// TupleDesc: schema-related information for HeapTuples

typedef struct tupleDesc 
{
  int         natts;       // # attributes in tuple 
  Oid         tdtypeid;    // composite type ID for tuple type 
  int32       tdtypmod;    // typmod for tuple type 
  bool        tdhasoid;    // does tuple have oid attribute? 
  int         tdrefcount;  // reference count (-1 if not counting)
  TupleConstr *constr;     // constraints, or NULL if none 
  FormData_pg_attribute attrs[];
  // attrs[N] is a pointer to description of attribute N+1 
} *TupleDesc;