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Oracle: Coalesce vs.  deallocate unused space

Oracle Database Tips by Donald Burleson


Question:  What is the difference between coalesce and "deallocate unused space"?


Answer:  Both the coalesce and "deallocate unused space" commands serve to free-up space, but they do it in different ways:


Deallocate unused space


Oracle notes that the "deallocate unused space" clause is used to to explicitly deallocate unused space at "the end" of a segment and makes that space available for other segments within the tablespace. 


alter table xxx deallocate unused space;

alter index xxx deallocate unused space;


Internally, Oracle deallocates unused space beginning from the end of the objects (allocated space) and moving downwards toward the beginning of the object, continuing down until it reaches the high water mark (HWM).  The high-water mark for the table determines the amount of data that is stored in the table.  For indexes, "deallocate unused space" coalesces all leaf blocks within same branch of b-tree, and quickly frees up index leaf blocks for use.


The coalesce statement


Unlike the "deallocate unused space" syntax, which removes space above the high-water mark, "coalesce" puts together discontiguous fragmented extents.


There are two type of space fragmentation in Oracle.  First is the honeycomb fragmentation, when the free extents are side by side, and the "Swiss Cheese" fragmentation, when the extents are separated by live segments.


alter table xxx coalesce;

alter index xxx coalesce;


The coalesce partition syntax


For partitioned tables, Oracle provides the "Coalesce partition" syntax.


alter table xxx
modify partition yyy
coalesce subpartition;

alter table xxx
modify partition yyy
coalesce partition;


The Oracle docs note that the coalesce partition syntax behave differently, depending on the type of partition:

"When a hash partition is coalesced, its contents are redistributed into one or more remaining partitions determined by the hash function.


The specific partition that is coalesced is selected by the database, and is dropped after its contents have been redistributed."



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