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Oracle Data block Size

When PCTUSED is ignored

PCTUSED is a storage parameter in Oracle which specifies when a database block is empty enough for Oracle to add it to the free list. When the percentage of used space in a block is greater than the PCTUSED parameter, Oracle will not add new rows to the block. The default settings for all Oracle tables are PCTUSED=40.
  1. PCTUSED is disregarded for all objects created in locally managed tablespaces when Automated Segment Space Management (ASSM) is enabled.
  2. PCTUSED is ignored for indexes: Oracle must manage the freelist re-link process for index tree management because the index node boundaries are the same as the index data block size.
Hence, PCTUSED is not used for index segments.

1) We allocate a tablespace called GENERAL with 8k blocksizes.

2) We allocate a table called TOM that has one extent of 16K and PCTFREE of 50. Three data blocks are now reserved for the TOM table.

3) We now create a JERRY table with one 16K extent and a PCTFREE of 10. Three data blocks are allocated for the JERRY table.

4) Note that TOM table and JERRY table have had varying amounts of free space reserved.