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Freelists Tips

Oracle Database Tips by Donald BurlesonFebruary 5, 2015

What is an Oracle freelist?  How does a freelist parameter determine what block a rows is stored inside?


Answer:  The freelist parameter is used when you create a table and it determines the number of freelists assigned to the table.  A freelist is used a "insert" time, when Oracle needs to place a row into a data block. 


At that time, Oracle grabs the first freelist, attempts the insert.  If the insert fails, Oracle tries another freelist, up to five time, before raising the high-water mark and grabbing brand new empty data blocks.  See the detail here on the relationship between ASSM freelists and pctfree.


 There are two types of freelists:


Standard freelists:  This is a one-way linked list anchored in the segment header block.


ASSM:  This is Automatic segment space management (ASSM), also known as bitmap freelists.  Bitmap freelists are defined when you specify "segment space management auto" in the "create table" statements.  See my notes here on bitmap freelists.

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