+----------------+---------------------------------+
| PageHeaderData | linp1 linp2 linp3 ...           |
+-----------+----+---------------------------------+
| ... linpN |                                      |
+-----------+--------------------------------------+
|           ^ pd_lower                             |
|                                                  |
|             v pd_upper                           |
+-------------+------------------------------------+
|             | tupleN ...                         |
+-------------+------------------+-----------------+
|       ... tuple3 tuple2 tuple1 | "special space" |
+--------------------------------+-----------------+
                                 ^ pd_special
                                 
*这部分代码在src/include/storage/bufpage.h

typedef struct PageHeaderData
{
	/* XXX LSN is member of *any* block, not only page-organized ones */
	PageXLogRecPtr pd_lsn;		/* LSN: next byte after last byte of xlog
								 * record for last change to this page */
	uint16		pd_checksum;	/* checksum */
	uint16		pd_flags;		/* flag bits, see below */
	LocationIndex pd_lower;		/* offset to start of free space */
	LocationIndex pd_upper;		/* offset to end of free space */
	LocationIndex pd_special;	/* offset to start of special space */
	uint16		pd_pagesize_version;
	TransactionId pd_prune_xid; /* oldest prunable XID, or zero if none */
	ItemIdData	pd_linp[FLEXIBLE_ARRAY_MEMBER]; /* line pointer array */
} PageHeaderData;

typedef PageHeaderData *PageHeader;

/*
 * pd_flags contains the following flag bits.  Undefined bits are initialized
 * to zero and may be used in the future.
 *
 * PD_HAS_FREE_LINES is set if there are any LP_UNUSED line pointers before
 * pd_lower.  This should be considered a hint rather than the truth, since
 * changes to it are not WAL-logged.
 *
 * PD_PAGE_FULL is set if an UPDATE doesn't find enough free space in the
 * page for its new tuple version; this suggests that a prune is needed.
 * Again, this is just a hint.
 */
#define PD_HAS_FREE_LINES	0x0001	/* are there any unused line pointers? */
#define PD_PAGE_FULL		0x0002	/* not enough free space for new tuple? */
#define PD_ALL_VISIBLE		0x0004	/* all tuples on page are visible to
									 * everyone */

#define PD_VALID_FLAG_BITS	0x0007	/* OR of all valid pd_flags bits */

/*
 * An item pointer (also called line pointer) on a buffer page
 *
 * In some cases an item pointer is "in use" but does not have any associated
 * storage on the page.  By convention, lp_len == 0 in every item pointer
 * that does not have storage, independently of its lp_flags state.
 */
typedef struct ItemIdData
{
	unsigned	lp_off:15,		/* offset to tuple (from start of page) */
				lp_flags:2,		/* state of item pointer, see below */
				lp_len:15;		/* byte length of tuple */
} ItemIdData;

typedef ItemIdData *ItemId;
/*
 * lp_flags has these possible states.  An UNUSED line pointer is available
 * for immediate re-use, the other states are not.
 */
#define LP_UNUSED		0		/* unused (should always have lp_len=0) */
#define LP_NORMAL		1		/* used (should always have lp_len>0) */
#define LP_REDIRECT		2		/* HOT redirect (should have lp_len=0) */
#define LP_DEAD			3		/* dead, may or may not have storage */

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;

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;

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 */
};

*这部分代码在src/include/access/htup_details.h

/*
 * information stored in t_infomask:
 */
#define HEAP_HASNULL			0x0001	/* has null attribute(s) */
#define HEAP_HASVARWIDTH		0x0002	/* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL		0x0004	/* has external stored attribute(s) */
#define HEAP_HASOID				0x0008	/* has an object-id field */
#define HEAP_XMAX_KEYSHR_LOCK	0x0010	/* xmax is a key-shared locker */
#define HEAP_COMBOCID			0x0020	/* t_cid is a combo cid */
#define HEAP_XMAX_EXCL_LOCK		0x0040	/* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY		0x0080	/* xmax, if valid, is only a locker */

/* xmax is a shared locker */
#define HEAP_XMAX_SHR_LOCK	(HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)

#define HEAP_LOCK_MASK	(HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
						 HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_XMIN_COMMITTED		0x0100	/* t_xmin committed */
#define HEAP_XMIN_INVALID		0x0200	/* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN		(HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_XMAX_COMMITTED		0x0400	/* t_xmax committed */
#define HEAP_XMAX_INVALID		0x0800	/* t_xmax invalid/aborted */
#define HEAP_XMAX_IS_MULTI		0x1000	/* t_xmax is a MultiXactId */
#define HEAP_UPDATED			0x2000	/* this is UPDATEd version of row */
#define HEAP_MOVED_OFF			0x4000	/* moved to another place by pre-9.0
										 * VACUUM FULL; kept for binary
										 * upgrade support */
#define HEAP_MOVED_IN			0x8000	/* moved from another place by pre-9.0
										 * VACUUM FULL; kept for binary
										 * upgrade support */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)

#define HEAP_XACT_MASK			0xFFF0	/* visibility-related bits */

postgres@307-> cd postgresql-10.4/contrib/pageinspect

postgres@307-> make
postgres@307-> make install

postgres@307-> psql
psql (10.4)
Type "help" for help.

postgres=# CREATE EXTENSION pageinspect;
CREATE EXTENSION

postgres=# \x
Expanded display is on.
postgres=# \dx
List of installed extensions
-[ RECORD 1 ]------------------------------------------------------
Name        | pageinspect
Version     | 1.6
Schema      | public
Description | inspect the contents of database pages at a low level
-[ RECORD 2 ]------------------------------------------------------
Name        | plpgsql
Version     | 1.0
Schema      | pg_catalog
Description | PL/pgSQL procedural language

postgres=# 

postgres=# create table t1(id int,name varchar(10));
CREATE TABLE

postgres=# insert into t1 select generate_series(1,1000),generate_series(1,1000)||'_x';
INSERT 0 1000
postgres=# 

postgres=# select * from t1 limit 10;
 id | name 
----+------
  1 | 1_x
  2 | 2_x
  3 | 3_x
  4 | 4_x
  5 | 5_x
  6 | 6_x
  7 | 7_x
  8 | 8_x
  9 | 9_x
 10 | 10_x
(10 rows)
postgres=# 

postgres=# select count(*) from t1;
 count 
-------
  1000
(1 row)

postgres=# select max(ctid) from t1;
  max   
--------
 (5,73)
(1 row)

postgres=# \d+
                  List of relations
 Schema | Name | Type  | Owner | Size  | Description 
--------+------+-------+-------+-------+-------------
 public | t1   | table | pg    | 72 kB | 
(1 row)

postgres=# 

postgres=# \x
Expanded display is on.
postgres=# select * from page_header(get_raw_page('t1',0));
-[ RECORD 1 ]--------
lsn       | 0/1671188
checksum  | 0
flags     | 0
lower     | 772
upper     | 784
special   | 8192
pagesize  | 8192
version   | 4
prune_xid | 0

postgres=# 

postgres=# select * from heap_page_items(get_raw_page('t1',0)) limit 1;
-[ RECORD 1 ]-------------------
lp          | 1
lp_off      | 8160
lp_flags    | 1
lp_len      | 32
t_xmin      | 557
t_xmax      | 0
t_field3    | 0
t_ctid      | (0,1)
t_infomask2 | 2
t_infomask  | 2306
t_hoff      | 24
t_bits      | 
t_oid       | 
t_data      | \x0100000009315f78

postgres=# 

postgres=# \d t1
                        Table "public.t1"
 Column |         Type          | Collation | Nullable | Default 
--------+-----------------------+-----------+----------+---------
 id     | integer               |           |          | 
 name   | character varying(10) |           |          | 

postgres=# 

postgres=# select * from page_header(get_raw_page('t1',0));
-[ RECORD 1 ]--------
lsn       | 0/1671188
checksum  | 0
flags     | 0
lower     | 772
upper     | 784
special   | 8192
pagesize  | 8192
version   | 4
prune_xid | 0

postgres=# delete from t1 where id=1;
DELETE 1
postgres=# select * from page_header(get_raw_page('t1',0));
-[ RECORD 1 ]--------
lsn       | 0/16852A0
checksum  | 0
flags     | 0
lower     | 772
upper     | 784
special   | 8192
pagesize  | 8192
version   | 4
prune_xid | 559

postgres=# 

postgres=# select * from heap_page_items(get_raw_page('t1',0)) limit 1;
-[ RECORD 1 ]-------------------
lp          | 1
lp_off      | 8160
lp_flags    | 1
lp_len      | 32
t_xmin      | 557
t_xmax      | 559
t_field3    | 0
t_ctid      | (0,1)
t_infomask2 | 8194
t_infomask  | 258
t_hoff      | 24
t_bits      | 
t_oid       | 
t_data      | \x0100000009315f78

postgres=#