Index Scan
- An Index Scan retrieves index rows from an index.
- The index row contains a row id, identifying the row in the heap.
- After each index row is retrieved, the row id is used to retrieve the
table row from the heap.
Bitmap Index Scan
- row id is (page address, row number within page).
- During the index scan, populate a bitmap recording page addresses. I.e.
bitmap[i] is 0 if page i of the heap is not needed, 1 if
it is needed.
- After the index scan, the heap pages can be read.
- Consolidates references to pages.
- Also orders them, which may trade random accesses for sequential accesses.