透过pageinspect了解PostgreSQl中的page

        博客 PostgreSQL的Page分析记录 有过对page的 大体介绍，可以参看之前的blog，下面主要通过插件 pageinspect 向大家进行介绍。

        在此之前需要了解的名词：
        page，物理文件的单位，默认大小为8K。
        tuple，PG中物理行。
        xid，事务号，执行操作时的顺序id。

        pageinspect里边有三个函数是本文用到的，他们分别是：
        a.get_raw_page，根据参数表明、数据文件类型（main、fsm、vm）以及page位置，将当前表文件中的page内容返回。还有一个函数于此同名，只有两个参数，是将第二个参数省略，直接使用'main'。
        b.page_header，参数是函数get_raw_page的返回值，返回值是将本page结构中的PageHeaderData详细信息。
        c.heap_page_items,参数是函数get_raw_page的返回值，返回值是将page内的项指针(ItemIddata)以及HeapTupleHeaderData的详细信息。

        下面通过例子来对page的PageHeaderData、ItemIddata和HeapTupleHeaderData进行说明。
        a.建表mvcc

[postgres@localhost bin]$ ./psql 
psql (9.4.5)
Type "help" for help.

postgres=# create table mvcc(id int);
CREATE TABLE
postgres=# insert into mvcc values (1),(2);
INSERT 0 2

         b.PageHeaderData

postgres=# select * from page_header(get_raw_page('mvcc', 'main', 0));
    lsn    | checksum | flags | lower | upper | special | pagesize | version | p
rune_xid 
-----------+----------+-------+-------+-------+---------+----------+---------+--
---------
 0/300F4D8 |        0 |     0 |    32 |  8128 |    8192 |     8192 |       4 |  
       0
(1 row)

        这是表mvcc的PageHeaderData信息，现在比对源码对各个字段进行解释：

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;

        pg_lsn：记录最后一次对page修改的xlog记录id。
        pg_checksum：页面的校验和，主要是通过函数pg_checksum_block函数生成的，0也是有效地，参数为PageHeaderData和BLCKSZ(page's size)。当校验和验证失败，即认为当前页面无效。
        pg_flags：page的flags，具体值为，可以叠加：

#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 */

        pg_lower和pg_upper：最后一个项指针的位置和最新的tuple位置。主要进行查找空闲位置，进行插入工作。
        pg_special：page预留的位置，可以存储索引等信息。
        pg_pagesize_version：page大小以及当前版本。page大小可以通过configure进行设置。version的意思是

/*
 * Page layout version number 0 is for pre-7.3 Postgres releases.
 * Releases 7.3 and 7.4 use 1, denoting a new HeapTupleHeader layout.
 * Release 8.0 uses 2; it changed the HeapTupleHeader layout again.
 * Release 8.1 uses 3; it redefined HeapTupleHeader infomask bits.
 * Release 8.3 uses 4; it changed the HeapTupleHeader layout again, and
 *		added the pd_flags field (by stealing some bits from pd_tli),
 *		as well as adding the pd_prune_xid field (which enlarges the header).
 *
 * As of Release 9.3, the checksum version must also be considered when
 * handling pages.
 */

          pg_prune_xid：一般是最后一次删除或者更新的xid。

        pg_linp：项指针。
        c.ItemIddata

postgres=# select lp,lp_off,lp_flags,lp_len from heap_page_items(get_raw_page('mvcc', 'main', 0));
 lp | lp_off | lp_flags | lp_len 
----+--------+----------+--------
  1 |   8160 |        1 |     28
  2 |   8128 |        1 |     28
(2 rows)

        这是表mvcc的项指针的信息，一样通过结合源码进行介绍：

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;

        lp：这是插件自己定义的列，在源码中其实没有，这个是项指针的顺序。
        lp_off：tuple在page中的位置。
        lp_flags：tuple的flags，具体为

#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 */

        lp_len： HeapTupleHeaderData 的长度+Oid的长度(8，因为要数据对齐，所以在这里会比原来预计的多4)。

        d.HeapTupleHeaderData

postgres=# select * from heap_page_items(get_raw_page('mvcc', 'main', 0));
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infom
ask2 | t_infomask | t_hoff | t_bits | t_oid 
----+--------+----------+--------+--------+--------+----------+--------+--------
-----+------------+--------+--------+-------
  1 |   8160 |        1 |     28 |   1831 |      0 |        0 | (0,1)  |        
   1 |       2048 |     24 |        |      
  2 |   8128 |        1 |     28 |   1831 |      0 |        0 | (0,2)  |        
   1 |       2048 |     24 |        |      
(2 rows)

         同样结合源码进行介绍：

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 */

	/* 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[1];		/* bitmap of NULLs -- VARIABLE LENGTH */

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

        xmin和xmax是插入、删除和更新时的事务ID，插入时会在xmin内写入当前事务ID，当删除时就会在xmax写入当前事务ID。更新是进行删除后再插入。
        t_cid：这个是指一个事务内的命令ID，每个事务都是从0开始。
        t_ctid：这个是指物理ID，结构如下：

typedef struct ItemPointerData
{
	BlockIdData ip_blkid;
	OffsetNumber ip_posid;
}

typedef struct BlockIdData
{
	uint16		bi_hi;
	uint16		bi_lo;
} BlockIdData;

        存储的为bi_hi（文件号） << 16 | bi_lo（page号），来获取磁盘顺序，ip_posid是在page的中序号。以此来准确定位数据。
        t_infomask2：表字段的个数以及一些flags，flags如下：

#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 */

         t_infomask：tuple的flags，如下：

#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

        t_hoff： HeapTupleHeaderData长度，如果有Oid会增加4，但由于受到对齐的影响，会增加8。
        t_bits：具体数据，可以参照 PostgreSQL的基础数据类型分析记录。