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You can use the space in an Oracle block for two purposes: inserting fresh data or updating existing data in the blocks When you delete data from a block, or an update statement changes the existing data length to a smaller size, there will be an increase in free space in the block Segment space management is how Oracle manages this free space within the individual Oracle data blocks If you specify manual segment space management (by using the keyword MANUAL), the database manages the free space of segments in the tablespace using entities known as free lists and a pair of storage parameters, PCTFREE and PCTUSED Oracle keeps track of how much free space is in its data blocks by maintaining freelists Every table and index maintains a list of all its data blocks with free space greater than PCTUSED.

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That is, freelists contain the list of all blocks eligible for data insertion Oracle first checks the appropriate freelist before making any insertions into tables or indexes The Oracle database has to do a lot of work to maintain the freelists, as blocks reach their PCTUSED threshold after insertions and fall below the threshold due to deletions The PCTFREE parameter lets you reserve a percentage of space in each data block for future updates to existing data For example, you may have some data on a person s address in a certain block If you update that address later, so that it is larger, there should ideally be room in the existing block for the enlarged address This is exactly what the PCTFREE parameter provides: room for the existing rows to grow.

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The PCTUSED parameter, on the other hand, deals with the threshold below which the used space must fall before new data can be placed in the blocks For example, if the PCTUSED parameter is set at 40 percent, Oracle can t insert new data into the block until the amount of used space falls below this threshold level You can see easily how the PCTFREE and PCTUSED parameters together optimize the use of space within an Oracle block Suppose 80 percent of the space in a block is filled with data This will be the maximum amount of data that you can insert inside the block if the PCTFREE parameter is set to 20 percent.

If some deletes take place in this block, there will be potential room to insert new rows, but Oracle uses the PCTUSED parameter in a clever way to keep any newly available free space from automatically being used for new inserts Oracle incurs an overhead when it tries to use newly available free space in data blocks, so Oracle waits until the used space falls below the PCTUSED setting before using that free space Until then, although there may be free spaces in partially used blocks, Oracle ignores them and goes to new data blocks to insert data The PCTFREE and PCTUSED parameters and the freelists comprise a manual way of checking for space, because you are making Oracle continually check for blocks with the right amount of free space In a database with heavy updates, inserts, and deletes, this could lead to a slowdown of your transactions.

If you choose automatic segment space management when creating a tablespace (by specifying AUTO), which is the default in Oracle Database 10g Release 2, Oracle ignores any specification for the free lists, free list groups, and PCTUSED parameters Instead, the database will use bitmaps to.

track free space availability in a segment. A bitmap, which is contained in a bitmap block, indicates whether free space in a data block is below 25 percent, between 25 and 50 percent, between 50 and 75 percent, or above 75 percent. For an index block, the bitmaps can tell you whether the blocks are empty or formatted.