Oracle define table blob lob storage Oracle Database Tips by Donald Burleson

Defining LOB Storage

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 CACHE status and if NOCACHE is specified whether the LOB storage should be LOGGING or NOLOGGING and the specification for the LOB index. The LOB Parameters section of the LOB storage clause is shown in figure 2.

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 the LOB 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.

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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.

Listing 1: Example Use of The LOB Storage Clause and LOB Use in Tables

BLOBs and CLOBs are identical in creation and use, their major difference is in that BLOBs are used for binary data (much like LONG RAW) while CLOBs are used for single byte character storage (like VARCHAR2). The  TYPE creation example in Listing 2 shows how a CLOB is specified for a TYPE.

Listing 2: Example Use of LOBs in a TYPE specification

Notice that no LOB storage is specified when a BLOB or a CLOB is used in a TYPE specification, the LOB storage clause is applied to the BLOB or CLOB TYPE only when it is used in a table.