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If LOB storage is not defined and LOB attribute size exceeds 4000 characters the attributes which overflow will automatically be placed in the tables tablespace in default configured LOB storage and index. If LOB storage is allowed to go to defaults you will not get optimal use of resources and could cause contention for disk resources, both of which will reduce your system performance.
LOB storage is defined by the CREATE or ALTER TABLE commands. The LOB storage clause is used to define the LOB storage profile. The LOB storage clause is shown in figure 1.
The LOB storage clause uses a LOB parameters section to specify the LOB tablespace, storage in row, CHUNK and PCTVERSION settings as well as CACHEstatus and if NOCACHE is specified whether the LOB storage should beLOGGING or NOLOGGING and the specification for the LOB index. The LOB Parameters section of the LOB storage clause is shown in figure 2.
The LOB index is specified through the LOB index clause. You can name or let the system name the index and then specify the normal index storage variables for the LOB index. The LOB index clause is shown in figure 3.
Whether the LOB datatype is BLOB, CLOB or NCLOB the storage clause format is identical. LOB storage specifications can be applied to BFILE LOB datatypes but it doesn't make sense to do so. If you do not specify the segname parameter the
Oracle system will give the segment some wonderful name such as 'SYS_LOB0000001562C0035$' so I suggest you name the segments.
BLOB and CLOB Creation
BLOB and CLOB datatypes are created by use of the CREATE or ALTER TABLE or the CREATE or ALTER TYPE commands. In fact, they are created identically to other non-sized datatypes such as DATE and LONG with the exception of theLOB storage clause. The LOB storage clause is not needed if the maximum size of the BLOB doesn't exceed 4000 bytes. Up to 4000 bytes can be stored in-line with the other data in the tablespace. If the length of the BLOB exceeds 4000 bytes it must be stored in either a system defaulted storage (the same as the default for the table it resides in) or in an explicitly defined LOB storage area.
TIP:
I suggest always specify the LOB storage clause, if you force the system to do a default storage each time a BLOB or CLOB exceeds 4000 bytes you could cause datafile fragmentation and performance problems. The LOB storage clause gives you control instead of the system.
An example creation of a table using a BLOB datatype is shown in Listing 1. It just as easily could have been a CLOB.
SYS@orcl11g>create table internal_graphics ( graphic_id number, graphic_desc varchar2(30), graphic_blob blob,
graphic_type VARCHAR2(4)) lob (graphic_blob) store as SECUREFILE glob_store ( tablespace ZBDBA storage
(initial 100k next 100k pctincrease 0) chunk 4 pctversion 10 INDEX glob_index1 ( tablespace zbdba))
TABLESPACE zbdba storage (initial 1m next 1m pctincrease 0);
Table created.
SYS@orcl11g>select dbms_metadata.get_ddl('TABLE','INTERNAL_GRAPHICS','SYS') FROM DUAL;
DBMS_METADATA.GET_DDL('TABLE','INTERNAL_GRAPHICS','SYS')
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CREATE TABLE "SYS"."INTERNAL_GRAPHICS"
( "GRAPHIC_ID" NUMBER,
"GRAPHIC_DESC" VARCHAR2(30),
"GRAPHIC_BLOB" BLOB,
"GRAPHIC_TYPE" VARCHAR2(4)
) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255
NOCOMPRESS LOGGING
STORAGE(INITIAL 1048576 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE |