bufpage.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * bufpage.h
 *    Standard POSTGRES buffer page definitions.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/bufpage.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef BUFPAGE_H
#define BUFPAGE_H

#include "access/xlogdefs.h"
#include "storage/block.h"
#include "storage/item.h"
#include "storage/off.h"

/*
 * A postgres disk page is an abstraction layered on top of a postgres
 * disk block (which is simply a unit of i/o, see block.h).
 *
 * specifically, while a disk block can be unformatted, a postgres
 * disk page is always a slotted page of the form:
 *
 * +----------------+---------------------------------+
 * | PageHeaderData | linp1 linp2 linp3 ...           |
 * +-----------+----+---------------------------------+
 * | ... linpN |                                      |
 * +-----------+--------------------------------------+
 * |           ^ pd_lower                             |
 * |                                                  |
 * |             v pd_upper                           |
 * +-------------+------------------------------------+
 * |             | tupleN ...                         |
 * +-------------+------------------+-----------------+
 * |       ... tuple3 tuple2 tuple1 | "special space" |
 * +--------------------------------+-----------------+
 *                                  ^ pd_special
 *
 * a page is full when nothing can be added between pd_lower and
 * pd_upper.
 *
 * all blocks written out by an access method must be disk pages.
 *
 * EXCEPTIONS:
 *
 * obviously, a page is not formatted before it is initialized by
 * a call to PageInit.
 *
 * NOTES:
 *
 * linp1..N form an ItemId (line pointer) array.  ItemPointers point
 * to a physical block number and a logical offset (line pointer
 * number) within that block/page.  Note that OffsetNumbers
 * conventionally start at 1, not 0.
 *
 * tuple1..N are added "backwards" on the page.  Since an ItemPointer
 * offset is used to access an ItemId entry rather than an actual
 * byte-offset position, tuples can be physically shuffled on a page
 * whenever the need arises.  This indirection also keeps crash recovery
 * relatively simple, because the low-level details of page space
 * management can be controlled by standard buffer page code during
 * logging, and during recovery.
 *
 * AM-generic per-page information is kept in PageHeaderData.
 *
 * AM-specific per-page data (if any) is kept in the area marked "special
 * space"; each AM has an "opaque" structure defined somewhere that is
 * stored as the page trailer.  an access method should always
 * initialize its pages with PageInit and then set its own opaque
 * fields.
 */

typedef Pointer Page;


/*
 * location (byte offset) within a page.
 *
 * note that this is actually limited to 2^15 because we have limited
 * ItemIdData.lp_off and ItemIdData.lp_len to 15 bits (see itemid.h).
 */
typedef uint16 LocationIndex;


/*
 * For historical reasons, the 64-bit LSN value is stored as two 32-bit
 * values.
 */
typedef struct
{
    uint32      xlogid;         /* high bits */
    uint32      xrecoff;        /* low bits */
} PageXLogRecPtr;

#define PageXLogRecPtrGet(val) \
    ((uint64) (val).xlogid << 32 | (val).xrecoff)
#define PageXLogRecPtrSet(ptr, lsn) \
    ((ptr).xlogid = (uint32) ((lsn) >> 32), (ptr).xrecoff = (uint32) (lsn))

/*
 * disk page organization
 *
 * space management information generic to any page
 *
 *      pd_lsn      - identifies xlog record for last change to this page.
 *      pd_checksum - page checksum, if set.
 *      pd_flags    - flag bits.
 *      pd_lower    - offset to start of free space.
 *      pd_upper    - offset to end of free space.
 *      pd_special  - offset to start of special space.
 *      pd_pagesize_version - size in bytes and page layout version number.
 *      pd_prune_xid - oldest XID among potentially prunable tuples on page.
 *
 * The LSN is used by the buffer manager to enforce the basic rule of WAL:
 * "thou shalt write xlog before data".  A dirty buffer cannot be dumped
 * to disk until xlog has been flushed at least as far as the page's LSN.
 *
 * pd_checksum stores the page checksum, if it has been set for this page;
 * zero is a valid value for a checksum. If a checksum is not in use then
 * we leave the field unset. This will typically mean the field is zero
 * though non-zero values may also be present if databases have been
 * pg_upgraded from releases prior to 9.3, when the same byte offset was
 * used to store the current timelineid when the page was last updated.
 * Note that there is no indication on a page as to whether the checksum
 * is valid or not, a deliberate design choice which avoids the problem
 * of relying on the page contents to decide whether to verify it. Hence
 * there are no flag bits relating to checksums.
 *
 * pd_prune_xid is a hint field that helps determine whether pruning will be
 * useful.  It is currently unused in index pages.
 *
 * The page version number and page size are packed together into a single
 * uint16 field.  This is for historical reasons: before PostgreSQL 7.3,
 * there was no concept of a page version number, and doing it this way
 * lets us pretend that pre-7.3 databases have page version number zero.
 * We constrain page sizes to be multiples of 256, leaving the low eight
 * bits available for a version number.
 *
 * Minimum possible page size is perhaps 64B to fit page header, opaque space
 * and a minimal tuple; of course, in reality you want it much bigger, so
 * the constraint on pagesize mod 256 is not an important restriction.
 * On the high end, we can only support pages up to 32KB because lp_off/lp_len
 * are 15 bits.
 */

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;

typedef PageHeaderData *PageHeader;

/*
 * pd_flags contains the following flag bits.  Undefined bits are initialized
 * to zero and may be used in the future.
 *
 * PD_HAS_FREE_LINES is set if there are any LP_UNUSED line pointers before
 * pd_lower.  This should be considered a hint rather than the truth, since
 * changes to it are not WAL-logged.
 *
 * PD_PAGE_FULL is set if an UPDATE doesn't find enough free space in the
 * page for its new tuple version; this suggests that a prune is needed.
 * Again, this is just a hint.
 */
#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 */

/*
 * 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.
 */
#define PG_PAGE_LAYOUT_VERSION      4
#define PG_DATA_CHECKSUM_VERSION    1

/* ----------------------------------------------------------------
 *                      page support macros
 * ----------------------------------------------------------------
 */

/*
 * PageIsValid
 *      True iff page is valid.
 */
#define PageIsValid(page) PointerIsValid(page)

/*
 * line pointer(s) do not count as part of header
 */
#define SizeOfPageHeaderData (offsetof(PageHeaderData, pd_linp))

/*
 * PageIsEmpty
 *      returns true iff no itemid has been allocated on the page
 */
#define PageIsEmpty(page) \
    (((PageHeader) (page))->pd_lower <= SizeOfPageHeaderData)

/*
 * PageIsNew
 *      returns true iff page has not been initialized (by PageInit)
 */
#define PageIsNew(page) (((PageHeader) (page))->pd_upper == 0)

/*
 * PageGetItemId
 *      Returns an item identifier of a page.
 */
#define PageGetItemId(page, offsetNumber) \
    ((ItemId) (&((PageHeader) (page))->pd_linp[(offsetNumber) - 1]))

/*
 * PageGetContents
 *      To be used in cases where the page does not contain line pointers.
 *
 * Note: prior to 8.3 this was not guaranteed to yield a MAXALIGN'd result.
 * Now it is.  Beware of old code that might think the offset to the contents
 * is just SizeOfPageHeaderData rather than MAXALIGN(SizeOfPageHeaderData).
 */
#define PageGetContents(page) \
    ((char *) (page) + MAXALIGN(SizeOfPageHeaderData))

/* ----------------
 *      macros to access page size info
 * ----------------
 */

/*
 * PageSizeIsValid
 *      True iff the page size is valid.
 */
#define PageSizeIsValid(pageSize) ((pageSize) == BLCKSZ)

/*
 * PageGetPageSize
 *      Returns the page size of a page.
 *
 * this can only be called on a formatted page (unlike
 * BufferGetPageSize, which can be called on an unformatted page).
 * however, it can be called on a page that is not stored in a buffer.
 */
#define PageGetPageSize(page) \
    ((Size) (((PageHeader) (page))->pd_pagesize_version & (uint16) 0xFF00))

/*
 * PageGetPageLayoutVersion
 *      Returns the page layout version of a page.
 */
#define PageGetPageLayoutVersion(page) \
    (((PageHeader) (page))->pd_pagesize_version & 0x00FF)

/*
 * PageSetPageSizeAndVersion
 *      Sets the page size and page layout version number of a page.
 *
 * We could support setting these two values separately, but there's
 * no real need for it at the moment.
 */
#define PageSetPageSizeAndVersion(page, size, version) \
( \
    AssertMacro(((size) & 0xFF00) == (size)), \
    AssertMacro(((version) & 0x00FF) == (version)), \
    ((PageHeader) (page))->pd_pagesize_version = (size) | (version) \
)

/* ----------------
 *      page special data macros
 * ----------------
 */
/*
 * PageGetSpecialSize
 *      Returns size of special space on a page.
 */
#define PageGetSpecialSize(page) \
    ((uint16) (PageGetPageSize(page) - ((PageHeader)(page))->pd_special))

/*
 * Using assertions, validate that the page special pointer is OK.
 *
 * This is intended to catch use of the pointer before page initialization.
 * It is implemented as a function due to the limitations of the MSVC
 * compiler, which choked on doing all these tests within another macro.  We
 * return true so that AssertMacro() can be used while still getting the
 * specifics from the macro failure within this function.
 */
static inline bool
PageValidateSpecialPointer(Page page)
{
    Assert(PageIsValid(page));
    Assert(((PageHeader) (page))->pd_special <= BLCKSZ);
    Assert(((PageHeader) (page))->pd_special >= SizeOfPageHeaderData);

    return true;
}

/*
 * PageGetSpecialPointer
 *      Returns pointer to special space on a page.
 */
#define PageGetSpecialPointer(page) \
( \
    AssertMacro(PageValidateSpecialPointer(page)), \
    (char *) ((char *) (page) + ((PageHeader) (page))->pd_special) \
)

/*
 * PageGetItem
 *      Retrieves an item on the given page.
 *
 * Note:
 *      This does not change the status of any of the resources passed.
 *      The semantics may change in the future.
 */
#define PageGetItem(page, itemId) \
( \
    AssertMacro(PageIsValid(page)), \
    AssertMacro(ItemIdHasStorage(itemId)), \
    (Item)(((char *)(page)) + ItemIdGetOffset(itemId)) \
)

/*
 * PageGetMaxOffsetNumber
 *      Returns the maximum offset number used by the given page.
 *      Since offset numbers are 1-based, this is also the number
 *      of items on the page.
 *
 *      NOTE: if the page is not initialized (pd_lower == 0), we must
 *      return zero to ensure sane behavior.  Accept double evaluation
 *      of the argument so that we can ensure this.
 */
#define PageGetMaxOffsetNumber(page) \
    (((PageHeader) (page))->pd_lower <= SizeOfPageHeaderData ? 0 : \
     ((((PageHeader) (page))->pd_lower - SizeOfPageHeaderData) \
      / sizeof(ItemIdData)))

/*
 * Additional macros for access to page headers. (Beware multiple evaluation
 * of the arguments!)
 */
#define PageGetLSN(page) \
    PageXLogRecPtrGet(((PageHeader) (page))->pd_lsn)
#define PageSetLSN(page, lsn) \
    PageXLogRecPtrSet(((PageHeader) (page))->pd_lsn, lsn)

#define PageHasFreeLinePointers(page) \
    (((PageHeader) (page))->pd_flags & PD_HAS_FREE_LINES)
#define PageSetHasFreeLinePointers(page) \
    (((PageHeader) (page))->pd_flags |= PD_HAS_FREE_LINES)
#define PageClearHasFreeLinePointers(page) \
    (((PageHeader) (page))->pd_flags &= ~PD_HAS_FREE_LINES)

#define PageIsFull(page) \
    (((PageHeader) (page))->pd_flags & PD_PAGE_FULL)
#define PageSetFull(page) \
    (((PageHeader) (page))->pd_flags |= PD_PAGE_FULL)
#define PageClearFull(page) \
    (((PageHeader) (page))->pd_flags &= ~PD_PAGE_FULL)

#define PageIsAllVisible(page) \
    (((PageHeader) (page))->pd_flags & PD_ALL_VISIBLE)
#define PageSetAllVisible(page) \
    (((PageHeader) (page))->pd_flags |= PD_ALL_VISIBLE)
#define PageClearAllVisible(page) \
    (((PageHeader) (page))->pd_flags &= ~PD_ALL_VISIBLE)

#define PageSetPrunable(page, xid) \
do { \
    Assert(TransactionIdIsNormal(xid)); \
    if (!TransactionIdIsValid(((PageHeader) (page))->pd_prune_xid) || \
        TransactionIdPrecedes(xid, ((PageHeader) (page))->pd_prune_xid)) \
        ((PageHeader) (page))->pd_prune_xid = (xid); \
} while (0)
#define PageClearPrunable(page) \
    (((PageHeader) (page))->pd_prune_xid = InvalidTransactionId)


/* ----------------------------------------------------------------
 *      extern declarations
 * ----------------------------------------------------------------
 */

/* flags for PageAddItemExtended() */
#define PAI_OVERWRITE           (1 << 0)
#define PAI_IS_HEAP             (1 << 1)

/* flags for PageIsVerifiedExtended() */
#define PIV_LOG_WARNING         (1 << 0)
#define PIV_REPORT_STAT         (1 << 1)

#define PageAddItem(page, item, size, offsetNumber, overwrite, is_heap) \
    PageAddItemExtended(page, item, size, offsetNumber, \
                        ((overwrite) ? PAI_OVERWRITE : 0) | \
                        ((is_heap) ? PAI_IS_HEAP : 0))

#define PageIsVerified(page, blkno) \
    PageIsVerifiedExtended(page, blkno, \
                           PIV_LOG_WARNING | PIV_REPORT_STAT)

/*
 * Check that BLCKSZ is a multiple of sizeof(size_t).  In
 * PageIsVerifiedExtended(), it is much faster to check if a page is
 * full of zeroes using the native word size.  Note that this assertion
 * is kept within a header to make sure that StaticAssertDecl() works
 * across various combinations of platforms and compilers.
 */
StaticAssertDecl(BLCKSZ == ((BLCKSZ / sizeof(size_t)) * sizeof(size_t)),
                 "BLCKSZ has to be a multiple of sizeof(size_t)");

extern void PageInit(Page page, Size pageSize, Size specialSize);
extern bool PageIsVerifiedExtended(Page page, BlockNumber blkno, int flags);
extern OffsetNumber PageAddItemExtended(Page page, Item item, Size size,
                                        OffsetNumber offsetNumber, int flags);
extern Page PageGetTempPage(Page page);
extern Page PageGetTempPageCopy(Page page);
extern Page PageGetTempPageCopySpecial(Page page);
extern void PageRestoreTempPage(Page tempPage, Page oldPage);
extern void PageRepairFragmentation(Page page);
extern void PageTruncateLinePointerArray(Page page);
extern Size PageGetFreeSpace(Page page);
extern Size PageGetFreeSpaceForMultipleTuples(Page page, int ntups);
extern Size PageGetExactFreeSpace(Page page);
extern Size PageGetHeapFreeSpace(Page page);
extern void PageIndexTupleDelete(Page page, OffsetNumber offset);
extern void PageIndexMultiDelete(Page page, OffsetNumber *itemnos, int nitems);
extern void PageIndexTupleDeleteNoCompact(Page page, OffsetNumber offset);
extern bool PageIndexTupleOverwrite(Page page, OffsetNumber offnum,
                                    Item newtup, Size newsize);
extern char *PageSetChecksumCopy(Page page, BlockNumber blkno);
extern void PageSetChecksumInplace(Page page, BlockNumber blkno);

#endif                          /* BUFPAGE_H */