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/16477/2480100000000  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.htypedef 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  Descriptiont_xmin          TransactionId   4 bytes 0       insert XID stampt_xmax          TransactionId   4 bytes 4       delete XID stampt_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 versiont_ctid          ItemPointerData 6 bytes 12      current TID of this or newer row versiont_infomask2     uint16          2 bytes 18      number of attributes, plus various flag bitst_infomask      uint16          2 bytes 20      various flag bitst_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：

[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8152 -n 400001fd8  e2 1b 18 00                                       |....|00001fdc[xdb@localhost ~]$ echo $((0x00181be2))1580002

t_xmax

[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8156 -n 400001fdc  00 00 00 00                                       |....|00001fe0

t_cid/t_xvac

[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8160 -n 400001fe0  00 00 00 00                                       |....|00001fe4

t_ctid

[xdb@localhost ~]$ hexdump -C $PGDATA/base/16477/24801 -s 8164 -n 600001fe4  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 200001fea  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 $PGDATA/base/16477/24801 -s 8172 -n 200001fec  02 08                                             |..|00001fee[xdb@localhost ~]$ echo $((0x0802))2050[xdb@localhost ~]$ echo "obase=2;2050"|bc100000000010//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 $PGDATA/base/16477/24801 -s 8174 -n 100001fee  18                                                |.|00001fef[xdb@localhost ~]$ 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 1500001ff0  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1c1=\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。