If a datatype is defined as LOB, CLOB or NLOB on the Dataype tab of the Table Editor, the Edit LOG Segment option is available, which when clicked opens the LOG Segment editor. The following describes the options available on the LOG Segment editor:
• Segment Name: Specifies the name of the segment on which the LOB data is stored.
• Tablespace: Specifies the name of the tablespace on which the LOB data is stored.
• Blocks Accessed at one time: (CHUNK) Determines the granularity of allocation for out of row LOBs.
• Percent Space Used for New Version: (PCTVERSION) Controls the mechanism that stores the previous image within the data block. The default value for PCTVersion is 10, reserving 10% of the LOB storage space for UNDO operations.
• Enable LOB Storage In Row: An LOB can be stored either along the row to which it belongs (in row storage) or in the lob segment (out of row storage). The maximum size for in row LOBs is 3964 bytes.
• LOB Cache: (CACHE) Select for data that is accessed frequently to specify that blocks retrieved for this column be placed at the most recently used end of the least recently used (LRU) list in the buffer cache when a full table scan is performed.
• Logging: (LOGGING) Select if you want the DLL operations to be logged in the redo file. This can increase index creation and updates by up to 30%.
• Initial Extent: (INITEXTENT) Specifies the initial number of data blocks that Oracle should reserve. Oracle will reserve the number of data blocks that correspond to the initial extent for that table’s rows.
• Next Extent: (NEXT) Specifies the size in kilobytes of the next extent. Monitor this figure against the largest available chunk of free space in the tablespace. If a table cannot allocate its next extent, it will no longer be able to extend and, therefore, cannot accept additional data.
• Pct Increase: (PCTINCREASE) Specifies the percentage by which the next extent should grow over the previous extent's size. Be careful when setting Percent Increase because it magnifies how an object grows and, therefore, can materially affect available free space in a tablespace.
• Min Extents: (MINEXTENT) Specifies the number of extents to allocate when the segment is created. Controls free space fragmentation by making sure that every used or free extent is at least as large as the value you specify.
• Max Extents: (MAXEXTENT) Species the maximum number of extents that Oracle can allocate to the materialized view. Once this limit is reached, Oracle prevents further growth of the cluster and cannot accept additional data. Carefully monitor the number extents already allocated to the table with this limit.
• Free Lists: (FREELISTS) Specifies the number of free lists to apply to the table. The default and minimum value is 1. Free lists can help manage the allocation of data blocks when concurrent processes are issued against the index. Oracle uses the free list to determine which data block to use when an INSERT operation occurs. Oracle allows table and indexes to be defined with multiple free lists. All tables and index free lists should be set to the high-water mark of concurrent INSERT or UPDATE activity. For example, if the table has up to 20 end users performing INSERTs at any time, then the index should have FREELISTS=20.Too low a value for free lists will cause poor Oracle performance. An increase in FREELISTS or FREELIST GROUPS can alleviate segment header contention.
• Free List Groups: (FREELIST GROUPS) Applicable only if you are using Oracle with the Parallel Server option in parallel mode. Specifies the number of free list groups which allow the table to have several segment headers. This enables multiple tasks to insert into the index; thereby alleviating segment header contention. Free list groups should be set to the number of Oracle Parallel Server instances that access the index. For partitioned objects and cases of segment header contention, free list groups my be set for non-RAC systems.