PCTFREE is a block storage parameter used to specify how much space should be left in a database block for future updates. For example, for PCTFREE=10, Oracle will keep on adding new rows to a block until it is 90% full. This leaves 10% for future updates (row expansion).
When using Oracle Advanced Compression, Oracle will trigger block compression when the PCTFREE is reached. This eliminates holes created by row deletions and maximizes contiguous free space in blocks.
See the PCTFREE setting for a table:
SQL> SELECT pct_free FROM user_tables WHERE table_name = 'EMP'; PCT_FREE ---------- 10
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