本篇内容主要讲解“PostgreSQL中PageAddItemExtended函数的逻辑是什么”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“PostgreSQL中PageAddItemExtended函数的逻辑是什么”吧!
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testdb=# drop table if exists t_insert;
NOTICE: table "t_insert" does not exist, skipping
DROP TABLE
testdb=# create table t_insert(id int,c1 char(10),c2 char(10),c3 char(10));
CREATE TABLE
插入数据主要的实现在bufpage.c中，主要的函数是PageAddItemExtended。该函数的英文注释已经说得非常清楚，不过为了更好理解，这里添加了中文注释。
变量、宏定义和结构体等说明
1、Page
指向char的指针：
typedef char *Pointer;
typedef Pointer Page;
2、Item
指向char的指针：
typedef Pointer Item;
3、Size
typedef size_t Size;
4、OffsetNumber
unsigned short（16bits）：
typedef uint16 OffsetNumber;
5、PageHeader
PageHeaderData结构体指针
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;
#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 */
6、SizeOfPageHeaderData
长整型，定义页头大小：
#define offsetof(type, field) ((long) &((type *)0)->field)
#define SizeOfPageHeaderData (offsetof(PageHeaderData, pd_linp))
7、BLCKSZ
#define BLCKSZ 8192
8、PageGetMaxOffsetNumber
如果空闲空间的lower小于等于页头大小，值为0，否则值为lower减去页头大小（24Bytes）后除以ItemId大小（4Bytes）
#define PageGetMaxOffsetNumber(page) \
(((PageHeader) (page))->pd_lower <= SizeOfPageHeaderData ? 0 : \
((((PageHeader) (page))->pd_lower - SizeOfPageHeaderData) \
/ sizeof(ItemIdData)))
9、InvalidOffsetNumber
是否无效的偏移值
#define InvalidOffsetNumber ((OffsetNumber) 0)
10、OffsetNumberNext
输入值+1
#define OffsetNumberNext(offsetNumber) \
((OffsetNumber) (1 + (offsetNumber)))
11、ItemId
结构体ItemIdData指针
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;
typedef ItemIdData *ItemId;
#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 */
12、PageGetItemId
获取相应的ItemIdData的指针
#define PageGetItemId(page, offsetNumber) \
((ItemId) (&((PageHeader) (page))->pd_linp[(offsetNumber) - 1]))
13、PAI_OVERWRITE
位标记，实际值为1
#define PAI_OVERWRITE (1 << 0)
14、PAI_IS_HEAP
位标记，实际值为2
#define PAI_IS_HEAP (1 << 1)
15、ItemIdIsUsed
ItemId是否已被占用
#define ItemIdIsUsed(itemId) \
((itemId)->lp_flags != LP_UNUSED)
16、ItemIdHasStorage
#define ItemIdHasStorage(itemId) \
((itemId)->lp_len != 0
17、PageHasFreeLinePointers/PageClearHasFreeLinePointers
判断Page从Header到Lower之间是否有空位
#define PageHasFreeLinePointers(page) \
(((PageHeader) (page))->pd_flags & PD_HAS_FREE_LINES)
清除空位标记（标记错误时清除）
#define PageClearHasFreeLinePointers(page) \
(((PageHeader) (page))->pd_flags &= ~PD_HAS_FREE_LINES)
18、MaxHeapTuplesPerPage
每个Page可以容纳的最大Tuple数，计算公式是：
（Block大小 - 页头大小） / (对齐后行头部大小 + 行指针大小）
#define MaxHeapTuplesPerPage \
((int) ((BLCKSZ - SizeOfPageHeaderData) / \
(MAXALIGN(SizeofHeapTupleHeader) + sizeof(ItemIdData))))PageAddItemExtended函数解读
/*
PageAddItemExtended函数：
输入：
page-指向页的指针
item-指向数据的指针
size-数据大小
offsetNumber-指定数据存储的偏移量
flags-标记位（是否覆盖/是否Heap数据）
输出：
OffsetNumber-数据存储实际的偏移量
*/
OffsetNumber
PageAddItemExtended(Page page,
Item item,
Size size,
OffsetNumber offsetNumber,
int flags)
{
PageHeader phdr = (PageHeader) page;//页头指针
Size alignedSize;//对齐大小
int lower;//Free space低位
int upper;//Free space高位
ItemId itemId;//行指针
OffsetNumber limit;//行偏移，Free space中第1个可用的位置偏移
bool needshuffle = false;//是否需要移动原有数据
/*
* Be wary about corrupted page pointers
*/
if (phdr->pd_lower < SizeOfPageHeaderData ||
phdr->pd_lower > phdr->pd_upper ||
phdr->pd_upper > phdr->pd_special ||
phdr->pd_special > BLCKSZ)
ereport(PANIC,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("corrupted page pointers: lower = %u, upper = %u, special = %u",
phdr->pd_lower, phdr->pd_upper, phdr->pd_special)));
/*
* Select offsetNumber to place the new item at
*/
//获取存储数据的偏移（位于Lower和Upper之间）
limit = OffsetNumberNext(PageGetMaxOffsetNumber(page));
/* was offsetNumber passed in? */
if (OffsetNumberIsValid(offsetNumber))
{
//如果指定了数据存储的偏移量（传入的偏移量参数有效）
/* yes, check it */
if ((flags & PAI_OVERWRITE) != 0) //不覆盖原有数据
{
if (offsetNumber < limit)
{
//获取指定偏移的ItemId
itemId = PageGetItemId(phdr, offsetNumber);
//指定的数据偏移已使用或者已分配存储空间，报错
if (ItemIdIsUsed(itemId) || ItemIdHasStorage(itemId))
{
elog(WARNING, "will not overwrite a used ItemId");
return InvalidOffsetNumber;
}
}
}
else//覆盖原有数据
{
//指定的行偏移不在空闲空间中，需要移动原数据为新数据腾位置
if (offsetNumber < limit)
needshuffle = true; /* need to move existing linp's */
}
}
else//没有指定数据存储行偏移
{
/* offsetNumber was not passed in, so find a free slot */
/* if no free slot, we'll put it at limit (1st open slot) */
if (PageHasFreeLinePointers(phdr))//页头标记提示存在已回收的空间
{
/*
* Look for "recyclable" (unused) ItemId. We check for no storage
* as well, just to be paranoid --- unused items should never have
* storage.
*/
//循环找出第1个可用的空闲行偏移
for (offsetNumber = 1; offsetNumber < limit; offsetNumber++)
{
itemId = PageGetItemId(phdr, offsetNumber);
if (!ItemIdIsUsed(itemId) && !ItemIdHasStorage(itemId))
break;
}
//没有找到，说明页头标记有误，需清除标记，以免误导
if (offsetNumber >= limit)
{
/* the hint is wrong, so reset it */
PageClearHasFreeLinePointers(phdr);
}
}
else//没有已回收的空间，行指针/数据存储到Free Space中
{
/* don't bother searching if hint says there's no free slot */
offsetNumber = limit;
}
}
/* Reject placing items beyond the first unused line pointer */
if (offsetNumber > limit)
{
//如果指定的偏移大于空闲空间可用的第1个位置，报错
elog(WARNING, "specified item offset is too large");
return InvalidOffsetNumber;
}
/* Reject placing items beyond heap boundary, if heap */
if ((flags & PAI_IS_HEAP) != 0 && offsetNumber > MaxHeapTuplesPerPage)
{
//Heap数据，但偏移大于一页可存储的最大Tuple数，报错
elog(WARNING, "can't put more than MaxHeapTuplesPerPage items in a heap page");
return InvalidOffsetNumber;
}
/*
* Compute new lower and upper pointers for page, see if it'll fit.
*
* Note: do arithmetic as signed ints, to avoid mistakes if, say,
* alignedSize > pd_upper.
*/
if (offsetNumber == limit || needshuffle)
//如果数据存储在Free space中，修改lower值
lower = phdr->pd_lower + sizeof(ItemIdData);
else
lower = phdr->pd_lower;//否则，找到了已回收的空闲位置，使用原有的lower
alignedSize = MAXALIGN(size);//大小对齐
upper = (int) phdr->pd_upper - (int) alignedSize;//申请存储空间
if (lower > upper)//校验
return InvalidOffsetNumber;
/*
* OK to insert the item. First, shuffle the existing pointers if needed.
*/
//获取行指针
itemId = PageGetItemId(phdr, offsetNumber);
//
if (needshuffle)
//如果需要腾位置，把原有的行指针往后挪一"格"
memmove(itemId + 1, itemId,
(limit - offsetNumber) * sizeof(ItemIdData));
/* set the item pointer */
//设置新数据行指针
ItemIdSetNormal(itemId, upper, size);
/*
* Items normally contain no uninitialized bytes. Core bufpage consumers
* conform, but this is not a necessary coding rule; a new index AM could
* opt to depart from it. However, data type input functions and other
* C-language functions that synthesize datums should initialize all
* bytes; datumIsEqual() relies on this. Testing here, along with the
* similar check in printtup(), helps to catch such mistakes.
*
* Values of the "name" type retrieved via index-only scans may contain
* uninitialized bytes; see comment in btrescan(). Valgrind will report
* this as an error, but it is safe to ignore.
*/
VALGRIND_CHECK_MEM_IS_DEFINED(item, size);
/* copy the item's data onto the page */
//把数据放在数据区
memcpy((char *) page + upper, item, size);
/* adjust page header */
//更新页头的lower & upper
phdr->pd_lower = (LocationIndex) lower;
phdr->pd_upper = (LocationIndex) upper;
//如成功，返回实际的行偏移
return offsetNumber;
}下面使用gdb对PageAddItemExtended函数跟踪分析。
测试场景：首先插入8行数据，然后删除第2行，接着插入1行数据：
testdb=# -- 插入8行数据 testdb=# insert into t_insert values(1,'11','12','13'); insert into t_insert values(6,'11','12','13'); insert into t_insert values(7,'11','12','13'); insert into t_insert values(8,'11','12','13'); checkpoint;INSERT 0 1 testdb=# insert into t_insert values(2,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(3,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(4,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(5,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(6,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(7,'11','12','13'); INSERT 0 1 testdb=# insert into t_insert values(8,'11','12','13'); INSERT 0 1 testdb=# testdb=# checkpoint; CHECKPOINT testdb=# -- 删除第2行 testdb=# delete from t_insert where id = 2; DELETE 1 testdb=# testdb=# checkpoint; CHECKPOINT testdb=# testdb=# -- 获取pid testdb=# select pg_backend_pid(); pg_backend_pid ---------------- 1572 (1 row)
使用gdb跟踪最后插入1行数据的过程：
