One.
what is
Freelists
This article is to explore the role, access mechanism, contention diagnosis and optimization method of FreeELISTS and FREELIST Groups, while overthrowing some misconceptions through theory and testing. The readers of this article should have more deeper Oracle knowledge, and there is not much help for the general developers.
We know that the read unit of the Oracle database is a block (block), and a block allows the written data to be based on a certain idleness, which is the PCTFREE and PCTUSED storage parameter settings.
Suppose PCTFREE = 10, PCTUSED = 40, which indicates that when a Block's spatial usage reaches 90% (100-PCTFREE), this block is no longer allowed to be used to add data (insert), but retain it. This 10% space is reserved for the space extension that may be needed by UPDATE. We said that this block was taken away from FreeElist (there is still another situation, that is, when the block remains The space is not sufficient to insert a record and the use of the block has exceeded the value of the PCTUSED definition and the block is taken away from Freeelist, which is called unlink. When there is data deletion (Delete), only the data in the block is deleted to a certain degree, the block is resended in Freelists, and this extent is the value defined by the PCTUSED parameter, such as our example, This block is re-permitted to add data when the data in the block is reduced below 40%.
Through the above description, you can know that the so-called Freelists is a list specified for all data blocks that can be used for INSERT. There is a data block in this list to be used for INSERT operations. Once a data block cannot be used for INSERT (limited to the limit of the PCTFree parameter), it will be removed from this list immediately. The Freelists' role is to manage free space below the High Water Level Sign (HWM).
Note: FreeLists just manages the free space below the high water level flag, and the actually available idle space includes two types:
1. Blocks that has been assigned to this segment but never used above high water level markers
2. Located under the high water level sign, Linked on Freeelists
As for the concept and role of Freelist Groups, the following sections will explain.
