now I found 3 possible types of structure in the source code:

1. in the excution plan node

   
   
   typedef struct List
   {
    NodeTag     type;           /* T_List, T_IntList, T_OidList, or T_XidList */
    int         length;         /* number of elements currently present */
    int         max_length;     /* allocated length of elements[] */
    ListCell   *elements;       /* re-allocatable array of cells */
    /* We may allocate some cells along with the List header: */
    ListCell    initial_elements[FLEXIBLE_ARRAY_MEMBER];
    /* If elements == initial_elements, it's not a separate allocation */
   } List;
   

2. in the scan node，Here are the contents of a relation cache entry.

/*
 * Here are the contents of a relation cache entry.
 */

typedef struct RelationData
{
    RelFileLocator rd_locator;  /* relation physical identifier */
    SMgrRelation rd_smgr;       /* cached file handle, or NULL */
    int         rd_refcnt;      /* reference count */
    ProcNumber  rd_backend;     /* owning backend's proc number, if temp rel */
    bool        rd_islocaltemp; /* rel is a temp rel of this session */
    bool        rd_isnailed;    /* rel is nailed in cache */
    bool        rd_isvalid;     /* relcache entry is valid */
    bool        rd_indexvalid;  /* is rd_indexlist valid? (also rd_pkindex and
                                 * rd_replidindex) */

3.HeapTupleData is an in-memory data structure that points to a tuple.There are several ways in which this data structure is used:

typedef struct HeapTupleData
{
    uint32      t_len;          /* length of *t_data */
    ItemPointerData t_self;     /* SelfItemPointer */
    Oid         t_tableOid;     /* table the tuple came from */
#define FIELDNO_HEAPTUPLEDATA_DATA 3
    HeapTupleHeader t_data;     /* -> tuple header and data */
} HeapTupleData;

Is it one of the above three, or is it some other structure?