Bitmap Heap Scan

Index can be clustered or unclustered

  • When index is clustered it means the records themselves are stored in index, not pointers
  • I.e. a clustered index ensures that all data is stored in some order
  • hash-ways-to-store.png
  • Usually there is only one clustered index per relation (otherwise the data will be duplicated)

If an index (say, B-Tree) is not clustered,

  • then instead of following each pointer
  • can use Bitmap Heap Scan


Algorithm

  • Collect all the pointers
  • Then group them by pages on disk
  • And retrieve them one-by-one

Reasons:

  • to avoid following each pointer separately
  • to make scans on clustered indexes more efficient

Sources

Machine Learning Bookcamp: Learn machine learning by doing projects. Get 40% off with code "grigorevpc".

Share your opinion