*
* Copyright (c) 1994, Regents of the University of California
*
- * $Id: bufpage.h,v 1.26 1999/07/16 17:07:37 momjian Exp $
+ * $Id: bufpage.h,v 1.27 2000/01/08 21:59:55 tgl Exp $
*
*-------------------------------------------------------------------------
*/
* disk page is always a slotted page of the form:
*
* +----------------+---------------------------------+
- * | PageHeaderData | linp0 linp1 linp2 ... |
+ * | PageHeaderData | linp1 linp2 linp3 ... |
* +-----------+----+---------------------------------+
* | ... linpN | |
* +-----------+--------------------------------------+
* +-------------+------------------------------------+
* | | tupleN ... |
* +-------------+------------------+-----------------+
- * | ... tuple2 tuple1 tuple0 | "special space" |
+ * | ... tuple3 tuple2 tuple1 | "special space" |
* +--------------------------------+-----------------+
* ^ pd_special
*
*
* NOTES:
*
- * linp0..N form an ItemId array. ItemPointers point into this array
- * rather than pointing directly to a tuple.
+ * linp1..N form an ItemId array. ItemPointers point into this array
+ * rather than pointing directly to a tuple. Note that OffsetNumbers
+ * conventionally start at 1, not 0.
*
- * tuple0..N are added "backwards" on the page. because a tuple's
+ * tuple1..N are added "backwards" on the page. because a tuple's
* ItemPointer points to its ItemId entry rather than its actual
* byte-offset position, tuples can be physically shuffled on a page
* whenever the need arises.
*
* AM-generic per-page information is kept in the pd_opaque field of
- * the PageHeaderData. (this is currently only the page size.)
- * AM-specific per-page data is kept in the area marked "special
+ * the PageHeaderData. (Currently, only the page size is kept here.)
+ *
+ * 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
/*
* location (byte offset) within a page.
*
- * note that this is actually limited to 2^13 because we have limited
- * ItemIdData.lp_off and ItemIdData.lp_len to 13 bits (see itemid.h).
- *
- * uint16 is still valid, but the limit has been raised to 15 bits.
- * 06 Jan 98 - darrenk
+ * 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;
* space management information generic to any page
*
* od_pagesize - size in bytes.
- * in reality, we need at least 64B to fit the
+ * Minimum possible page size is perhaps 64B to fit
* page header, opaque space and a minimal tuple;
- * on the high end, we can only support pages up
- * to 8KB because lp_off/lp_len are 13 bits.
- *
- * see above comment. Now use 15 bits and pages
- * up to 32KB at your own risk.
+ * of course, in reality you want it much bigger.
+ * On the high end, we can only support pages up
+ * to 32KB because lp_off/lp_len are 15 bits.
*/
typedef struct OpaqueData
{
LocationIndex pd_upper; /* offset to end of free space */
LocationIndex pd_special; /* offset to start of special space */
OpaqueData pd_opaque; /* AM-generic information */
- ItemIdData pd_linp[1]; /* line pointers */
+ ItemIdData pd_linp[1]; /* beginning of line pointer array */
} PageHeaderData;
typedef PageHeaderData *PageHeader;
* returns true iff no itemid has been allocated on the page
*/
#define PageIsEmpty(page) \
- (((PageHeader) (page))->pd_lower == \
- (sizeof(PageHeaderData) - sizeof(ItemIdData)) ? true : false)
+ (((PageHeader) (page))->pd_lower <= \
+ (sizeof(PageHeaderData) - sizeof(ItemIdData)))
/*
* PageIsNew
* Returns an item identifier of a page.
*/
#define PageGetItemId(page, offsetNumber) \
- ((ItemId) (&((PageHeader) (page))->pd_linp[(-1) + (offsetNumber)]))
+ ((ItemId) (&((PageHeader) (page))->pd_linp[(offsetNumber) - 1]))
/* ----------------
* macros to access opaque space
* Sets the page size of a page.
*/
#define PageSetPageSize(page, size) \
- ((PageHeader) (page))->pd_opaque.od_pagesize = (size)
+ (((PageHeader) (page))->pd_opaque.od_pagesize = (size))
/* ----------------
* page special data macros
* Retrieves an item on the given page.
*
* Note:
- * This does change the status of any of the resources passed.
+ * This does not change the status of any of the resources passed.
* The semantics may change in the future.
*/
#define PageGetItem(page, itemId) \
* The buffer can be a raw disk block and need not contain a valid
* (formatted) disk page.
*/
-/* XXX dig out of buffer descriptor */
+/* XXX should dig out of buffer descriptor */
#define BufferGetPageSize(buffer) \
( \
AssertMacro(BufferIsValid(buffer)), \
/*
* 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: The offset is invalid if the page is non-empty.
- * Test whether PageIsEmpty before calling this routine
- * and/or using its return value.
+ * NOTE: to ensure sane behavior if the page is not initialized
+ * (pd_lower == 0), cast the unsigned values to int before dividing.
+ * That way we get -1 or so, not a huge positive number...
*/
#define PageGetMaxOffsetNumber(page) \
-( \
- (((PageHeader) (page))->pd_lower - \
- (sizeof(PageHeaderData) - sizeof(ItemIdData))) \
- / sizeof(ItemIdData) \
-)
+ (((int) (((PageHeader) (page))->pd_lower - \
+ (sizeof(PageHeaderData) - sizeof(ItemIdData)))) \
+ / ((int) sizeof(ItemIdData)))
/* ----------------------------------------------------------------
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