PCTFree & PCTUsed

PCTFree:
It is used to mention how much space should be Free in the Data Block for future update:

PCTUsed:
Its is used to mention that how much Data Block should be filled before any new insertion. This parameter worked when the data block reached PCT Free threshold and new insertion stop filling and then this block will start insertion when data block get empty until reached PCTUsed limit.

Lets suppose you have specified PCTFree as 10 % and PCTUsed as 20 %.

• Oracle will keep inserting new rows until the space is reached 90 %. It will reserve the remaining 10%(PCTFree) for future updates.
• To start adding new rows again to the data block, Oracle will check the space in the Data Block and the PCTUsed parameter.
• When the Data Block space falls below 20 %(PCTUsed, Oracle will start adding new rows to the Data Block.