SELECT ... WHERE status = 'paid'; -- used inside a Bitmap Heap Scan or BitmapAnd/Or
Index (idx_status)
root
[cancelled]
[paid]
[pending]
leaf: paid
Output — TID bitmap (one bit per page)
The bitmap is handed up to a parent plan node — a Bitmap Heap Scan, or a BitmapAnd / BitmapOr that will combine it with other bitmaps.