Research on freelists and Freelist groups (revised edition)-What is freelists

A              What is freelists this paper is to explore freelists and freelist Groups's role, access mechanism, contention for diagnostic and optimization methods, and the use of theory and testing to overturn some long-standing erroneous ideas. The reader of this article should have a deeper knowledge of Oracle, which may not be of much help to the general developer. We know that the Oracle database reads in units of data blocks, and whether a block allows written data is based on a certain degree of idleness, which is known as Pctfree and pctused storage parameter settings. Assuming pctfree=10, pctused=40, this means that when the space usage of a block reaches the 90% (100-pctfree), it is no longer allowed to be used for new data (insert), and the remaining 10% Space is reserved for the space extensions that the row update (update) may require, and we say that the block is picked up from the freelist (there is actually another case That is, when the block space is not enough to insert a record and the usage of the block exceeds the value defined by pctused and the block is located at the Freelist header, the block is also removed from the freelist, the term unlink. When there is data deletion (delete), only if the data in the block is deleted to a certain extent, the chunk is then added to the freelists, which is the value defined by the pctused parameter, as in our case, only the data in the block is reduced below 40%. , the block is again allowed for new data. By the description above, you know that the so-called freelists is a list of all the blocks of data that you can use for insert operations. A block of data that exists in this list can be used for insert operations, and once a block of data cannot be used for inserts (reaching the limit specified by the Pctfree parameter), it is removed from the list immediately. The Freelists function is to manage the free space below the high water mark (HWM). Note: freelists only manages the free space below the high watermark flag, while in fact a segment free space available includes two types: 1.   has been allocated to this segment but has never been used in the high water mark above the BLOCKS2.   is located below the high water mark and is linked to the freelists Blocks  As to the concept and role of freelist groups, the following sections will explain the appropriate place.