HeadTupleHeaderData.c

说明：
1. 内容参考pg9.5源码src/include/access/htup_details.h
2. 参考《PostgreSQL数据库内核分析》P59
3. 需要加载contrib/pageinspect
4. 参考：http://my.oschina.net/Suregogo/blog/176279
         http://my.oschina.net/Suregogo/blog/595335?fromerr=4Ru4VQhW

HeadTupleHeaderData结构体：
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! */

	uint16		t_infomask2;	/* number of attributes + various flags */

	uint16		t_infomask;		/* various flag bits, see below */

	uint8		t_hoff;			/* sizeof header incl. bitmap, padding */

	/* ^ - 23 bytes - ^ */

	bits8		t_bits[FLEXIBLE_ARRAY_MEMBER];	/* bitmap of NULLs */

	/* MORE DATA FOLLOWS AT END OF STRUCT */
};

通过pg_relation_filepath('tbname')找出其对应的表数据文件，通过hex 文件名 即可查看
下面简单画出该结构体示意图(由于hexdump每16个字节输出1行，所以下面也是16字节1行):

 xmin(4bytes)                         xmax（4bytes）                                    cmin/cmax(4bytes)         ctid(4bytes+..)
 ctid(..+2bytes)  t_infomask2(2bytes) t_informask(2bytes)   t_bits(1byte) t_hoff(1byte)  values(...)
 
 操作示例；
 wln@iZ232ngsvp8Z:~> psql
psql (9.5beta1)
Type "help" for help.

postgres=# \d
       List of relations
 Schema | Name | Type  | Owner 
--------+------+-------+-------
 public | c1   | table | wln
 public | t1   | table | wln
 public | t2   | table | wln
 public | t4   | table | wln
(4 rows)

postgres=# select * from t2;
 id1 | id2 | id3 | id4 | id5 | id6 
-----+-----+-----+-----+-----+-----
   1 |   2 |   3 |   4 |     |    
   2 |   2 |   3 |   4 |     |    
   3 |   2 |   3 |   4 |     |    
   4 |   2 |   3 |   4 |     |    
   5 |   2 |   3 |   4 |     |    
   6 |   2 |   3 |   4 |     |    
   7 |   2 |   3 |   4 |     |    
   8 |   2 |   3 |   4 |     |    
   9 |   2 |   3 |   4 |     |    
  11 |     |     |     |     |    
  12 |     |     |     |     |    
  13 |     |     |     |     |    
  14 |     |     |     |     |    
  15 |     |     |     |     |    
  16 |     |     |     |     |    
  11 |  12 |  13 |  14 |  15 |  16
(16 rows)

postgres=# select pg_relation_filepath('t2');
 pg_relation_filepath 
----------------------
 base/12974/41017
(1 row)

postgres=# select * from heap_page_items(get_raw_page('t2', 'main', 0));
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff |  t_bits  | t_oid 
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+----------+-------
  1 |   8152 |        1 |     40 |   1320 |      0 |        0 | (0,1)  |           6 |       2305 |     24 | 11110000 |      
  2 |   8112 |        1 |     40 |   1320 |      0 |        0 | (0,2)  |           6 |       2305 |     24 | 11110000 |      
  3 |   8072 |        1 |     40 |   1320 |      0 |        0 | (0,3)  |           6 |       2305 |     24 | 11110000 |      
  4 |   8032 |        1 |     40 |   1320 |      0 |        0 | (0,4)  |           6 |       2305 |     24 | 11110000 |      
  5 |   7992 |        1 |     40 |   1320 |      0 |        0 | (0,5)  |           6 |       2305 |     24 | 11110000 |      
  6 |   7952 |        1 |     40 |   1320 |      0 |        0 | (0,6)  |           6 |       2305 |     24 | 11110000 |      
  7 |   7912 |        1 |     40 |   1320 |      0 |        0 | (0,7)  |           6 |       2305 |     24 | 11110000 |      
  8 |   7872 |        1 |     40 |   1320 |      0 |        0 | (0,8)  |           6 |       2305 |     24 | 11110000 |      
  9 |   7832 |        1 |     40 |   1320 |      0 |        0 | (0,9)  |           6 |       2305 |     24 | 11110000 |      
 10 |   7792 |        1 |     40 |   1320 |   1321 |        0 | (0,10) |        8198 |       1281 |     24 | 11110000 |      
 11 |   7760 |        1 |     28 |   1332 |      0 |        0 | (0,11) |           6 |       2305 |     24 | 10000000 |      
 12 |   7728 |        1 |     28 |   1332 |      0 |        0 | (0,12) |           6 |       2305 |     24 | 10000000 |      
 13 |   7696 |        1 |     28 |   1332 |      0 |        0 | (0,13) |           6 |       2305 |     24 | 10000000 |      
 14 |   7664 |        1 |     28 |   1332 |      0 |        0 | (0,14) |           6 |       2305 |     24 | 10000000 |      
 15 |   7632 |        1 |     28 |   1332 |      0 |        0 | (0,15) |           6 |       2305 |     24 | 10000000 |      
 16 |   7600 |        1 |     28 |   1332 |      0 |        0 | (0,16) |           6 |       2305 |     24 | 10000000 |      
 17 |   7552 |        1 |     48 |   1333 |      0 |        0 | (0,17) |           6 |       2304 |     24 |          |      
(17 rows)

postgres=# \q
wln@iZ232ngsvp8Z:~> hexdump pg95/data/base/12974/41017 
0000000 0000 0000 ff20 12d2 0000 0000 005c 1d80
0000010 2000 2004 0529 0000 9fd8 0050 9fb0 0050
0000020 9f88 0050 9f60 0050 9f38 0050 9f10 0050
0000030 9ee8 0050 9ec0 0050 9e98 0050 9e70 0050
0000040 9e50 0038 9e30 0038 9e10 0038 9df0 0038
0000050 9dd0 0038 9db0 0038 9d80 0060 0000 0000
0000060 0000 0000 0000 0000 0000 0000 0000 0000
*
0001d80 0535 0000 0000 0000 0000 0000 0000 0000
0001d90 0011 0006 0900 0018 000b 0000 000c 0000
0001da0 000d 0000 000e 0000 000f 0000 0010 0000
0001db0 0534 0000 0000 0000 0000 0000 0000 0000
0001dc0 0010 0006 0901 0118 0010 0000 0000 0000
0001dd0 0534 0000 0000 0000 0000 0000 0000 0000
0001de0 000f 0006 0901 0118 000f 0000 0000 0000
0001df0 0534 0000 0000 0000 0000 0000 0000 0000
0001e00 000e 0006 0901 0118 000e 0000 0000 0000
0001e10 0534 0000 0000 0000 0000 0000 0000 0000
0001e20 000d 0006 0901 0118 000d 0000 0000 0000
0001e30 0534 0000 0000 0000 0000 0000 0000 0000
0001e40 000c 0006 0901 0118 000c 0000 0000 0000
0001e50 0534 0000 0000 0000 0000 0000 0000 0000
0001e60 000b 0006 0901 0118 000b 0000 0000 0000
0001e70 0528 0000 0529 0000 0000 0000 0000 0000
0001e80 000a 2006 0501 0f18 000a 0000 0002 0000
0001e90 0003 0000 0004 0000 0528 0000 0000 0000
0001ea0 0000 0000 0000 0000 0009 0006 0901 0f18
0001eb0 0009 0000 0002 0000 0003 0000 0004 0000
0001ec0 0528 0000 0000 0000 0000 0000 0000 0000
0001ed0 0008 0006 0901 0f18 0008 0000 0002 0000
0001ee0 0003 0000 0004 0000 0528 0000 0000 0000
0001ef0 0000 0000 0000 0000 0007 0006 0901 0f18
0001f00 0007 0000 0002 0000 0003 0000 0004 0000
0001f10 0528 0000 0000 0000 0000 0000 0000 0000
0001f20 0006 0006 0901 0f18 0006 0000 0002 0000
0001f30 0003 0000 0004 0000 0528 0000 0000 0000
0001f40 0000 0000 0000 0000 0005 0006 0901 0f18
0001f50 0005 0000 0002 0000 0003 0000 0004 0000
0001f60 0528 0000 0000 0000 0000 0000 0000 0000
0001f70 0004 0006 0901 0f18 0004 0000 0002 0000
0001f80 0003 0000 0004 0000 0528 0000 0000 0000
0001f90 0000 0000 0000 0000 0003 0006 0901 0f18
0001fa0 0003 0000 0002 0000 0003 0000 0004 0000
0001fb0 0528 0000 0000 0000 0000 0000 0000 0000
0001fc0 0002 0006 0901 0f18 0002 0000 0002 0000
0001fd0 0003 0000 0004 0000 0528 0000 0000 0000
0001fe0 0000 0000 0000 0000 0001 0006 0901 0f18
0001ff0 0001 0000 0002 0000 0003 0000 0004 0000
0002000
wln@iZ232ngsvp8Z:~> hexdump pg95/data/base/12974/41017  -d
0000000   00000   00000   65312   04818   00000   00000   00092   07552
0000010   08192   08196   01321   00000   40920   00080   40880   00080
0000020   40840   00080   40800   00080   40760   00080   40720   00080
0000030   40680   00080   40640   00080   40600   00080   40560   00080
0000040   40528   00056   40496   00056   40464   00056   40432   00056
0000050   40400   00056   40368   00056   40320   00096   00000   00000
0000060   00000   00000   00000   00000   00000   00000   00000   00000
*
0001d80   01333   00000   00000   00000   00000   00000   00000   00000
0001d90   00017   00006   02304   00024   00011   00000   00012   00000
0001da0   00013   00000   00014   00000   00015   00000   00016   00000
0001db0   01332   00000   00000   00000   00000   00000   00000   00000
0001dc0   00016   00006   02305   00280   00016   00000   00000   00000
0001dd0   01332   00000   00000   00000   00000   00000   00000   00000
0001de0   00015   00006   02305   00280   00015   00000   00000   00000
0001df0   01332   00000   00000   00000   00000   00000   00000   00000
0001e00   00014   00006   02305   00280   00014   00000   00000   00000
0001e10   01332   00000   00000   00000   00000   00000   00000   00000
0001e20   00013   00006   02305   00280   00013   00000   00000   00000
0001e30   01332   00000   00000   00000   00000   00000   00000   00000
0001e40   00012   00006   02305   00280   00012   00000   00000   00000
0001e50   01332   00000   00000   00000   00000   00000   00000   00000
0001e60   00011   00006   02305   00280   00011   00000   00000   00000
0001e70   01320   00000   01321   00000   00000   00000   00000   00000
0001e80   00010   08198   01281   03864   00010   00000   00002   00000
0001e90   00003   00000   00004   00000   01320   00000   00000   00000
0001ea0   00000   00000   00000   00000   00009   00006   02305   03864
0001eb0   00009   00000   00002   00000   00003   00000   00004   00000
0001ec0   01320   00000   00000   00000   00000   00000   00000   00000
0001ed0   00008   00006   02305   03864   00008   00000   00002   00000
0001ee0   00003   00000   00004   00000   01320   00000   00000   00000
0001ef0   00000   00000   00000   00000   00007   00006   02305   03864
0001f00   00007   00000   00002   00000   00003   00000   00004   00000
0001f10   01320   00000   00000   00000   00000   00000   00000   00000
0001f20   00006   00006   02305   03864   00006   00000   00002   00000
0001f30   00003   00000   00004   00000   01320   00000   00000   00000
0001f40   00000   00000   00000   00000   00005   00006   02305   03864
0001f50   00005   00000   00002   00000   00003   00000   00004   00000
0001f60   01320   00000   00000   00000   00000   00000   00000   00000
0001f70   00004   00006   02305   03864   00004   00000   00002   00000
0001f80   00003   00000   00004   00000   01320   00000   00000   00000
0001f90   00000   00000   00000   00000   00003   00006   02305   03864
0001fa0   00003   00000   00002   00000   00003   00000   00004   00000
0001fb0   01320   00000   00000   00000   00000   00000   00000   00000
0001fc0   00002   00006   02305   03864   00002   00000   00002   00000
0001fd0   00003   00000   00004   00000   01320   00000   00000   00000
0001fe0   00000   00000   00000   00000   00001   00006   02305   03864
0001ff0   00001   00000   00002   00000   00003   00000   00004   00000
0002000

可以看出，元组头长度均为24（xmin+xmax+cmin+cmax+ctid+t_infomask2+t_infomask+t_bits+t_hoff）,
元组中数据长度lp_len 为48,40,28。可以看出，有5列为空时长度最短28（24+4），无空列则为24+486=48。




//代码、
//HeapTupleHeaderData.c 
#include <stdio.h>
#include <fcntl.h>
#include<stdlib.h>
#include<string.h>


#ifndef HAVE_UINT8
typedef unsigned char uint8;    /* == 8 bits */
typedef unsigned short uint16;  /* == 16 bits */
typedef unsigned int uint32;    /* == 32 bits */
typedef uint8 bits8;                    /* >= 8 bits */
typedef uint16 bits16;                  /* >= 16 bits */
typedef uint32 bits32;
typedef signed int int32;                       /* >= 32 bits */
#endif   /* not HAVE_UINT8 */


typedef uint32 TransactionId;
typedef uint32 CommandId;
typedef uint16 OffsetNumber;
typedef unsigned int Oid;



typedef struct DatumTupleFields
{
        int32           datum_len_;             /* varlena header (do not touch directly!) */

        int32           datum_typmod;   /* -1, or identifier of a record type */

        Oid             datum_typeid;   /* composite type OID, or RECORDOID */

        /*
         * Note: field ordering is chosen with thought that Oid might someday
         * widen to 64 bits.
         */
}DatumTupleFields;

/*
 * BlockId:
 *
 * this is a storage type for BlockNumber.  in other words, this type
 * is used for on-disk structures (e.g., in HeapTupleData) whereas
 * BlockNumber is the type on which calculations are performed (e.g.,
 * in access method code).
 *
 * there doesn't appear to be any reason to have separate types except
 * for the fact that BlockIds can be SHORTALIGN'd (and therefore any
 * structures that contains them, such as ItemPointerData, can also be
 * SHORTALIGN'd).  this is an important consideration for reducing the
 * space requirements of the line pointer (ItemIdData) array on each
 * page and the header of each heap or index tuple, so it doesn't seem
 * wise to change this without good reason.
 */
typedef struct BlockIdData
{
        uint16          bi_hi;
        uint16          bi_lo;
} BlockIdData;

/*
 * ItemPointer:
 *
 * This is a pointer to an item within a disk page of a known file
 * (for example, a cross-link from an index to its parent table).
 * blkid tells us which block, posid tells us which entry in the linp
 * (ItemIdData) array we want.
 *
 * Note: because there is an item pointer in each tuple header and index
 * tuple header on disk, it's very important not to waste space with
 * structure padding bytes.  The struct is designed to be six bytes long
 * (it contains three int16 fields) but a few compilers will pad it to
 * eight bytes unless coerced.  We apply appropriate persuasion where
 * possible, and to cope with unpersuadable compilers, we try to use
 * "SizeOfIptrData" rather than "sizeof(ItemPointerData)" when computing
 * on-disk sizes.
 */
typedef struct ItemPointerData
{
        BlockIdData ip_blkid;
        OffsetNumber ip_posid;
}ItemPointerData;



/*
 * Heap tuple header.  To avoid wasting space, the fields should be
 * laid out in such a way as to avoid structure padding.
 *
 * Datums of composite types (row types) share the same general structure
 * as on-disk tuples, so that the same routines can be used to build and
 * examine them.  However the requirements are slightly different: a Datum
 * does not need any transaction visibility information, and it does need
 * a length word and some embedded type information.  We can achieve this
 * by overlaying the xmin/cmin/xmax/cmax/xvac fields of a heap tuple
 * with the fields needed in the Datum case.  Typically, all tuples built
 * in-memory will be initialized with the Datum fields; but when a tuple is
 * about to be inserted in a table, the transaction fields will be filled,
 * overwriting the datum fields.
 *
 * The overall structure of a heap tuple looks like:
 *                      fixed fields (HeapTupleHeaderData struct)
 *                      nulls bitmap (if HEAP_HASNULL is set in t_infomask)
 *                      alignment padding (as needed to make user data MAXALIGN'd)
 *                      object ID (if HEAP_HASOID is set in t_infomask)
 *                      user data fields
 *
 * We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in three
 * physical fields.  Xmin and Xmax are always really stored, but Cmin, Cmax
 * and Xvac share a field.  This works because we know that Cmin and Cmax
 * are only interesting for the lifetime of the inserting and deleting
 * transaction respectively.  If a tuple is inserted and deleted in the same
 * transaction, we store a "combo" command id that can be mapped to the real
 * cmin and cmax, but only by use of local state within the originating
 * backend.  See combocid.c for more details.  Meanwhile, Xvac is only set by
 * old-style VACUUM FULL, which does not have any command sub-structure and so
 * does not need either Cmin or Cmax.  (This requires that old-style VACUUM
 * FULL never try to move a tuple whose Cmin or Cmax is still interesting,
 * ie, an insert-in-progress or delete-in-progress tuple.)
 *
 * A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
 * is initialized with its own TID (location).  If the tuple is ever updated,
 * its t_ctid is changed to point to the replacement version of the tuple.
 * Thus, a tuple is the latest version of its row iff XMAX is invalid or
 * t_ctid points to itself (in which case, if XMAX is valid, the tuple is
 * either locked or deleted).  One can follow the chain of t_ctid links
 * to find the newest version of the row.  Beware however that VACUUM might
 * erase the pointed-to (newer) tuple before erasing the pointing (older)
 * tuple.  Hence, when following a t_ctid link, it is necessary to check
 * to see if the referenced slot is empty or contains an unrelated tuple.
 * Check that the referenced tuple has XMIN equal to the referencing tuple's
 * XMAX to verify that it is actually the descendant version and not an
 * unrelated tuple stored into a slot recently freed by VACUUM.  If either
 * check fails, one may assume that there is no live descendant version.
 *
 * t_ctid is sometimes used to store a speculative insertion token, instead
 * of a real TID.  A speculative token is set on a tuple that's being
 * inserted, until the inserter is sure that it wants to go ahead with the
 * insertion.  Hence a token should only be seen on a tuple with an XMAX
 * that's still in-progress, or invalid/aborted.  The token is replaced with
 * the tuple's real TID when the insertion is confirmed.  One should never
 * see a speculative insertion token while following a chain of t_ctid links,
 * because they are not used on updates, only insertions.
 *
 * Following the fixed header fields, the nulls bitmap is stored (beginning
 * at t_bits).  The bitmap is *not* stored if t_infomask shows that there
 * are no nulls in the tuple.  If an OID field is present (as indicated by
 * t_infomask), then it is stored just before the user data, which begins at
 * the offset shown by t_hoff.  Note that t_hoff must be a multiple of
 * MAXALIGN.
 */

typedef struct HeapTupleFields
{
        TransactionId t_xmin;           /* inserting xact ID */
        TransactionId t_xmax;           /* deleting or locking xact ID */

        union
        {
                CommandId       t_cid;          /* inserting or deleting command ID, or both */
                TransactionId t_xvac;   /* old-style VACUUM FULL xact ID */
        }                       t_field3;
}HeapTupleFields ;

//add by wln
#define FLEXIBLE_ARRAY_MEMBER 1

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! */

        uint16          t_infomask2;    /* number of attributes + various flags */

        uint16          t_infomask;             /* various flag bits, see below */

        uint8           t_hoff;                 /* sizeof header incl. bitmap, padding */

        /* ^ - 23 bytes - ^ */

        bits8           t_bits[FLEXIBLE_ARRAY_MEMBER];  /* bitmap of NULLs */

        /* MORE DATA FOLLOWS AT END OF STRUCT */
};

/*
*  this macro used to print variable
*/
#define print(n)  printf(#n" = %d\n",n)


int main(int argc,char *argv[])
{

 if(argc<2 || (argc>=2 && access(argv[1], F_OK)))
  {
    printf("file:%s doesnot exist.\n",argv[1]);
    exit(1);
  }

 char filename[1024];
 strncpy(filename,argv[1],strlen(argv[1])+1); //must +1
 printf("filename:%s,%d\n",filename,strlen(filename));
 int fd;

 if((fd=open(filename,O_RDONLY))==-1)
 {
    printf("%m,%s,line:%d\n",__FILE__,__LINE__);
    exit(1);
 }

  struct HeapTupleHeaderData HeapTupleHeaderDataEle;

  lseek(fd,7872,SEEK_SET);
  if(read(fd,&HeapTupleHeaderDataEle,sizeof(HeapTupleHeaderDataEle))==-1)
 {
    printf("%m,%s,line:%d\n",__FILE__,__LINE__);
    exit(1);
 }
  printf("sizeof(HeapTupleHeaderData)=%d\n",sizeof(HeapTupleHeaderDataEle));

 print(HeapTupleHeaderDataEle.t_choice.t_heap.t_xmin);
 print(HeapTupleHeaderDataEle.t_choice.t_heap.t_xmax); 
 print(HeapTupleHeaderDataEle.t_ctid.ip_blkid);
 print(HeapTupleHeaderDataEle.t_ctid.ip_posid);
 print(HeapTupleHeaderDataEle.t_infomask2);
 print(HeapTupleHeaderDataEle.t_infomask);
 print(HeapTupleHeaderDataEle.t_hoff);
 print(HeapTupleHeaderDataEle.t_bits[0]);



 close(fd);

}

//HeapTupleHeaderData.c 

测试：
wln@iZ232ngsvp8Z:~/pg95> ./HeapTupleHeaderData data/base/12974/41021 
filename:data/base/12974/41021,21
sizeof(HeapTupleHeaderData)=24
HeapTupleHeaderDataEle.t_choice.t_heap.t_xmin = 1329
HeapTupleHeaderDataEle.t_choice.t_heap.t_xmax = 0
HeapTupleHeaderDataEle.t_ctid.ip_blkid = 0
HeapTupleHeaderDataEle.t_ctid.ip_posid = 10
HeapTupleHeaderDataEle.t_infomask2 = 1
HeapTupleHeaderDataEle.t_infomask = 2304
HeapTupleHeaderDataEle.t_hoff = 24
HeapTupleHeaderDataEle.t_bits[0] = 0
wln@iZ232ngsvp8Z:~/pg95> psql
psql (9.5beta1)
Type "help" for help.

postgres=# select * from heap_page_items(get_raw_page('t1', 'main', 0));
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits | t_oid 
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------+-------
  1 |   8160 |        1 |     28 |   1326 |      0 |        0 | (0,1)  |           1 |       2304 |     24 |        |      
  2 |   8128 |        1 |     28 |   1326 |   1326 |        0 | (0,2)  |        8193 |       1312 |     24 |        |      
  3 |   8096 |        1 |     28 |   1327 |      0 |        0 | (0,3)  |           1 |       2304 |     24 |        |      
  4 |   8064 |        1 |     28 |   1327 |      0 |        1 | (0,4)  |           1 |       2304 |     24 |        |      
  5 |   8032 |        1 |     28 |   1328 |      0 |        0 | (0,5)  |           1 |       2304 |     24 |        |      
  6 |   8000 |        1 |     28 |   1328 |      0 |        1 | (0,6)  |           1 |       2304 |     24 |        |      
  7 |   7968 |        1 |     28 |   1329 |      0 |        0 | (0,7)  |           1 |       2304 |     24 |        |      
  8 |   7936 |        1 |     28 |   1329 |      0 |        1 | (0,8)  |           1 |       2304 |     24 |        |      
  9 |   7904 |        1 |     28 |   1329 |      0 |        2 | (0,9)  |           1 |       2304 |     24 |        |      
 10 |   7872 |        1 |     28 |   1329 |      0 |        3 | (0,10) |           1 |       2304 |     24 |        |      
(10 rows)


可以看出，和用插件查看的是一致的。lseek 位置是可以通过hexdump查看具体行位置，也可以通过PageHeaderData.c
函数取得各个tuple对应的行在page中的offset。