PostgreSQL 原始碼解讀（1）- 插入資料#1
本文簡單介紹了PG插入資料部分的原始碼，主要是PageAddItemExtended函式的邏輯，同時結合先前介紹的頁儲存結構透過gdb進行跟蹤分析其中的資料結構。
一、測試資料表
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
二、原始碼分析
插入資料主要的實現在bufpage.c中，主要的函式是PageAddItemExtended。該函式的英文註釋已經說得非常清楚，不過為了更好理解，這裡新增了中文註釋。
變數、宏定義和結構體等說明
1、Page
指向char的指標：
typedef char *Pointer;
typedef Pointer Page;
2、Item
指向char的指標：
typedef Pointer Item;
3、Size
typedef size_t Size;
4、OffsetNumber
unsigned short（16bits）：
typedef uint16 OffsetNumber;
5、PageHeader
PageHeaderData結構體指標
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
長整型，定義頁頭大小：
#define offsetof(type, field) ((long) &((type *)0)->field)
#define SizeOfPageHeaderData (offsetof(PageHeaderData, pd_linp))
7、BLCKSZ
#define BLCKSZ 8192
8、PageGetMaxOffsetNumber
如果空閒空間的lower小於等於頁頭大小，值為0，否則值為lower減去頁頭大小（24Bytes）後除以ItemId大小（4Bytes）
#define PageGetMaxOffsetNumber(page) \
(((PageHeader) (page))->pd_lower <= SizeOfPageHeaderData ? 0 : \
((((PageHeader) (page))->pd_lower - SizeOfPageHeaderData) \
/ sizeof(ItemIdData)))
9、InvalidOffsetNumber
是否無效的偏移值
#define InvalidOffsetNumber ((OffsetNumber) 0)
10、OffsetNumberNext
輸入值+1
#define OffsetNumberNext(offsetNumber) \
((OffsetNumber) (1 + (offsetNumber)))
11、ItemId
結構體ItemIdData指標
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
獲取相應的ItemIdData的指標
#define PageGetItemId(page, offsetNumber) \
((ItemId) (&((PageHeader) (page))->pd_linp[(offsetNumber) - 1]))
13、PAI_OVERWRITE
位標記，實際值為1
#define PAI_OVERWRITE (1 << 0)
14、PAI_IS_HEAP
位標記，實際值為2
#define PAI_IS_HEAP (1 << 1)
15、ItemIdIsUsed
ItemId是否已被佔用
#define ItemIdIsUsed(itemId) \
((itemId)->lp_flags != LP_UNUSED)
16、ItemIdHasStorage
#define ItemIdHasStorage(itemId) \
((itemId)->lp_len != 0
17、PageHasFreeLinePointers/PageClearHasFreeLinePointers
判斷Page從Header到Lower之間是否有空位
#define PageHasFreeLinePointers(page) \
(((PageHeader) (page))->pd_flags & PD_HAS_FREE_LINES)
清除空位標記（標記錯誤時清除）
#define PageClearHasFreeLinePointers(page) \
(((PageHeader) (page))->pd_flags &= ~PD_HAS_FREE_LINES)
18、MaxHeapTuplesPerPage
每個Page可以容納的最大Tuple數，計算公式是：
（Block大小 - 頁頭大小） / (對齊後行頭部大小 + 行指標大小）
#define MaxHeapTuplesPerPage \
((int) ((BLCKSZ - SizeOfPageHeaderData) / \
(MAXALIGN(SizeofHeapTupleHeader) + sizeof(ItemIdData))))
PageAddItemExtended函式解讀
/*
PageAddItemExtended函式：
輸入：
page-指向頁的指標
item-指向資料的指標
size-資料大小
offsetNumber-指定資料儲存的偏移量
flags-標記位（是否覆蓋/是否Heap資料）
輸出：
OffsetNumber-資料儲存實際的偏移量
*/
OffsetNumber
PageAddItemExtended(Page page,
Item item,
Size size,
OffsetNumber offsetNumber,
int flags)
{
PageHeader phdr = (PageHeader) page;//頁頭指標
Size alignedSize;//對齊大小
int lower;//Free space低位
int upper;//Free space高位
ItemId itemId;//行指標
OffsetNumber limit;//行偏移，Free space中第1個可用的位置偏移
bool needshuffle = false;//是否需要移動原有資料
/*
* 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
*/
//獲取儲存資料的偏移（位於Lower和Upper之間）
limit = OffsetNumberNext(PageGetMaxOffsetNumber(page));
/* was offsetNumber passed in? */
if (OffsetNumberIsValid(offsetNumber))
{
//如果指定了資料儲存的偏移量（傳入的偏移量引數有效）
/* yes, check it */
if ((flags & PAI_OVERWRITE) != 0) //不覆蓋原有資料
{
if (offsetNumber < limit)
{
//獲取指定偏移的ItemId
itemId = PageGetItemId(phdr, offsetNumber);
//指定的資料偏移已使用或者已分配儲存空間，報錯
if (ItemIdIsUsed(itemId) || ItemIdHasStorage(itemId))
{
elog(WARNING, "will not overwrite a used ItemId");
return InvalidOffsetNumber;
}
}
}
else//覆蓋原有資料
{
//指定的行偏移不在空閒空間中，需要移動原資料為新資料騰位置
if (offsetNumber < limit)
needshuffle = true; /* need to move existing linp's */
}
}
else//沒有指定資料儲存行偏移
{
/* 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))//頁頭標記提示存在已回收的空間
{
/*
* Look for "recyclable" (unused) ItemId. We check for no storage
* as well, just to be paranoid --- unused items should never have
* storage.
*/
//迴圈找出第1個可用的空閒行偏移
for (offsetNumber = 1; offsetNumber < limit; offsetNumber++)
{
itemId = PageGetItemId(phdr, offsetNumber);
if (!ItemIdIsUsed(itemId) && !ItemIdHasStorage(itemId))
break;
}
//沒有找到，說明頁頭標記有誤，需清除標記，以免誤導
if (offsetNumber >= limit)
{
/* the hint is wrong, so reset it */
PageClearHasFreeLinePointers(phdr);
}
}
else//沒有已回收的空間，行指標/資料儲存到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)
{
//如果指定的偏移大於空閒空間可用的第1個位置，報錯
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資料，但偏移大於一頁可儲存的最大Tuple數，報錯
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)
//如果資料儲存在Free space中，修改lower值
lower = phdr->pd_lower + sizeof(ItemIdData);
else
lower = phdr->pd_lower;//否則，找到了已回收的空閒位置，使用原有的lower
alignedSize = MAXALIGN(size);//大小對齊
upper = (int) phdr->pd_upper - (int) alignedSize;//申請儲存空間
if (lower > upper)//校驗
return InvalidOffsetNumber;
/*
* OK to insert the item. First, shuffle the existing pointers if needed.
*/
//獲取行指標
itemId = PageGetItemId(phdr, offsetNumber);
//
if (needshuffle)
//如果需要騰位置，把原有的行指標往後挪一"格"
memmove(itemId + 1, itemId,
(limit - offsetNumber) * sizeof(ItemIdData));
/* set the item pointer */
//設定新資料行指標
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 */
//把資料放在資料區
memcpy((char *) page + upper, item, size);
/* adjust page header */
//更新頁頭的lower & upper
phdr->pd_lower = (LocationIndex) lower;
phdr->pd_upper = (LocationIndex) upper;
//如成功，返回實際的行偏移
return offsetNumber;
}
三、跟蹤分析
下面使用gdb對PageAddItemExtended函式跟蹤分析。
測試場景：首先插入8行資料，然後刪除第2行，接著插入1行資料：
testdb=# -- 插入8行資料
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=# -- 刪除第2行
testdb=# delete from t_insert where id = 2;
DELETE 1
testdb=#
testdb=# checkpoint;
CHECKPOINT
testdb=#
testdb=# -- 獲取pid
testdb=# select pg_backend_pid();
pg_backend_pid
----------------
1572
(1 row)
使用gdb跟蹤最後插入1行資料的過程：
#啟動gdb，繫結程式
[root@localhost demo]# gdb -p 1572
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-100.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<
Attaching to process 1572
...
(gdb)
#設定斷點
(gdb) b PageAddItemExtended
Breakpoint 1 at 0x845119: file bufpage.c, line 196.
(gdb)
#切換到psql，執行插入語句
testdb=# insert into t_insert values(9,'11','12','13');
（掛起）
#切換回gdb
(gdb) c
Continuing.
Breakpoint 1, PageAddItemExtended (page=0x7feaaefac300 "\001", item=0x29859f8 "2\234\030", size=61, offsetNumber=0, flags=2) at bufpage.c:196
196 PageHeader phdr = (PageHeader) page;
#斷點在函式的第一行程式碼上
#bt列印呼叫棧
(gdb) bt
#0 PageAddItemExtended (page=0x7feaaefac300 "\001", item=0x29859f8 "2\234\030", size=61, offsetNumber=0, flags=2) at bufpage.c:196
#1 0x00000000004cf4f9 in RelationPutHeapTuple (relation=0x7feac6e2ccb8, buffer=141, tuple=0x29859e0, token=false) at hio.c:53
#2 0x00000000004c34ec in heap_insert (relation=0x7feac6e2ccb8, tup=0x29859e0, cid=0, options=0, bistate=0x0) at heapam.c:2487
#3 0x00000000006c076b in ExecInsert (mtstate=0x2984c10, slot=0x2985250, planSlot=0x2985250, estate=0x29848c0, canSetTag=true) at nodeModifyTable.c:529
#4 0x00000000006c29f3 in ExecModifyTable (pstate=0x2984c10) at nodeModifyTable.c:2126
#5 0x000000000069a7d8 in ExecProcNodeFirst (node=0x2984c10) at execProcnode.c:445
#6 0x0000000000690994 in ExecProcNode (node=0x2984c10) at ../../../src/include/executor/executor.h:237
#7 0x0000000000692e5e in ExecutePlan (estate=0x29848c0, planstate=0x2984c10, use_parallel_mode=false, operation=CMD_INSERT, sendTuples=false, numberTuples=0, direction=ForwardScanDirection,
dest=0x2990dc8, execute_once=true) at execMain.c:1726
#8 0x0000000000690e58 in standard_ExecutorRun (queryDesc=0x2981020, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:363
#9 0x0000000000690cef in ExecutorRun (queryDesc=0x2981020, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:306
#10 0x0000000000851d84 in ProcessQuery (plan=0x2990c68, sourceText=0x28c5ef0 "insert into t_insert values(9,'11','12','13');", params=0x0, queryEnv=0x0, dest=0x2990dc8, completionTag=0x7ffdbc052d10 "")
at pquery.c:161
#11 0x00000000008534f4 in PortalRunMulti (portal=0x292b490, isTopLevel=true, setHoldSnapshot=false, dest=0x2990dc8, altdest=0x2990dc8, completionTag=0x7ffdbc052d10 "") at pquery.c:1286
#12 0x0000000000852b32 in PortalRun (portal=0x292b490, count=9223372036854775807, isTopLevel=true, run_once=true, dest=0x2990dc8, altdest=0x2990dc8, completionTag=0x7ffdbc052d10 "") at pquery.c:799
#13 0x000000000084cebc in exec_simple_query (query_string=0x28c5ef0 "insert into t_insert values(9,'11','12','13');") at postgres.c:1122
#14 0x0000000000850f3c in PostgresMain (argc=1, argv=0x28efaa8, dbname=0x28ef990 "testdb", username=0x28ef978 "xdb") at postgres.c:4153
#15 0x00000000007c0168 in BackendRun (port=0x28e7970) at postmaster.c:4361
#16 0x00000000007bf8fc in BackendStartup (port=0x28e7970) at postmaster.c:4033
#17 0x00000000007bc139 in ServerLoop () at postmaster.c:1706
#18 0x00000000007bb9f9 in PostmasterMain (argc=1, argv=0x28c0b60) at postmaster.c:1379
#19 0x00000000006f19e8 in main (argc=1, argv=0x28c0b60) at main.c:228
#使用next命令，單步除錯
(gdb) next
207 if (phdr->pd_lower < SizeOfPageHeaderData ||
(gdb) next
208 phdr->pd_lower > phdr->pd_upper ||
(gdb) next
207 if (phdr->pd_lower < SizeOfPageHeaderData ||
(gdb) next
209 phdr->pd_upper > phdr->pd_special ||
(gdb) next
208 phdr->pd_lower > phdr->pd_upper ||
(gdb) next
210 phdr->pd_special > BLCKSZ)
(gdb) next
209 phdr->pd_upper > phdr->pd_special ||
(gdb) next
219 limit = OffsetNumberNext(PageGetMaxOffsetNumber(page));
(gdb) next
222 if (OffsetNumberIsValid(offsetNumber))
(gdb) p offsetNumber
$1 = 0
#offsetNumber為0，沒有指定插入的行偏移
(gdb) next
247 if (PageHasFreeLinePointers(phdr))
#檢視頁頭資訊
(gdb) p *phdr
$2 = {pd_lsn = {xlogid = 1, xrecoff = 3677462648}, pd_checksum = 0, pd_flags = 0, pd_lower = 56, pd_upper = 7680, pd_special = 8192, pd_pagesize_version = 8196, pd_prune_xid = 1612849,
pd_linp = 0x7feaaefac318}
(gdb)
#檢視行指標資訊
(gdb) p phdr->pd_linp[0]
$3 = {lp_off = 8128, lp_flags = 1, lp_len = 61}
(gdb) p phdr->pd_linp[1]
$4 = {lp_off = 8064, lp_flags = 1, lp_len = 61}
...
(gdb) p phdr->pd_linp[8]
$11 = {lp_off = 0, lp_flags = 0, lp_len = 0}
#行指標偏移的lp_flags均為1（LP_NORMAL），表示正在使用。
...
298 alignedSize = MAXALIGN(size);
(gdb) p size
$17 = 61
(gdb) p alignedSize
$18 = 5045956
(gdb) next
300 upper = (int) phdr->pd_upper - (int) alignedSize;
(gdb) p alignedSize
$19 = 64 #原為61，對齊為64
(gdb) next
302 if (lower > upper)
(gdb) p lower
$20 = 60
(gdb) p upper
$21 = 7616
(gdb) next
308 itemId = PageGetItemId(phdr, offsetNumber);
(gdb)
310 if (needshuffle)
(gdb) next
315 ItemIdSetNormal(itemId, upper, size);
(gdb) next
332 memcpy((char *) page + upper, item, size);
(gdb) p *itemId
$23 = {lp_off = 7616, lp_flags = 1, lp_len = 61}
...
(gdb) next
338 return offsetNumber;
(gdb)
#資料插入在行偏移為9（行指標下標=8）的位置上。
(gdb) p offsetNumber
$24 = 9
(gdb) p phdr->pd_linp[8]
$25 = {lp_off = 7616, lp_flags = 1, lp_len = 61}
...
#說明：之所以新插入的資料沒有放在2號偏移上面，是因為被刪除的第2行沒有被回收。
#檢視Page中的資料
testdb=# select * from heap_page_items(get_raw_page('t_insert',0));
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits | t_oid | t_data
----+--------+----------+--------+---------+---------+----------+--------+-------------+------------+--------+--------+-------+---------------------------------------------------------------------------
---
1 | 8128 | 1 | 61 | 1612841 | 0 | 0 | (0,1) | 4 | 2306 | 24 | | | \x010000001731312020202020202020173132202020202020202017313320202020202020
20
2 | 8064 | 1 | 61 | 1612842 | 1612849 | 0 | (0,2) | 8196 | 258 | 24 | | | \x020000001731312020202020202020173132202020202020202017313320202020202020
20
3 | 8000 | 1 | 61 | 1612843 | 0 | 0 | (0,3) | 4 | 2306 | 24 | | | \x030000001731312020202020202020173132202020202020202017313320202020202020
20
4 | 7936 | 1 | 61 | 1612844 | 0 | 0 | (0,4) | 4 | 2306 | 24 | | | \x040000001731312020202020202020173132202020202020202017313320202020202020
20
5 | 7872 | 1 | 61 | 1612845 | 0 | 0 | (0,5) | 4 | 2306 | 24 | | | \x050000001731312020202020202020173132202020202020202017313320202020202020
20
6 | 7808 | 1 | 61 | 1612846 | 0 | 0 | (0,6) | 4 | 2306 | 24 | | | \x060000001731312020202020202020173132202020202020202017313320202020202020
20
7 | 7744 | 1 | 61 | 1612847 | 0 | 0 | (0,7) | 4 | 2306 | 24 | | | \x070000001731312020202020202020173132202020202020202017313320202020202020
20
8 | 7680 | 1 | 61 | 1612848 | 0 | 0 | (0,8) | 4 | 2306 | 24 | | | \x080000001731312020202020202020173132202020202020202017313320202020202020
20
9 | 7616 | 1 | 61 | 1612850 | 0 | 0 | (0,9) | 4 | 2050 | 24 | | | \x090000001731312020202020202020173132202020202020202017313320202020202020
20
(9 rows)
四、小結
得益於PG的開放性，可以在原始碼層次深入瞭解其實現。
知識要點：
1、基本瞭解PageAddItemExtended函式的實現邏輯；
2、基本懂得使用gdb輔助跟蹤分析實現邏輯。
下一節，按照自下往上的分析思路，將會解析RelationPutHeapTuple 函式
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/6906/viewspace-2374915/，如需轉載，請註明出處，否則將追究法律責任。
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