typedef struct HeapScanDescData
{
  // scan parameters 
  Relation      rs_rd;        // heap relation descriptor 
  Snapshot      rs_snapshot;  // snapshot ... tuple visibility 
  int           rs_nkeys;     // number of scan keys 
  ScanKey       rs_key;       // array of scan key descriptors 
  ...
  // state set up at initscan time 
  PageNumber    rs_npages;    // number of pages to scan 
  PageNumber    rs_startpage; // page # to start at 
  ...
  // scan current state, initally set to invalid 
  HeapTupleData rs_ctup;      // current tuple in scan
  PageNumber    rs_cpage;     // current page # in scan
  Buffer        rs_cbuf;      // current buffer in scan
   ...
} HeapScanDescData;