Standard Index Scan vs. Bitmap Index Scan:
- A Standard Index Scan works by traversing the index, finding the TIDs of matching rows, and then fetching each row directly from the heap (the main table data) using its TID. This involves potentially random disk I/O to jump to different pages for each tuple.
- A Bitmap Index Scan is different. It scans the index (or multiple indexes) to find matching TIDs, but instead of fetching the rows immediately, it builds a bitmap in memory. This bitmap is used to represent the locations of the matching tuples.