htup.h

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

/*-------------------------------------------------------------------------
 *
 * htup.h
 *	  POSTGRES heap tuple definitions.
 *
 *
 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.78 2005/10/15 02:49:42 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#ifndef HTUP_H
#define HTUP_H

#include "storage/bufpage.h"
#include "storage/relfilenode.h"
#include "access/transam.h"


/*
 * MaxTupleAttributeNumber limits the number of (user) columns in a tuple.
 * The key limit on this value is that the size of the fixed overhead for
 * a tuple, plus the size of the null-values bitmap (at 1 bit per column),
 * plus MAXALIGN alignment, must fit into t_hoff which is uint8.  On most
 * machines the upper limit without making t_hoff wider would be a little
 * over 1700.  We use round numbers here and for MaxHeapAttributeNumber
 * so that alterations in HeapTupleHeaderData layout won't change the
 * supported max number of columns.
 */
#define MaxTupleAttributeNumber 1664	/* 8 * 208 */

/*----------
 * MaxHeapAttributeNumber limits the number of (user) columns in a table.
 * This should be somewhat less than MaxTupleAttributeNumber.  It must be
 * at least one less, else we will fail to do UPDATEs on a maximal-width
 * table (because UPDATE has to form working tuples that include CTID).
 * In practice we want some additional daylight so that we can gracefully
 * support operations that add hidden "resjunk" columns, for example
 * SELECT * FROM wide_table ORDER BY foo, bar, baz.
 * In any case, depending on column data types you will likely be running
 * into the disk-block-based limit on overall tuple size if you have more
 * than a thousand or so columns.  TOAST won't help.
 *----------
 */
#define MaxHeapAttributeNumber	1600	/* 8 * 200 */

/*----------
 * Heap tuple header.  To avoid wasting space, the fields should be
 * layed out in such a way to avoid structure padding.
 *
 * Datums of composite types (row types) share the same general structure
 * as on-disk tuples, so that the same routines can be used to build and
 * examine them.  However the requirements are slightly different: a Datum
 * does not need any transaction visibility information, and it does need
 * a length word and some embedded type information.  We can achieve this
 * by overlaying the xmin/cmin/xmax/cmax/xvac fields of a heap tuple
 * with the fields needed in the Datum case.  Typically, all tuples built
 * in-memory will be initialized with the Datum fields; but when a tuple is
 * about to be inserted in a table, the transaction fields will be filled,
 * overwriting the datum fields.
 *
 * The overall structure of a heap tuple looks like:
 *			fixed fields (HeapTupleHeaderData struct)
 *			nulls bitmap (if HEAP_HASNULL is set in t_infomask)
 *			alignment padding (as needed to make user data MAXALIGN'd)
 *			object ID (if HEAP_HASOID is set in t_infomask)
 *			user data fields
 *
 * We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in four
 * physical fields.  Xmin, Cmin and Xmax are always really stored, but
 * Cmax and Xvac share a field.  This works because we know that there are
 * only a limited number of states that a tuple can be in, and that Cmax
 * is only interesting for the lifetime of the deleting transaction.
 * This assumes that VACUUM FULL never tries to move a tuple whose Cmax
 * is still interesting (ie, delete-in-progress).
 *
 * Note that in 7.3 and 7.4 a similar idea was applied to Xmax and Cmin.
 * However, with the advent of subtransactions, a tuple may need both Xmax
 * and Cmin simultaneously, so this is no longer possible.
 *
 * A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
 * is initialized with its own TID (location).	If the tuple is ever updated,
 * its t_ctid is changed to point to the replacement version of the tuple.
 * Thus, a tuple is the latest version of its row iff XMAX is invalid or
 * t_ctid points to itself (in which case, if XMAX is valid, the tuple is
 * either locked or deleted).  One can follow the chain of t_ctid links
 * to find the newest version of the row.  Beware however that VACUUM might
 * erase the pointed-to (newer) tuple before erasing the pointing (older)
 * tuple.  Hence, when following a t_ctid link, it is necessary to check
 * to see if the referenced slot is empty or contains an unrelated tuple.
 * Check that the referenced tuple has XMIN equal to the referencing tuple's
 * XMAX to verify that it is actually the descendant version and not an
 * unrelated tuple stored into a slot recently freed by VACUUM.  If either
 * check fails, one may assume that there is no live descendant version.
 *
 * Following the fixed header fields, the nulls bitmap is stored (beginning
 * at t_bits).	The bitmap is *not* stored if t_infomask shows that there
 * are no nulls in the tuple.  If an OID field is present (as indicated by
 * t_infomask), then it is stored just before the user data, which begins at
 * the offset shown by t_hoff.	Note that t_hoff must be a multiple of
 * MAXALIGN.
 *----------
 */

typedef struct HeapTupleFields
{
	TransactionId t_xmin;		/* inserting xact ID */
	CommandId	t_cmin;			/* inserting command ID */
	TransactionId t_xmax;		/* deleting or locking xact ID */

	union
	{
		CommandId	t_cmax;		/* deleting or locking command ID */
		TransactionId t_xvac;	/* VACUUM FULL xact ID */
	}			t_field4;
} HeapTupleFields;

typedef struct DatumTupleFields
{
	int32		datum_len;		/* required to be a varlena type */

	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;

typedef struct HeapTupleHeaderData
{
	union
	{
		HeapTupleFields t_heap;
		DatumTupleFields t_datum;
	}			t_choice;

	ItemPointerData t_ctid;		/* current TID of this or newer tuple */

	int16		t_natts;		/* number of attributes */

	uint16		t_infomask;		/* various flag bits, see below */

	uint8		t_hoff;			/* sizeof header incl. bitmap, padding */

	/* ^ - 27 bytes - ^ */

	bits8		t_bits[1];		/* bitmap of NULLs -- VARIABLE LENGTH */

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

typedef HeapTupleHeaderData *HeapTupleHeader;

/*
 * information stored in t_infomask:
 */
#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_HASCOMPRESSED		0x0008	/* has compressed stored attribute(s) */
#define HEAP_HASEXTENDED		0x000C	/* the two above combined */
#define HEAP_HASOID				0x0010	/* has an object-id field */
/* 0x0020 is presently unused */
#define HEAP_XMAX_EXCL_LOCK		0x0040	/* xmax is exclusive locker */
#define HEAP_XMAX_SHARED_LOCK	0x0080	/* xmax is shared locker */
/* if either LOCK bit is set, xmax hasn't deleted the tuple, only locked it */
#define HEAP_IS_LOCKED	(HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_SHARED_LOCK)
#define HEAP_XMIN_COMMITTED		0x0100	/* t_xmin committed */
#define HEAP_XMIN_INVALID		0x0200	/* t_xmin invalid/aborted */
#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 VACUUM
										 * FULL */
#define HEAP_MOVED_IN			0x8000	/* moved from another place by VACUUM
										 * FULL */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)

#define HEAP_XACT_MASK			0xFFC0	/* visibility-related bits */


/*
 * HeapTupleHeader accessor macros
 *
 * Note: beware of multiple evaluations of "tup" argument.	But the Set
 * macros evaluate their other argument only once.
 */

#define HeapTupleHeaderGetXmin(tup) \
( \
	(tup)->t_choice.t_heap.t_xmin \
)

#define HeapTupleHeaderSetXmin(tup, xid) \
( \
	TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_xmin) \
)

#define HeapTupleHeaderGetXmax(tup) \
( \
	(tup)->t_choice.t_heap.t_xmax \
)

#define HeapTupleHeaderSetXmax(tup, xid) \
( \
	TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_xmax) \
)

#define HeapTupleHeaderGetCmin(tup) \
( \
	(tup)->t_choice.t_heap.t_cmin \
)

#define HeapTupleHeaderSetCmin(tup, cid) \
( \
	(tup)->t_choice.t_heap.t_cmin = (cid) \
)

/*
 * Note: GetCmax will produce wrong answers after SetXvac has been executed
 * by a transaction other than the inserting one.  We could check
 * HEAP_XMAX_INVALID and return FirstCommandId if it's clear, but since that
 * bit will be set again if the deleting transaction aborts, there'd be no
 * real gain in safety from the extra test.  So, just rely on the caller not
 * to trust the value unless it's meaningful.
 */
#define HeapTupleHeaderGetCmax(tup) \
( \
	(tup)->t_choice.t_heap.t_field4.t_cmax \
)

#define HeapTupleHeaderSetCmax(tup, cid) \
do { \
	Assert(!((tup)->t_infomask & HEAP_MOVED)); \
	(tup)->t_choice.t_heap.t_field4.t_cmax = (cid); \
} while (0)

#define HeapTupleHeaderGetXvac(tup) \
( \
	((tup)->t_infomask & HEAP_MOVED) ? \
		(tup)->t_choice.t_heap.t_field4.t_xvac \
	: \
		InvalidTransactionId \
)

#define HeapTupleHeaderSetXvac(tup, xid) \
do { \
	Assert((tup)->t_infomask & HEAP_MOVED); \
	TransactionIdStore((xid), &(tup)->t_choice.t_heap.t_field4.t_xvac); \
} while (0)

#define HeapTupleHeaderGetDatumLength(tup) \
( \
	(tup)->t_choice.t_datum.datum_len \
)

#define HeapTupleHeaderSetDatumLength(tup, len) \
( \
	(tup)->t_choice.t_datum.datum_len = (len) \
)

#define HeapTupleHeaderGetTypeId(tup) \
( \
	(tup)->t_choice.t_datum.datum_typeid \
)

#define HeapTupleHeaderSetTypeId(tup, typeid) \
( \
	(tup)->t_choice.t_datum.datum_typeid = (typeid) \