When records are inserted into the segment, Oracle examines the freelist to determine which data blocks have available space to store the new rows. When rows are deleted (or updated), and the amount of free space in the data block falls below a threshold value (PCTUSED), the data block will be returned to the freelist.
CREATE TABLE tab1 (col1 NUMBER) STORAGE (FREELISTS 10);
ASSM - Automatic Segment Space Management
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