PCTFREE, PCTUSED, and Row Chaining
For manually managed tablespaces, two space management parameters, PCTFREE and PCTUSED, enable you to control the use of free space for inserts and updates to the rows in all the data blocks of a particular segment. Specify these parameters when you create or alter a table or cluster (which has its own data segment). You can also specify the storage parameter PCTFREE when creating or altering an index (which has its own index segment).
This discussion does not apply to LOB datatypes (BLOB, CLOB, NCLOB, and BFILE). They do not use the PCTFREE storage parameter or free lists.
See "Overview of LOB Datatypes" for information.
The PCTFREE Parameter
The PCTFREE parameter sets the minimum percentage of a data block to be reserved as free space for possible updates to rows that already exist in that block. For example, assume that you specify the following parameter within a CREATE TABLE statement:
This states that 20% of each data block in this table's data segment be kept free and available for possible updates to the existing rows already within each block. New rows can be added to the row data area, and corresponding information can be added to the variable portions of the overhead area, until the row data and overhead total 80% of the total block size. Figure 2-3illustratesPCTFREE.
The PCTUSED Parameter
The PCTUSED parameter sets the minimum percentage of a block that can be used for row data plus overhead before new rows are added to the block. After a data block is filled to the limit determined by PCTFREE, Oracle considers the block unavailable for the insertion of new rows until the percentage of that block falls beneath the parameter PCTUSED. Until this value is achieved, Oracle uses the free space of the data block only for updates to rows already contained in the data block. For example, assume that you specify the following parameter in a CREATE TABLEstatement:
In this case, a data block used for this table's data segment is considered unavailable for the insertion of any new rows until the amount of used space in the block falls to 39% or less (assuming that the block's used space has previously reached PCTFREE). Figure 2-4 illustrates this.
How PCTFREE and PCTUSED Work Together
PCTFREE and PCTUSED work together to optimize the use of space in the data blocks of the extents within a data segment. Figure 2-5 illustrates the interaction of these two parameters.
In a newly allocated data block, the space available for inserts is the block size minus the sum of the block overhead and free space (PCTFREE). Updates to existing data can use any available space in the block. Therefore, updates can reduce the available space of a block to less than PCTFREE, the space reserved for updates but not accessible to inserts.
For each data and index segment, Oracle maintains one or more free lists—lists of data blocks that have been allocated for that segment's extents and have free space greater than PCTFREE. These blocks are available for inserts. When you issue an INSERT statement, Oracle checks a free list of the table for the first available data block and uses it if possible. If the free space in that block is not large enough to accommodate the INSERT statement, and the block is at least PCTUSED, then Oracle takes the block off the free list. Multiple free lists for each segment can reduce contention for free lists when concurrent inserts take place.
After you issue a DELETE or UPDATE statement, Oracle processes the statement and checks to see if the space being used in the block is now less than PCTUSED. If it is, then the block goes to the beginning of the transaction free list, and it is the first of the available blocks to be used in that transaction. When the transaction commits, free space in the block becomes available for other transactions.
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/8747612/viewspace-1972290/，如需转载，请注明出处，否则将追究法律责任。