The Oracle Database server automatically and transparently coalesces the free space of a data block when:•An INSERTor UPDATEstatement attempts to use a block with sufficient free space for a new row piece•The free space is fragmented, so that the row piece cannot be inserted in a contiguous section of the blockAfter coalescing, the amount of free space is identical to the amount before the operation, but the space is now contiguous.Oracle Database 12c: Administration Workshop 8 - 6
Free space can be managed automatically inside database segments. The in-segment free or used space is tracked with bitmaps. To take advantage of this feature, specify Automatic Segment Space Management when you create a locally managed tablespace. Your specification then applies to all segments subsequently created in this tablespace.