ASSM and PCTUSED

ASSM and pctused I believe many people know that when using ASSM (auto segment space management), PCTUSED will not be valid. However, how many people tell you why PCTUSED is ignored in ASSM. Before ASSM, Oracle used freelist and freegroup to manage idle blocks. How can I determine whether a block can be added to freelist? Assume that if the free size of the block is less than or equal to PCTFREE, that is, the block is removed from freelist. After a delete or update operation, the free size of the block is greater than 20%, that is to say, this block is added to freelist. Although the block space can be maximized, the maintenance cost of freelist is increased (the block is removed and added frequently from freelist, in addition, it is the operation of arranging it ). This is why PCTUSED is introduced, sacrificing space to maintain freelist efficiency. There is a buffer space between PCTFREE and 1-PCTUSED. ASSM discards freelist's method of managing idle blocks. Instead, it uses a third-level bitmap and uses the extent header block (not each extent header block) to manage free blocks. In this design, modifying the idle status of any block has a very small impact. Frequent changes do not cause any problems. Therefore, PCTUSED is ignored in ASSM.