How does Space allocation table place within a block ?

Each block contains entries as followsFixed block headerVariable block headerRow Header,row date (multiple rows may exists)PCTEREE (% of free space for row updation in future)

Interview Candidate
Sep 6th, 2004
1
1793

Memory Management

Answer

Showing Answers 1 - 1 of 1 Answers

Anu

Oct 3rd, 2006

A block is divided into
1. Header
2.free space
3.data space
The free space is in the middle shrinks as header and data space grow.
Space usage in the data space :
The data space is managed with 2 parameters
a.pctused    b.pctfree
suppose pctused=40 and pctfree=20
We know data is stored in the data block in database.The data or rows are filled in the pctused area and they get filled until 80% of the dataspace is full ie100-pctfree.ie Row inserts occur until 80% of dataspace is full.
     The use of pctfree is to accomadate any growth in dataspace that might occur due to row updation or any previous null values replaced with values.
       A pctused can also shrink due to any deletion.Suppose a deletion causes pctused to a value 60% from 80%.Now this block is not available for new inserts until pctused is 40%.So when pctused=40 then the block is included into freelist and made available for new inserts.
100-pctfree.