PostgreSQL 数据页Page解析（2）- 页头和行数据指针

来源：这里教程网时间：2026-03-14 19:28:38 作者：

本文简单介绍了PG数据页Page中存储的原始内容以及如何阅读它们，包括页头PageHeader和行数据指针ItemId（Line Pointer）。

一、测试数据

-- 创建一张表，插入几行数据 drop table if exists t_page; create table t_page (id int,c1 char(8),c2 varchar(16)); insert into t_page values(1,'1','a'); insert into t_page values(2,'2','b'); insert into t_page values(3,'3','c'); insert into t_page values(4,'4','d'); -- 获取该表对应的数据文件 testdb=# select pg_relation_filepath('t_page'); pg_relation_filepath ---------------------- base/16477/24801 (1 row) -- Dump数据文件中的数据 [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、PageHeader

上一节提到过PageHeaderData，其数据结构如下： 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[1]; /* beginning of line pointer array */ } PageHeaderData; 下面根据数据文件中的数据使用hexdump查看并逐个进行解析。

pd_lsn(8bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 0 -n 8 00000000 01 00 00 00 88 20 2a 12 |..... *.| 00000008 数据文件的8个Bytes存储的是LSN，其中最开始的4个Bytes是TimelineID，在这里是\x0000 0001（即数字1），后面的4个Bytes是\x122a2088，组合起来LSN为1/122A2088 注意： A、0000000&0000008是hexdump工具的输出，不是数据内容 B、X86使用小端模式，阅读字节码时注意高低位变换

pd_checksum(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 8 -n 2 00000008 00 00 |..| 0000000a checksum为\x0000

pd_flags(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 10 -n 2 0000000a 00 00 |..| 0000000c flags为\x0000

pd_lower(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 12 -n 2 0000000c 28 00 |(.| 0000000e lower为\x0028，十进制值为40

pd_upper(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 14 -n 2 0000000e 60 1f |`.| 00000010 [xdb@localhost utf8db]$ echo $((0x1f60)) 8032 upper为\x1f60，十进制为8032

pd_special(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 16 -n 2 00000010 00 20 |. | 00000012 Special Space为\x2000，十进制值为8192

pd_pagesize_version(2bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 18 -n 2 00000012 04 20 |. | 00000014 pagesize_version为\x2004，十进制为8196（即版本4）

pd_prune_xid(4bytes)

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 20 -n 4 00000014 00 00 00 00 |....| 00000018 prune_xid为\x0000，即0

三、ItemIds

PageHeaderData之后是ItemId数组，每个元素占用的空间为4Bytes，数据结构： 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_off

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 24 -n 2 00000018 d8 9f |..| 0000001a 取低15位 [xdb@localhost utf8db]$ echo $((0x9fd8 & ~$((1<<15)))) 8152 表示第1个Item（tuple）从8152开始

lp_len

[xdb@localhost utf8db]$ hexdump -C $PGDATA/base/16477/24801 -s 26 -n 2 0000001a 4e 00 |N.| 0000001c 取高15位 [xdb@localhost utf8db]$ echo $((0x004e >> 1)) 39 表示第1个Item（tuple）的大小为39