The image cncpt031.gif shows how PCTFREE and PCTUSED work together to maintain free space in a data block.

In step 1, rows are inserted up to 80% only, because PCTFREE specifies that 20% of the block must remain open for updates of existing rows.

In step 2, updates to existing rows use the free space reserved in the block. No new rows can be inserted into the block until the amount of used space is 39% or less.

In step 3, after the amount of used space falls below 40%, new rows can again be inserted into this block.

In step 4, rows are inserted up to 80% only, because PCTFREE specifies that 20% of the block must reamin open for updates of existing rows. This cycle continues.

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