typedef HeapTupleHeaderData *HeapTupleHeader;
//这个定义在postgresql-11.1\src\include\access\htup.h
//里的HeapTuple指针和HeapTupleData结构体文章里面
struct HeapTupleHeaderData
{
union
{
HeapTupleFields t_heap;
DatumTupleFields t_datum;
} t_choice;
ItemPointerData t_ctid; /* current TID of this or newer tuple (or a
* speculative insertion token) */
/* Fields below here must match MinimalTupleData! */
#define FIELDNO_HEAPTUPLEHEADERDATA_INFOMASK2 2
uint16 t_infomask2; /* number of attributes + various flags */
#define FIELDNO_HEAPTUPLEHEADERDATA_INFOMASK 3
uint16 t_infomask; /* various flag bits, see below */
#define FIELDNO_HEAPTUPLEHEADERDATA_HOFF 4
uint8 t_hoff; /* sizeof header incl. bitmap, padding */
/* ^ - 23 bytes - ^ */
#define FIELDNO_HEAPTUPLEHEADERDATA_BITS 5
bits8 t_bits[FLEXIBLE_ARRAY_MEMBER]; /* bitmap of NULLs */
/* MORE DATA FOLLOWS AT END OF STRUCT */
};