2.8 Describe a serial scan
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Key points
- Remember that a page table page is basically a table of contents for the
next 252 pages.
- When a serial scan is performed for a table, all DBEFiles in the
DBEFileSet containing the table are examined. Each entry on every page table
page is checked: if the object id on the entry matches the table on which the
scan is being performed, the appropriate data page is accessed directly by
using the pointer stored on the entry. Finally, all rows on each retrieved
data page are examined to see if they qualify for the query.
- All of the rows in the table must be read to see if they qualify for the
query. Rows are read in the order that they are physically stored.
- The I/O cost for a serial scan is equal to the number of page table pages
that exist in the DBEFileSet, plus the number of pages that exist in the table.
- A serial scan usually requires more I/O than any other type of scan,
unless all or most rows in the table will qualify for the query; in this case,
a serial scan is actually the most efficient method for accessing the rows.
Page last updated on November 29, 1995
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