本文介绍了PG数据页Page中存储的原始内容以及如何阅读它们，这一节主要介绍行数据（Items）。
一、测试数据
详见上一节，数据文件中的内容如下：
[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801
00000000 01 00 00 00 88 20 2a 12 00 00 00 00 28 00 60 1f |..... *.....(.`.|
00000010 00 20 04 20 00 00 00 00 d8 9f 4e 00 b0 9f 4e 00 |. . ......N...N.|
00000020 88 9f 4e 00 60 9f 4e 00 00 00 00 00 00 00 00 00 |..N.`.N.........|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001f60 e5 1b 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001f70 04 00 03 00 02 08 18 00 04 00 00 00 13 34 20 20 |.............4 |
00001f80 20 20 20 20 20 05 64 00 e4 1b 18 00 00 00 00 00 | .d.........|
00001f90 00 00 00 00 00 00 00 00 03 00 03 00 02 08 18 00 |................|
00001fa0 03 00 00 00 13 33 20 20 20 20 20 20 20 05 63 00 |.....3 .c.|
00001fb0 e3 1b 18 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001fc0 02 00 03 00 02 08 18 00 02 00 00 00 13 32 20 20 |.............2 |
00001fd0 20 20 20 20 20 05 62 00 e2 1b 18 00 00 00 00 00 | .b.........|
00001fe0 00 00 00 00 00 00 00 00 01 00 03 00 02 08 18 00 |................|
00001ff0 01 00 00 00 13 31 20 20 20 20 20 20 20 05 61 00 |.....1 .a.|
00002000
二、Items（Tuples）
每个Tuple包括两部分，第一部分是Tuple头部信息，第二部分是实际的数据。
1、HeapTupleHeader
相关数据结构如下：
//--------------------- src/include/storage/off.h
/*
* OffsetNumber:
*
* this is a 1-based index into the linp (ItemIdData) array in the
* header of each disk page.
*/
typedef uint16 OffsetNumber;
//--------------------- src/include/storage/block.h
/*
* 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;
typedef BlockIdData *BlockId; /* block identifier */
//--------------------- src/include/storage/itemptr.h
/*
* 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. If your compiler can't be made to play along, you'll waste
* lots of space.
*/
typedef struct ItemPointerData
{
BlockIdData ip_blkid;
OffsetNumber ip_posid;
}
//--------------------- src/include/access/htup_details.h
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 */
/*
* datum_typeid cannot be a domain over composite, only plain composite,
* even if the datum is meant as a value of a domain-over-composite type.
* This is in line with the general principle that CoerceToDomain does not
* change the physical representation of the base type value.
*
* 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! */
#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 */
};
结构体展开，详见下表：
Field Type Length Offset Description
t_xmin TransactionId 4 bytes 0 insert XID stamp
t_xmax TransactionId 4 bytes 4 delete XID stamp
t_cid CommandId 4 bytes 8 insert and/or delete CID stamp (overlays with t_xvac)
t_xvac TransactionId 4 bytes 8 XID for VACUUM operation moving a row version
t_ctid ItemPointerData 6 bytes 12 current TID of this or newer row version
t_infomask2 uint16 2 bytes 18 number of attributes, plus various flag bits
t_infomask uint16 2 bytes 20 various flag bits
t_hoff uint8 1 byte 22 offset to user data
//注意：t_cid和t_xvac为联合体，共用存储空间
从上一节我们已经得出第1个Tuple的偏移为8152，下面使用hexdump对其中的数据逐个解析：
t_xmin
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8152 -n 4
00001fd8 e2 1b 18 00 |....|
00001fdc
[xdb@localhost ~]$ echo $((0x00181be2))
1580002
t_xmax
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8156 -n 4
00001fdc 00 00 00 00 |....|
00001fe0
t_cid/t_xvac
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8160 -n 4
00001fe0 00 00 00 00 |....|
00001fe4
t_ctid
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8164 -n 6
00001fe4 00 00 00 00 01 00 |......|
00001fea
//ip_blkid=\x0000，即blockid=0
//ip_posid=\x0001，即posid=1，第1个tuple
t_infomask2
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8170 -n 2
00001fea 03 00 |..|
00001fec
//t_infomask2=\x0003，3代表什么意思？我们看看t_infomask2的说明
/*
* information stored in t_infomask2:
*/
#define HEAP_NATTS_MASK 0x07FF /* 11 bits for number of attributes */
/* bits 0x1800 are available */
#define HEAP_KEYS_UPDATED 0x2000 /* tuple was updated and key cols
* modified, or tuple deleted */
#define HEAP_HOT_UPDATED 0x4000 /* tuple was HOT-updated */
#define HEAP_ONLY_TUPLE 0x8000 /* this is heap-only tuple */
#define HEAP2_XACT_MASK 0xE000 /* visibility-related bits */
//根把十六进制值转换为二进制显示
11111111111 #define HEAP_NATTS_MASK 0x07FF
10000000000000 #define HEAP_KEYS_UPDATED 0x2000
100000000000000 #define HEAP_HOT_UPDATED 0x4000
1000000000000000 #define HEAP_ONLY_TUPLE 0x8000
1110000000000000 #define HEAP2_XACT_MASK 0xE000
1111111111111110 #define SpecTokenOffsetNumber 0xfffe
//前（低）11位为属性的个数，3意味着有3个属性（字段）
t_infomask
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8172 -n 2
00001fec 02 08 |..|
00001fee
[xdb@localhost ~]$ echo $((0x0802))
2050
[xdb@localhost ~]$ echo "obase=2;2050"|bc
100000000010
//t_infomask=\x0802，十进制值为2050，二进制值为100000000010
//t_infomask说明
1 #define HEAP_HASNULL 0x0001 /* has null attribute(s) */
10 #define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
100 #define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
1000 #define HEAP_HASOID 0x0008 /* has an object-id field */
10000 #define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
100000 #define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */
1000000 #define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
10000000 #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)
100000000 #define HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
1000000000 #define HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
10000000000 #define HEAP_XMAX_COMMITTED 0x0400 /* t_xmax committed */
100000000000 #define HEAP_XMAX_INVALID 0x0800 /* t_xmax invalid/aborted */
1000000000000 #define HEAP_XMAX_IS_MULTI 0x1000 /* t_xmax is a MultiXactId */
10000000000000 #define HEAP_UPDATED 0x2000 /* this is UPDATEd version of row */
100000000000000 #define HEAP_MOVED_OFF 0x4000 /* moved to another place by pre-9.0
* VACUUM FULL; kept for binary
* upgrade support */
1000000000000000 #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)
1111111111110000 #define HEAP_XACT_MASK 0xFFF0 /* visibility-related bits */
//\x0802，二进制100000000010表示第2位和第12位为1，
//意味着存在可变长属性（HEAP_HASVARWIDTH），XMAX无效（HEAP_XMAX_INVALID）
t_hoff
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8174 -n 1
00001fee 18 |.|
00001fef
[xdb@localhost ~]$ echo $((0x18))
24
//用户数据开始偏移为24，即8152+24
2、Tuple
说完了Tuple的头部数据，接下来我们看看实际的数据存储。上一节我们得到Tuple总的长度是39，计算得到数据大小为39-24=15。
[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8176 -n 15
00001ff0 01 00 00 00 13 31 20 20 20 20 20 20 20 05 61 |.....1 .a|
00001fff
回顾我们的表结构：
create table t_page (id int,c1 char(8),c2 varchar(16));
第1个字段为int，第2个字段为定长字符，第3个字段为变长字符。
相应的数据：
id=\x00000001，数字1
c1=\x133120202020202020，字符串，无需高低位变换，第1个字节\x13为标志位，后面是字符'1'+7个空格
c2=\x0561，字符串，第1个字节\x05为标志位，后面是字符'a'
三、小结
以上简单介绍了如何阅读Raw Datafile中的数据行信息，包括Tuple头部信息和用户数据。在空间使用上面，可以看PG到为了进行实际的数据查询和MVCC等机制，数据库添加了很多的额外信息，实际占用的空间大小比实际的数据要大很多，如果使用列式存储应可有效的压缩空间。
