBMS_LOB package applies to both SecureFiles LOB and BasicFiles LOB.

DBMS_LOB Constants Used with SecureFiles LOBs and DBFS and DBMS_LOB Subprograms Used with SecureFiles LOBs and DBFS describe modifications made to the DBMS_LOB constants and subprograms with the addition of SecureFiles LOB and Database File System (DBFS).

See Also:

DBMS_LOB Constants Used with SecureFiles LOBs and DBFS

Certain constants support DBFS link interfaces.

Table 3-3 lists constants that support DBFS Link interfaces.

See Also:

Oracle Database PL/SQL Packages and Types Reference for complete information about constants used in the PL/SQL DBMS_LOB package

Table 3-3 DBMS_LOB Constants That Support DBFS Link Interfaces

Constant Description 
DBFS_LINK_NEVER

DBFS link state value

 
DBFS_LINK_YES

DBFS link state value

 
DBFS_LINK_NO

DBFS link state value

 
DBFS_LINK_CACHE

Flag used by COPY_DBFS_LINK() and MOVE_DBFS_LINK(). 

DBFS_LINK_NOCACHE

Flag used by COPY_DBFS_LINK() and MOVE_DBFS_LINK(). 

DBFS_LINK_PATH_MAX_SIZE

The maximum length of DBFS pathnames; 1024. 

CONTENTTYPE_MAX_SIZE

The maximum 1-byte ASCII characters for content type; 128.

DBMS_LOB Subprograms Used with SecureFiles LOBs and DBFS

You should note that some changes have been made to the DBMS_LOB subprograms over time.

Table 3-4 summarizes changes made to PL/SQL package DBMS_LOB subprograms.

Be aware that some of the DBMS_LOB operations that existed before Oracle Database 11g Release 2 throw an exception error if the LOB is a DBFS link. To remedy this problem, modify your applications to explicitly replace the DBFS link with a LOB by calling the DBMS_LOB.COPY_FROM_LINK procedure before they make these calls. When the call completes, then the application can move the updated LOB back to DBFS using the DBMS_LOB.MOVE_TO_DBFS_LINK procedure, if necessary.

Other DBMS_LOB operations that existed before Oracle Database 11g Release 2 work transparently if the DBFS Link is in a file system that supports streaming. Note that these operations fail if streaming is either not supported or disabled.

Table 3-4 DBMS_LOB Subprograms

Subprogram Description

APPEND

Appends the contents of the source LOB to the destination LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

COMPARE

Compares two LOBs in full or in parts

See Also:

Oracle Database PL/SQL Packages and Types Reference

CONVERTTOBLOB

Converts the character data of a CLOB or NCLOB into the specified character set and writes it in binary format to a destination BLOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

CONVERTTOCLOB

Converts the binary data of a BLOB into the specified character set and writes it in character format to a destination CLOB or NCLOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

COPY

Copies all or part of the source LOB to the destination LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

COPY_DBFS_LINK

Copies an existing DBFS link into a new LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

COPY_FROM_DBFS_LINK

Copies the specified LOB data from DBFS HSM Store into the database

See Also:

Oracle Database PL/SQL Packages and Types Reference

DBFS_LINK_GENERATE_PATHNAME

Returns a unique file path name for creating a DBFS Link

See Also:

Oracle Database PL/SQL Packages and Types Reference

ERASE

Erases all or part of a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

FRAGMENT_DELETE

Deletes a specified fragment of the LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

FRAGMENT_INSERT

Inserts a fragment of data into the LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

FRAGMENT_MOVE

Moves a fragment of a LOB from one location in the LOB to another location

See Also:

Oracle Database PL/SQL Packages and Types Reference

FRAGMENT_REPLACE

Replaces a fragment of a LOB with new data

See Also:

Oracle Database PL/SQL Packages and Types Reference

GET_DBFS_LINK

Returns the DBFS path name for a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

GET_DBFS_LINK_STATE

Returns the linking state of a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

GETCONTENTTYPE

Retrieves the content type string of the LOB data

See Also:

Oracle Database PL/SQL Packages and Types Reference

GETOPTIONS

Retrieves the previously set options of a specific LOB

See Also:

ISSECUREFILE

Determines if a LOB is a SecureFiles LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

LOADBLOBFROMFILE

Loads BFILE data into a BLOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

LOADCLOBFROMFILE

Loads BFILE data into a CLOB

If the CLOB is linked, an exception is thrown.

See Also:

Oracle Database PL/SQL Packages and Types Reference

LOADFROMFILE

Loads BFILE data into a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

MOVE_TO_DBFS_LINK

Moves the specified LOB data from the database into DBFS HSM Store

See Also:

Oracle Database PL/SQL Packages and Types Reference

READ

Reads data from a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

SET_DBFS_LINK

Links a LOB with a DBFS path name

See Also:

Oracle Database PL/SQL Packages and Types Reference

SETCONTENTTYPE

Sets the content type string of the LOB data

See Also:

Oracle Database PL/SQL Packages and Types Reference

SETOPTIONS

Sets new options for a specific LOB

See Also:

SUBSTR

Returns a fragment of a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

TRIM

Trims the LOB to a specified length

See Also:

Oracle Database PL/SQL Packages and Types Reference

WRITE

Writes data to a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

WRITEAPPEND

Appends data to the end of a LOB

See Also:

Oracle Database PL/SQL Packages and Types Reference

DBMS_SPACE Package

You can analyze segment growth and space requirements using the DBMS_SPACE PL/SQL package.

The DBMS_SPACE PL/SQL package enables you to analyze segment growth and space requirements.

DBMS_SPACE.SPACE_USAGE()

The existing DBMS_SPACE.SPACE_USAGE procedure is overloaded to return information about LOB space usage.

It returns the amount of disk space in blocks used by all the SecureFiles LOBs in the LOB segment.

See Also:

Oracle Database PL/SQL Packages and Types Reference