PostgreSQL源码学习（1）Page页

  • Z时代
  • 2024-01-10
  • 分类：综合

database

The Internals of PostgreSQL中对于存储的描述http://www.interdb.jp/pg/pgsql01.html#_1.2.

每个表或索引都作为一个page数组存储于物理数据文件中（page大小默认为8k，编译时可以指定），单个的表数据文件中包含多个page页（默认单个表文件最大1G，超过1G后会新建同名+“.1”后缀的数据文件，依次累加，同样在编译时可以指定单个文件的最大值）。page页的结构如下图所示。

而对于page页中存储的行数据Tuple，其结构为下图所示。

相关数据结构

// 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;

// src/include/storage/itemid.h
typedef struct ItemIdData
{
	unsigned	lp_off:15,		/* offset to tuple (from start of page) */
				lp_flags:2,		/* state of line pointer, see below */
				lp_len:15;		/* byte length of tuple */
} ItemIdData;
/*
 * 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 */

// src/include/access/htup_details.h
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 */
};

// src/include/access/htup.h
typedef struct HeapTupleData
{
	uint32		t_len;			/* length of *t_data */
	ItemPointerData t_self;		/* SelfItemPointer */
	Oid			t_tableOid;		/* table the tuple came from */
#define FIELDNO_HEAPTUPLEDATA_DATA 3
	HeapTupleHeader t_data;		/* -> tuple header and data */
} HeapTupleData;

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