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PostgreSQL源码解读（4）-插入数据#3（heap_insert）
本文简单介绍了PG插入数据部分的源码，这是第三部分，主要内容包括heap_insert函数的实现逻辑，该函数在源文件heapam.c中。
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一、基础信息
heap_insert使用的数据结构、宏定义以及依赖的函数等。
数据结构/宏定义
1、CommandId
32bit无符号整型
typedef uint32 CommandId;
2、options
整型，标记bits
/* "options" flag bits for heap_insert */
#define HEAP_INSERT_SKIP_WAL 0x0001
#define HEAP_INSERT_SKIP_FSM 0x0002
#define HEAP_INSERT_FROZEN 0x0004
#define HEAP_INSERT_SPECULATIVE 0x0008
3、BulkInsertState
批量插入状态指针
/*
* state for bulk inserts --- private to heapam.c and hio.c
*
* If current_buf isn't InvalidBuffer, then we are holding an extra pin
* on that buffer.
*
* "typedef struct BulkInsertStateData *BulkInsertState" is in heapam.h
*/
typedef struct BulkInsertStateData
{
BufferAccessStrategy strategy; /* our BULKWRITE strategy object */
Buffer current_buf; /* current insertion target page */
} BulkInsertStateData;
typedef struct BulkInsertStateData *BulkInsertState;
4、TransactionId
32bit无符号整型
typedef uint32 TransactionId;
typedef uint32 LocalTransactionId;
typedef uint32 SubTransactionId;
5、xl_heap_insert
typedef struct xl_heap_insert
{
OffsetNumber offnum; /* inserted tuple's offset */
uint8 flags;
/* xl_heap_header & TUPLE DATA in backup block 0 */
} xl_heap_insert;
#define SizeOfHeapInsert (offsetof(xl_heap_insert, flags) + sizeof(uint8))
6、xl_heap_header
typedef struct xl_heap_header
{
uint16 t_infomask2;
uint16 t_infomask;
uint8 t_hoff;
} xl_heap_header;
#define SizeOfHeapHeader (offsetof(xl_heap_header, t_hoff) + sizeof(uint8))
7、XLogRecPtr
64bit无符号长整型
typedef uint64 XLogRecPtr;
依赖的函数
1、heap_prepare_insert
/*
* Subroutine for heap_insert(). Prepares a tuple for insertion. This sets the
* tuple header fields, assigns an OID, and toasts the tuple if necessary.
* Returns a toasted version of the tuple if it was toasted, or the original
* tuple if not. Note that in any case, the header fields are also set in
* the original tuple.
*/
static HeapTuple
heap_prepare_insert(Relation relation, HeapTuple tup, TransactionId xid,
CommandId cid, int options)
{
/*
* Parallel operations are required to be strictly read-only in a parallel
* worker. Parallel inserts are not safe even in the leader in the
* general case, because group locking means that heavyweight locks for
* relation extension or GIN page locks will not conflict between members
* of a lock group, but we don't prohibit that case here because there are
* useful special cases that we can safely allow, such as CREATE TABLE AS.
*/
//暂不支持并行操作
if (IsParallelWorker())
ereport(ERROR,
(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
errmsg("cannot insert tuples in a parallel worker")));
//设置Oid
if (relation->rd_rel->relhasoids)
{
#ifdef NOT_USED
/* this is redundant with an Assert in HeapTupleSetOid */
Assert(tup->t_data->t_infomask & HEAP_HASOID);
#endif
/*
* If the object id of this tuple has already been assigned, trust the
* caller. There are a couple of ways this can happen. At initial db
* creation, the backend program sets oids for tuples. When we define
* an index, we set the oid. Finally, in the future, we may allow
* users to set their own object ids in order to support a persistent
* object store (objects need to contain pointers to one another).
*/
if (!OidIsValid(HeapTupleGetOid(tup)))
HeapTupleSetOid(tup, GetNewOid(relation));
}
else
{
/* check there is not space for an OID */
Assert(!(tup->t_data->t_infomask & HEAP_HASOID));
}
//设置标记位t_infomask/t_infomask2
tup->t_data->t_infomask &= ~(HEAP_XACT_MASK);//HEAP_XACT_MASK=0xFFF0，取反
tup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);//HEAP2_XACT_MASK=0xE000，取反
tup->t_data->t_infomask |= HEAP_XMAX_INVALID;//插入数据，XMAX设置为invalid
HeapTupleHeaderSetXmin(tup->t_data, xid);//设置xmin为当前事务id
if (options & HEAP_INSERT_FROZEN)//冻结型插入（在事务id回卷时发生）
HeapTupleHeaderSetXminFrozen(tup->t_data);
//设置cid
HeapTupleHeaderSetCmin(tup->t_data, cid);
//设置xmax=0
HeapTupleHeaderSetXmax(tup->t_data, 0); /* for cleanliness */
//设置Oid
tup->t_tableOid = RelationGetRelid(relation);
/*
* If the new tuple is too big for storage or contains already toasted
* out-of-line attributes from some other relation, invoke the toaster.
*/
if (relation->rd_rel->relkind != RELKIND_RELATION &&
relation->rd_rel->relkind != RELKIND_MATVIEW)
{
/* toast table entries should never be recursively toasted */
Assert(!HeapTupleHasExternal(tup));
return tup;
}
else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
return toast_insert_or_update(relation, tup, NULL, options);
else
return tup;
}
2、RelationGetBufferForTuple
稍长，请耐心阅读，如能读懂，必有收获
/*
输入：
relation-数据表
len-需要的空间大小
otherBuffer-用于update场景，上一次pinned的buffer
options-处理选项
bistate-BulkInsert标记
vmbuffer-第1个vm(visibilitymap)
vmbuffer_other-用于update场景，上一次pinned的buffer对应的vm(visibilitymap)
注意:
otherBuffer这个参数让人觉得困惑，原因是PG的机制使然
Update时，不是原地更新，而是原数据保留（更新xmax），新数据插入
原数据&新数据如果在不同Block中，锁定Block的时候可能会出现Deadlock
举个例子：Session A更新表T的第一行，第一行在Block 0中，新数据存储在Block 2中
Session B更新表T的第二行，第二行在Block 0中，新数据存储在Block 2中
Block 0/2均要锁定才能完整实现Update操作：
如果Session A先锁定了Block 2，Session B先锁定了Block 0，
然后Session A尝试锁定Block 0，Session B尝试锁定Block 2，这时候就会出现死锁
为了避免这种情况，PG规定锁定时，同一个Relation，按Block的编号顺序锁定，
如需要锁定0和2，那必须先锁定Block 0，再锁定2
输出：
为Tuple分配的Buffer
附：
Pinned buffers：means buffers are currently being used,it should not be flushed out.
*/
Buffer
RelationGetBufferForTuple(Relation relation, Size len,
Buffer otherBuffer, int options,
BulkInsertState bistate,
Buffer *vmbuffer, Buffer *vmbuffer_other)
{
bool use_fsm = !(options & HEAP_INSERT_SKIP_FSM);//是否使用FSM寻找空闲空间
Buffer buffer = InvalidBuffer;//
Page page;//
Size pageFreeSpace = 0,//page空闲空间
saveFreeSpace = 0;//page需要预留的空间
BlockNumber targetBlock,//目标Block
otherBlock;//上一次pinned的buffer对应的Block
bool needLock;//是否需要上锁
len = MAXALIGN(len); /* be conservative *///大小对齐
/* Bulk insert is not supported for updates, only inserts. */
Assert(otherBuffer == InvalidBuffer || !bistate);//otherBuffer有效，说明是update操作，不支持bi(BulkInsert)
/*
* If we're gonna fail for oversize tuple, do it right away
*/
if (len > MaxHeapTupleSize)
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("row is too big: size %zu, maximum size %zu",
len, MaxHeapTupleSize)));
/* Compute desired extra freespace due to fillfactor option */
//获取预留空间
saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
HEAP_DEFAULT_FILLFACTOR);
//update操作,获取上次pinned buffer对应的Block
if (otherBuffer != InvalidBuffer)
otherBlock = BufferGetBlockNumber(otherBuffer);
else
otherBlock = InvalidBlockNumber; /* just to keep compiler quiet */
/*
* We first try to put the tuple on the same page we last inserted a tuple
* on, as cached in the BulkInsertState or relcache entry. If that
* doesn't work, we ask the Free Space Map to locate a suitable page.
* Since the FSM's info might be out of date, we have to be prepared to
* loop around and retry multiple times. (To insure this isn't an infinite
* loop, we must update the FSM with the correct amount of free space on
* each page that proves not to be suitable.) If the FSM has no record of
* a page with enough free space, we give up and extend the relation.
*
* When use_fsm is false, we either put the tuple onto the existing target
* page or extend the relation.
*/
if (len + saveFreeSpace > MaxHeapTupleSize)
{
//如果需要的大小+预留空间大于可容纳的最大Tuple大小，不使用FSM，扩展后再尝试
/* can't fit, don't bother asking FSM */
targetBlock = InvalidBlockNumber;
use_fsm = false;
}
else if (bistate && bistate->current_buf != InvalidBuffer)//BulkInsert模式
targetBlock = BufferGetBlockNumber(bistate->current_buf);
else
targetBlock = RelationGetTargetBlock(relation);//普通Insert模式
if (targetBlock == InvalidBlockNumber && use_fsm)//还没有找到合适的BlockNumber，需要使用FSM
{
/*
* We have no cached target page, so ask the FSM for an initial
* target.
*/
//使用FSM申请空闲空间=len + saveFreeSpace的块
targetBlock = GetPageWithFreeSpace(relation, len + saveFreeSpace);
/*
* If the FSM knows nothing of the rel, try the last page before we
* give up and extend. This avoids one-tuple-per-page syndrome during
* bootstrapping or in a recently-started system.
*/
//申请不到，使用最后一个块，否则扩展或者放弃
if (targetBlock == InvalidBlockNumber)
{
BlockNumber nblocks = RelationGetNumberOfBlocks(relation);
if (nblocks > 0)
targetBlock = nblocks - 1;
}
}
loop:
while (targetBlock != InvalidBlockNumber)//已成功获取插入数据的块号
{
/*
* Read and exclusive-lock the target block, as well as the other
* block if one was given, taking suitable care with lock ordering and
* the possibility they are the same block.
*
* If the page-level all-visible flag is set, caller will need to
* clear both that and the corresponding visibility map bit. However,
* by the time we return, we'll have x-locked the buffer, and we don't
* want to do any I/O while in that state. So we check the bit here
* before taking the lock, and pin the page if it appears necessary.
* Checking without the lock creates a risk of getting the wrong
* answer, so we'll have to recheck after acquiring the lock.
*/
if (otherBuffer == InvalidBuffer)//非Update操作
{
/* easy case */
buffer = ReadBufferBI(relation, targetBlock, bistate);//获取Buffer
if (PageIsAllVisible(BufferGetPage(buffer)))
//如果Page全局可见，那么把Page Pin在内存中（Pin的意思是固定/保留）
visibilitymap_pin(relation, targetBlock, vmbuffer);
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);//锁定buffer
}
else if (otherBlock == targetBlock)//Update操作，新记录跟原记录在同一个Block中
{
/* also easy case */
buffer = otherBuffer;
if (PageIsAllVisible(BufferGetPage(buffer)))
visibilitymap_pin(relation, targetBlock, vmbuffer);
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
}
else if (otherBlock < targetBlock)//Update操作，原记录所在的Block < 新记录的Block
{
/* lock other buffer first */
buffer = ReadBuffer(relation, targetBlock);
if (PageIsAllVisible(BufferGetPage(buffer)))
visibilitymap_pin(relation, targetBlock, vmbuffer);
LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);//优先锁定BlockNumber小的那个
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
}
else//Update操作，原记录所在的Block > 新记录的Block
{
/* lock target buffer first */
buffer = ReadBuffer(relation, targetBlock);
if (PageIsAllVisible(BufferGetPage(buffer)))
visibilitymap_pin(relation, targetBlock, vmbuffer);
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);//优先锁定BlockNumber小的那个
LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
}
/*
* We now have the target page (and the other buffer, if any) pinned
* and locked. However, since our initial PageIsAllVisible checks
* were performed before acquiring the lock, the results might now be
* out of date, either for the selected victim buffer, or for the
* other buffer passed by the caller. In that case, we'll need to
* give up our locks, go get the pin(s) we failed to get earlier, and
* re-lock. That's pretty painful, but hopefully shouldn't happen
* often.
*
* Note that there's a small possibility that we didn't pin the page
* above but still have the correct page pinned anyway, either because
* we've already made a previous pass through this loop, or because
* caller passed us the right page anyway.
*
* Note also that it's possible that by the time we get the pin and
* retake the buffer locks, the visibility map bit will have been
* cleared by some other backend anyway. In that case, we'll have
* done a bit of extra work for no gain, but there's no real harm
* done.
*/
if (otherBuffer == InvalidBuffer || buffer <= otherBuffer)
GetVisibilityMapPins(relation, buffer, otherBuffer,
targetBlock, otherBlock, vmbuffer,
vmbuffer_other);//Pin VM在内存中
else
GetVisibilityMapPins(relation, otherBuffer, buffer,
otherBlock, targetBlock, vmbuffer_other,
vmbuffer);//Pin VM在内存中
/*
* Now we can check to see if there's enough free space here. If so,
* we're done.
*/
page = BufferGetPage(buffer);
pageFreeSpace = PageGetHeapFreeSpace(page);
if (len + saveFreeSpace <= pageFreeSpace)//有足够的空间存储数据，返回此Buffer
{
/* use this page as future insert target, too */
RelationSetTargetBlock(relation, targetBlock);
return buffer;
}
/*
* Not enough space, so we must give up our page locks and pin (if
* any) and prepare to look elsewhere. We don't care which order we
* unlock the two buffers in, so this can be slightly simpler than the
* code above.
*/
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
if (otherBuffer == InvalidBuffer)
ReleaseBuffer(buffer);
else if (otherBlock != targetBlock)
{
LockBuffer(otherBuffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
}
/* Without FSM, always fall out of the loop and extend */
if (!use_fsm)//不使用FSM定位空闲空间，跳出循环，执行扩展
break;
/*
* Update FSM as to condition of this page, and ask for another page
* to try.
*/
//使用FSM获取下一个备选的Block
//注意：如果全部扫描后发现没有满足条件的Block，targetBlock = InvalidBlockNumber，跳出循环
targetBlock = RecordAndGetPageWithFreeSpace(relation,
targetBlock,
pageFreeSpace,
len + saveFreeSpace);
}
//没有获取满足条件的Block，扩展表
/*
* Have to extend the relation.
*
* We have to use a lock to ensure no one else is extending the rel at the
* same time, else we will both try to initialize the same new page. We
* can skip locking for new or temp relations, however, since no one else
* could be accessing them.
*/
needLock = !RELATION_IS_LOCAL(relation);//新创建的数据表或者临时表，无需Lock
/*
* If we need the lock but are not able to acquire it immediately, we'll
* consider extending the relation by multiple blocks at a time to manage
* contention on the relation extension lock. However, this only makes
* sense if we're using the FSM; otherwise, there's no point.
*/
if (needLock)//需要锁定
{
if (!use_fsm)
LockRelationForExtension(relation, ExclusiveLock);
else if (!ConditionalLockRelationForExtension(relation, ExclusiveLock))
{
/* Couldn't get the lock immediately; wait for it. */
LockRelationForExtension(relation, ExclusiveLock);
/*
* Check if some other backend has extended a block for us while
* we were waiting on the lock.
*/
//如有其它进程扩展了数据表，那么可以成功获取满足条件的targetBlock
targetBlock = GetPageWithFreeSpace(relation, len + saveFreeSpace);
/*
* If some other waiter has already extended the relation, we
* don't need to do so; just use the existing freespace.
*/
if (targetBlock != InvalidBlockNumber)
{
UnlockRelationForExtension(relation, ExclusiveLock);
goto loop;
}
/* Time to bulk-extend. */
//其它进程没有扩展
//Just extend it!
RelationAddExtraBlocks(relation, bistate);
}
}
/*
* In addition to whatever extension we performed above, we always add at
* least one block to satisfy our own request.
*
* XXX This does an lseek - rather expensive - but at the moment it is the
* only way to accurately determine how many blocks are in a relation. Is
* it worth keeping an accurate file length in shared memory someplace,
* rather than relying on the kernel to do it for us?
*/
//扩展表后，New Page！
buffer = ReadBufferBI(relation, P_NEW, bistate);
/*
* We can be certain that locking the otherBuffer first is OK, since it
* must have a lower page number.
*/
if (otherBuffer != InvalidBuffer)
LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);//otherBuffer的顺序一定在扩展的Block之后，Lock it！
/*
* Now acquire lock on the new page.
*/
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);//锁定New Page
/*
* Release the file-extension lock; it's now OK for someone else to extend
* the relation some more. Note that we cannot release this lock before
* we have buffer lock on the new page, or we risk a race condition
* against vacuumlazy.c --- see comments therein.
*/
if (needLock)
UnlockRelationForExtension(relation, ExclusiveLock);//释放扩展锁
/*
* We need to initialize the empty new page. Double-check that it really
* is empty (this should never happen, but if it does we don't want to
* risk wiping out valid data).
*/
page = BufferGetPage(buffer);//获取相应的Page
if (!PageIsNew(page))//不是New Page，那一定某个地方搞错了！
elog(ERROR, "page %u of relation \"%s\" should be empty but is not",
BufferGetBlockNumber(buffer),
RelationGetRelationName(relation));
//初始化New Page
PageInit(page, BufferGetPageSize(buffer), 0);
//New Page也满足不了要求的大小，报错
if (len > PageGetHeapFreeSpace(page))
{
/* We should not get here given the test at the top */
elog(PANIC, "tuple is too big: size %zu", len);
}
/*
* Remember the new page as our target for future insertions.
*
* XXX should we enter the new page into the free space map immediately,
* or just keep it for this backend's exclusive use in the short run
* (until VACUUM sees it)? Seems to depend on whether you expect the
* current backend to make more insertions or not, which is probably a
* good bet most of the time. So for now, don't add it to FSM yet.
*/
//终于找到了可用于存储数据的Block
RelationSetTargetBlock(relation, BufferGetBlockNumber(buffer));
//返回
return buffer;
}
//-------------------------------------------------------------------------------
/*
* Read in a buffer, using bulk-insert strategy if bistate isn't NULL.
*/
static Buffer
ReadBufferBI(Relation relation, BlockNumber targetBlock,
BulkInsertState bistate)
{
Buffer buffer;
/* If not bulk-insert, exactly like ReadBuffer */
if (!bistate)
return ReadBuffer(relation, targetBlock);//非BulkInsert模式，使用常规方法获取
//TODO 以下为BI模式
/* If we have the desired block already pinned, re-pin and return it */
if (bistate->current_buf != InvalidBuffer)
{
if (BufferGetBlockNumber(bistate->current_buf) == targetBlock)
{
IncrBufferRefCount(bistate->current_buf);
return bistate->current_buf;
}
/* ... else drop the old buffer */
ReleaseBuffer(bistate->current_buf);
bistate->current_buf = InvalidBuffer;
}
/* Perform a read using the buffer strategy */
buffer = ReadBufferExtended(relation, MAIN_FORKNUM, targetBlock,
RBM_NORMAL, bistate->strategy);
/* Save the selected block as target for future inserts */
IncrBufferRefCount(buffer);
bistate->current_buf = buffer;
return buffer;
}
/*
* ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
* fork with RBM_NORMAL mode and default strategy.
*/
Buffer
ReadBuffer(Relation reln, BlockNumber blockNum)
{
return ReadBufferExtended(reln, MAIN_FORKNUM, blockNum, RBM_NORMAL, NULL);
}
typedef enum ForkNumber
{
InvalidForkNumber = -1,
MAIN_FORKNUM = 0,
FSM_FORKNUM,
VISIBILITYMAP_FORKNUM,
INIT_FORKNUM
/*
* NOTE: if you add a new fork, change MAX_FORKNUM and possibly
* FORKNAMECHARS below, and update the forkNames array in
* src/common/relpath.c
*/
} ForkNumber;
//参考url : https://www.postgresql.org/docs/11/static/storage-file-layout.html
/*
* ReadBufferExtended -- returns a buffer containing the requested
* block of the requested relation. If the blknum
* requested is P_NEW, extend the relation file and
* allocate a new block. (Caller is responsible for
* ensuring that only one backend tries to extend a
* relation at the same time!)
*
* Returns: the buffer number for the buffer containing
* the block read. The returned buffer has been pinned.
* Does not return on error --- elog's instead.
*
* Assume when this function is called, that reln has been opened already.
*
* In RBM_NORMAL mode, the page is read from disk, and the page header is
* validated. An error is thrown if the page header is not valid. (But
* note that an all-zero page is considered "valid"; see PageIsVerified().)
*
* RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not
* valid, the page is zeroed instead of throwing an error. This is intended
* for non-critical data, where the caller is prepared to repair errors.
*
* In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
* filled with zeros instead of reading it from disk. Useful when the caller
* is going to fill the page from scratch, since this saves I/O and avoids
* unnecessary failure if the page-on-disk has corrupt page headers.
* The page is returned locked to ensure that the caller has a chance to
* initialize the page before it's made visible to others.
* Caution: do not use this mode to read a page that is beyond the relation's
* current physical EOF; that is likely to cause problems in md.c when
* the page is modified and written out. P_NEW is OK, though.
*
* RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
* a cleanup-strength lock on the page.
*
* RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
*
* If strategy is not NULL, a nondefault buffer access strategy is used.
* See buffer/README for details.
*/
Buffer
ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
ReadBufferMode mode, BufferAccessStrategy strategy)
{
bool hit;
Buffer buf;
/* Open it at the smgr level if not already done */
RelationOpenSmgr(reln);//Smgr=Storage Manager，数据表存储管理封装
/*
* Reject attempts to read non-local temporary relations; we would be
* likely to get wrong data since we have no visibility into the owning
* session's local buffers.
*/
if (RELATION_IS_OTHER_TEMP(reln))
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot access temporary tables of other sessions")));
/*
* Read the buffer, and update pgstat counters to reflect a cache hit or
* miss.
*/
pgstat_count_buffer_read(reln);//统计信息
//TODO Buffer管理后续再行解读
buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
forkNum, blockNum, mode, strategy, &hit);
if (hit)
pgstat_count_buffer_hit(reln);//统计信息
return buf;
}
3、CheckForSerializableConflictIn
检查序列化操作是否会出现冲突。比如并发执行delete & update操作的时候。
/*
* CheckForSerializableConflictIn
* We are writing the given tuple. If that indicates a rw-conflict
* in from another serializable transaction, take appropriate action.
*
* Skip checking for any granularity for which a parameter is missing.
*
* A tuple update or delete is in conflict if we have a predicate lock
* against the relation or page in which the tuple exists, or against the
* tuple itself.
*/
void
CheckForSerializableConflictIn(Relation relation, HeapTuple tuple,
Buffer buffer)
{
PREDICATELOCKTARGETTAG targettag;
if (!SerializationNeededForWrite(relation))
return;
/* Check if someone else has already decided that we need to die */
if (SxactIsDoomed(MySerializableXact))
ereport(ERROR,
(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
errmsg("could not serialize access due to read/write dependencies among transactions"),
errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict in checking."),
errhint("The transaction might succeed if retried.")));
/*
* We're doing a write which might cause rw-conflicts now or later.
* Memorize that fact.
*/
MyXactDidWrite = true;
/*
* It is important that we check for locks from the finest granularity to
* the coarsest granularity, so that granularity promotion doesn't cause
* us to miss a lock. The new (coarser) lock will be acquired before the
* old (finer) locks are released.
*
* It is not possible to take and hold a lock across the checks for all
* granularities because each target could be in a separate partition.
*/
if (tuple != NULL)
{
SET_PREDICATELOCKTARGETTAG_TUPLE(targettag,
relation->rd_node.dbNode,
relation->rd_id,
ItemPointerGetBlockNumber(&(tuple->t_self)),
ItemPointerGetOffsetNumber(&(tuple->t_self)));
CheckTargetForConflictsIn(&targettag);
}
if (BufferIsValid(buffer))
{
SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
relation->rd_node.dbNode,
relation->rd_id,
BufferGetBlockNumber(buffer));
CheckTargetForConflictsIn(&targettag);
}
SET_PREDICATELOCKTARGETTAG_RELATION(targettag,
relation->rd_node.dbNode,
relation->rd_id);
CheckTargetForConflictsIn(&targettag);
}
4、START_CRIT_SECTION
extern PGDLLIMPORT volatile uint32 CritSectionCount;
#define START_CRIT_SECTION() (CritSectionCount++)
5、PageIsAllVisible
通过位操作判断Page是否All Visible
#define PageIsAllVisible(page) \
(((PageHeader) (page))->pd_flags & PD_ALL_VISIBLE)
6、PageClearAllVisible
通过位操作清除All Visible标记
#define PageClearAllVisible(page) \
(((PageHeader) (page))->pd_flags &= ~PD_ALL_VISIBLE)
7、visibilitymap_clear
//TODO 缓冲区管理相关的设置，待进一步理解
/*
* visibilitymap_clear - clear specified bits for one page in visibility map
*
* You must pass a buffer containing the correct map page to this function.
* Call visibilitymap_pin first to pin the right one. This function doesn't do
* any I/O. Returns true if any bits have been cleared and false otherwise.
*/
bool
visibilitymap_clear(Relation rel, BlockNumber heapBlk, Buffer buf, uint8 flags)
{
BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
int mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
int mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
uint8 mask = flags << mapOffset;
char *map;
bool cleared = false;
Assert(flags & VISIBILITYMAP_VALID_BITS);
#ifdef TRACE_VISIBILITYMAP
elog(DEBUG1, "vm_clear %s %d", RelationGetRelationName(rel), heapBlk);
#endif
if (!BufferIsValid(buf) || BufferGetBlockNumber(buf) != mapBlock)
elog(ERROR, "wrong buffer passed to visibilitymap_clear");
LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
map = PageGetContents(BufferGetPage(buf));
if (map[mapByte] & mask)
{
map[mapByte] &= ~mask;
MarkBufferDirty(buf);
cleared = true;
}
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
return cleared;
}
8、MarkBufferDirty
//设置缓冲块为Dirty（待Flush到数据文件）
//TODO 缓冲区相关管理
/*
* MarkBufferDirty
*
* Marks buffer contents as dirty (actual write happens later).
*
* Buffer must be pinned and exclusive-locked. (If caller does not hold
* exclusive lock, then somebody could be in process of writing the buffer,
* leading to risk of bad data written to disk.)
*/
void
MarkBufferDirty(Buffer buffer)
{
BufferDesc *bufHdr;
uint32 buf_state;
uint32 old_buf_state;
if (!BufferIsValid(buffer))
elog(ERROR, "bad buffer ID: %d", buffer);
if (BufferIsLocal(buffer))
{
MarkLocalBufferDirty(buffer);
return;
}
bufHdr = GetBufferDescriptor(buffer - 1);
Assert(BufferIsPinned(buffer));
Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
LW_EXCLUSIVE));
old_buf_state = pg_atomic_read_u32(&bufHdr->state);
for (;;)
{
if (old_buf_state & BM_LOCKED)
old_buf_state = WaitBufHdrUnlocked(bufHdr);
buf_state = old_buf_state;
Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
if (pg_atomic_compare_exchange_u32(&bufHdr->state, &old_buf_state,
buf_state))
break;
}
/*
* If the buffer was not dirty already, do vacuum accounting.
*/
if (!(old_buf_state & BM_DIRTY))
{
VacuumPageDirty++;
pgBufferUsage.shared_blks_dirtied++;
if (VacuumCostActive)
VacuumCostBalance += VacuumCostPageDirty;
}
}
9、RelationNeedsWAL
非临时表，需持久化的数据表
/*
* RelationNeedsWAL
* True if relation needs WAL.
*/
#define RelationNeedsWAL(relation) \
((relation)->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT)
10、RelationIsAccessibleInLogicalDecoding
/*
* RelationIsAccessibleInLogicalDecoding
* True if we need to log enough information to have access via
* decoding snapshot.
*/
#define RelationIsAccessibleInLogicalDecoding(relation) \
(XLogLogicalInfoActive() && \ //处于逻辑复制活动状态
RelationNeedsWAL(relation) && \ //需要写WAL日志
(IsCatalogRelation(relation) || RelationIsUsedAsCatalogTable(relation)))//Catalog类型表
11、log_heap_new_cid
/*
* Perform XLogInsert of an XLOG_HEAP2_NEW_CID record
*
* This is only used in wal_level >= WAL_LEVEL_LOGICAL, and only for catalog
* tuples.
*/
static XLogRecPtr
log_heap_new_cid(Relation relation, HeapTuple tup)
{
xl_heap_new_cid xlrec;
XLogRecPtr recptr;
HeapTupleHeader hdr = tup->t_data;
Assert(ItemPointerIsValid(&tup->t_self));
Assert(tup->t_tableOid != InvalidOid);
xlrec.top_xid = GetTopTransactionId();
xlrec.target_node = relation->rd_node;
xlrec.target_tid = tup->t_self;
/*
* If the tuple got inserted & deleted in the same TX we definitely have a
* combocid, set cmin and cmax.
*/
if (hdr->t_infomask & HEAP_COMBOCID)
{
Assert(!(hdr->t_infomask & HEAP_XMAX_INVALID));
Assert(!HeapTupleHeaderXminInvalid(hdr));
xlrec.cmin = HeapTupleHeaderGetCmin(hdr);
xlrec.cmax = HeapTupleHeaderGetCmax(hdr);
xlrec.combocid = HeapTupleHeaderGetRawCommandId(hdr);
}
/* No combocid, so only cmin or cmax can be set by this TX */
else
{
/*
* Tuple inserted.
*
* We need to check for LOCK ONLY because multixacts might be
* transferred to the new tuple in case of FOR KEY SHARE updates in
* which case there will be an xmax, although the tuple just got
* inserted.
*/
if (hdr->t_infomask & HEAP_XMAX_INVALID ||
HEAP_XMAX_IS_LOCKED_ONLY(hdr->t_infomask))
{
xlrec.cmin = HeapTupleHeaderGetRawCommandId(hdr);
xlrec.cmax = InvalidCommandId;
}
/* Tuple from a different tx updated or deleted. */
else
{
xlrec.cmin = InvalidCommandId;
xlrec.cmax = HeapTupleHeaderGetRawCommandId(hdr);
}
xlrec.combocid = InvalidCommandId;
}
/*
* Note that we don't need to register the buffer here, because this
* operation does not modify the page. The insert/update/delete that
* called us certainly did, but that's WAL-logged separately.
*/
XLogBeginInsert();
XLogRegisterData((char *) &xlrec, SizeOfHeapNewCid);
/* will be looked at irrespective of origin */
recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_NEW_CID);
return recptr;
}
12、RelationIsLogicallyLogged
判断数据表是否正在可用于逻辑复制，如需要，则需要记录足够信息用于后续的日志解析
/*
* RelationIsLogicallyLogged
* True if we need to log enough information to extract the data from the
* WAL stream.
*
* We don't log information for unlogged tables (since they don't WAL log
* anyway) and for system tables (their content is hard to make sense of, and
* it would complicate decoding slightly for little gain). Note that we *do*
* log information for user defined catalog tables since they presumably are
* interesting to the user...
*/
#define RelationIsLogicallyLogged(relation) \
(XLogLogicalInfoActive() && \
RelationNeedsWAL(relation) && \
!IsCatalogRelation(relation))
13、XLog*
XLogBeginInsert
XLogRegisterData
XLogRegisterBuffer
XLogRegisterBufData
XLogSetRecordFlags
XLogInsert
14、PageSetLSN
设置PageHeader的LSN（先前已解析）
#define PageSetLSN(page, lsn) \
PageXLogRecPtrSet(((PageHeader) (page))->pd_lsn, lsn)
15、END_CRIT_SECTION
#define END_CRIT_SECTION() \
do { \
Assert(CritSectionCount > 0); \
CritSectionCount--; \
} while(0)
16、UnlockReleaseBuffer
释放Buffer锁
/*
* UnlockReleaseBuffer -- release the content lock and pin on a buffer
*
* This is just a shorthand for a common combination.
*/
void
UnlockReleaseBuffer(Buffer buffer)
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
}
17、ReleaseBuffer
Unpin Buffer，意味着Buffer可Flush用于其他地方
/*
* ReleaseBuffer -- release the pin on a buffer
*/
void
ReleaseBuffer(Buffer buffer)
{
if (!BufferIsValid(buffer))
elog(ERROR, "bad buffer ID: %d", buffer);
if (BufferIsLocal(buffer))
{
ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
Assert(LocalRefCount[-buffer - 1] > 0);
LocalRefCount[-buffer - 1]--;
return;
}
UnpinBuffer(GetBufferDescriptor(buffer - 1), true);
}
18、CacheInvalidateHeapTuple
缓存那些已“无用”的Tuple，比如Update操作的原记录，Delete操作的原记录等。
/*
* CacheInvalidateHeapTuple
* Register the given tuple for invalidation at end of command
* (ie, current command is creating or outdating this tuple).
* Also, detect whether a relcache invalidation is implied.
*
* For an insert or delete, tuple is the target tuple and newtuple is NULL.
* For an update, we are called just once, with tuple being the old tuple
* version and newtuple the new version. This allows avoidance of duplicate
* effort during an update.
*/
void
CacheInvalidateHeapTuple(Relation relation,
HeapTuple tuple,
HeapTuple newtuple)
{
Oid tupleRelId;
Oid databaseId;
Oid relationId;
/* Do nothing during bootstrap */
if (IsBootstrapProcessingMode())
return;
/*
* We only need to worry about invalidation for tuples that are in system
* catalogs; user-relation tuples are never in catcaches and can't affect
* the relcache either.
*/
if (!IsCatalogRelation(relation))
return;
/*
* IsCatalogRelation() will return true for TOAST tables of system
* catalogs, but we don't care about those, either.
*/
if (IsToastRelation(relation))
return;
/*
* If we're not prepared to queue invalidation messages for this
* subtransaction level, get ready now.
*/
PrepareInvalidationState();
/*
* First let the catcache do its thing
*/
tupleRelId = RelationGetRelid(relation);
if (RelationInvalidatesSnapshotsOnly(tupleRelId))
{
databaseId = IsSharedRelation(tupleRelId) ? InvalidOid : MyDatabaseId;
RegisterSnapshotInvalidation(databaseId, tupleRelId);
}
else
PrepareToInvalidateCacheTuple(relation, tuple, newtuple,
RegisterCatcacheInvalidation);
/*
* Now, is this tuple one of the primary definers of a relcache entry? See
* comments in file header for deeper explanation.
*
* Note we ignore newtuple here; we assume an update cannot move a tuple
* from being part of one relcache entry to being part of another.
*/
if (tupleRelId == RelationRelationId)
{
Form_pg_class classtup = (Form_pg_class) GETSTRUCT(tuple);
relationId = HeapTupleGetOid(tuple);
if (classtup->relisshared)
databaseId = InvalidOid;
else
databaseId = MyDatabaseId;
}
else if (tupleRelId == AttributeRelationId)
{
Form_pg_attribute atttup = (Form_pg_attribute) GETSTRUCT(tuple);
relationId = atttup->attrelid;
/*
* KLUGE ALERT: we always send the relcache event with MyDatabaseId,
* even if the rel in question is shared (which we can't easily tell).
* This essentially means that only backends in this same database
* will react to the relcache flush request. This is in fact
* appropriate, since only those backends could see our pg_attribute
* change anyway. It looks a bit ugly though. (In practice, shared
* relations can't have schema changes after bootstrap, so we should
* never come here for a shared rel anyway.)
*/
databaseId = MyDatabaseId;
}
else if (tupleRelId == IndexRelationId)
{
Form_pg_index indextup = (Form_pg_index) GETSTRUCT(tuple);
/*
* When a pg_index row is updated, we should send out a relcache inval
* for the index relation. As above, we don't know the shared status
* of the index, but in practice it doesn't matter since indexes of
* shared catalogs can't have such updates.
*/
relationId = indextup->indexrelid;
databaseId = MyDatabaseId;
}
else
return;
/*
* Yes. We need to register a relcache invalidation event.
*/
RegisterRelcacheInvalidation(databaseId, relationId);
}
19、heap_freetuple
释放内存
/*
* heap_freetuple
*/
void
heap_freetuple(HeapTuple htup)
{
pfree(htup);
}
二、源码解读
heap_insert函数本身不复杂，最为复杂的子函数RelationGetBufferForTuple已在上一小节解析
/*
输入：
relation-数据表结构体
tup-Heap Tuple数据（包括头部数据等），亦即数据行
cid-命令ID（顺序）
options-选项
bistate-BulkInsert状态
输出：
Oid-数据表Oid
*/
Oid
heap_insert(Relation relation, HeapTuple tup, CommandId cid,
int options, BulkInsertState bistate)
{
TransactionId xid = GetCurrentTransactionId();//事务id
HeapTuple heaptup;//Heap Tuple数据，亦即数据行
Buffer buffer;//数据缓存块
Buffer vmbuffer = InvalidBuffer;//vm缓冲块
bool all_visible_cleared = false;//标记
/*
* Fill in tuple header fields, assign an OID, and toast the tuple if
* necessary.
*
* Note: below this point, heaptup is the data we actually intend to store
* into the relation; tup is the caller's original untoasted data.
*/
//插入前准备工作，比如设置t_infomask标记等
heaptup = heap_prepare_insert(relation, tup, xid, cid, options);
/*
* Find buffer to insert this tuple into. If the page is all visible,
* this will also pin the requisite visibility map page.
*/
//获取相应的buffer，详见上面的子函数解析
buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
InvalidBuffer, options, bistate,
&vmbuffer, NULL);
/*
* We're about to do the actual insert -- but check for conflict first, to
* avoid possibly having to roll back work we've just done.
*
* This is safe without a recheck as long as there is no possibility of
* another process scanning the page between this check and the insert
* being visible to the scan (i.e., an exclusive buffer content lock is
* continuously held from this point until the tuple insert is visible).
*
* For a heap insert, we only need to check for table-level SSI locks. Our
* new tuple can't possibly conflict with existing tuple locks, and heap
* page locks are only consolidated versions of tuple locks; they do not
* lock "gaps" as index page locks do. So we don't need to specify a
* buffer when making the call, which makes for a faster check.
*/
//检查序列化是否冲突
CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);
/* NO EREPORT(ERROR) from here till changes are logged */
//开始，变量+1
START_CRIT_SECTION();
//插入数据（详见上一节对该函数的解析）
RelationPutHeapTuple(relation, buffer, heaptup,
(options & HEAP_INSERT_SPECULATIVE) != 0);
//如Page is All Visible
if (PageIsAllVisible(BufferGetPage(buffer)))
{
//复位
all_visible_cleared = true;
PageClearAllVisible(BufferGetPage(buffer));
visibilitymap_clear(relation,
ItemPointerGetBlockNumber(&(heaptup->t_self)),
vmbuffer, VISIBILITYMAP_VALID_BITS);
}
/*
* XXX Should we set PageSetPrunable on this page ?
*
* The inserting transaction may eventually abort thus making this tuple
* DEAD and hence available for pruning. Though we don't want to optimize
* for aborts, if no other tuple in this page is UPDATEd/DELETEd, the
* aborted tuple will never be pruned until next vacuum is triggered.
*
* If you do add PageSetPrunable here, add it in heap_xlog_insert too.
*/
//设置缓冲块为脏块
MarkBufferDirty(buffer);
/* XLOG stuff */
//记录日志
if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
{
xl_heap_insert xlrec;
xl_heap_header xlhdr;
XLogRecPtr recptr;
Page page = BufferGetPage(buffer);
uint8 info = XLOG_HEAP_INSERT;
int bufflags = 0;
/*
* If this is a catalog, we need to transmit combocids to properly
* decode, so log that as well.
*/
if (RelationIsAccessibleInLogicalDecoding(relation))
log_heap_new_cid(relation, heaptup);
/*
* If this is the single and first tuple on page, we can reinit the
* page instead of restoring the whole thing. Set flag, and hide
* buffer references from XLogInsert.
*/
if (ItemPointerGetOffsetNumber(&(heaptup->t_self)) == FirstOffsetNumber &&
PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
{
info |= XLOG_HEAP_INIT_PAGE;
bufflags |= REGBUF_WILL_INIT;
}
xlrec.offnum = ItemPointerGetOffsetNumber(&heaptup->t_self);
xlrec.flags = 0;
if (all_visible_cleared)
xlrec.flags |= XLH_INSERT_ALL_VISIBLE_CLEARED;
if (options & HEAP_INSERT_SPECULATIVE)
xlrec.flags |= XLH_INSERT_IS_SPECULATIVE;
Assert(ItemPointerGetBlockNumber(&heaptup->t_self) == BufferGetBlockNumber(buffer));
/*
* For logical decoding, we need the tuple even if we're doing a full
* page write, so make sure it's included even if we take a full-page
* image. (XXX We could alternatively store a pointer into the FPW).
*/
if (RelationIsLogicallyLogged(relation))
{
xlrec.flags |= XLH_INSERT_CONTAINS_NEW_TUPLE;
bufflags |= REGBUF_KEEP_DATA;
}
XLogBeginInsert();
XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);
xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
xlhdr.t_infomask = heaptup->t_data->t_infomask;
xlhdr.t_hoff = heaptup->t_data->t_hoff;
/*
* note we mark xlhdr as belonging to buffer; if XLogInsert decides to
* write the whole page to the xlog, we don't need to store
* xl_heap_header in the xlog.
*/
XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);
XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
/* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
XLogRegisterBufData(0,
(char *) heaptup->t_data + SizeofHeapTupleHeader,
heaptup->t_len - SizeofHeapTupleHeader);
/* filtering by origin on a row level is much more efficient */
XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);
recptr = XLogInsert(RM_HEAP_ID, info);
PageSetLSN(page, recptr);
}
//完成！
END_CRIT_SECTION();
//解锁Buffer，包括vm buffer
UnlockReleaseBuffer(buffer);
if (vmbuffer != InvalidBuffer)
ReleaseBuffer(vmbuffer);
/*
* If tuple is cachable, mark it for invalidation from the caches in case
* we abort. Note it is OK to do this after releasing the buffer, because
* the heaptup data structure is all in local memory, not in the shared
* buffer.
*/
//缓存操作后变“无效”的Tuple
CacheInvalidateHeapTuple(relation, heaptup, NULL);
/* Note: speculative insertions are counted too, even if aborted later */
//更新统计信息
pgstat_count_heap_insert(relation, 1);
/*
* If heaptup is a private copy, release it. Don't forget to copy t_self
* back to the caller's image, too.
*/
if (heaptup != tup)
{
tup->t_self = heaptup->t_self;
heap_freetuple(heaptup);
}
return HeapTupleGetOid(tup);
}
三、跟踪分析
插入一条记录，使用gdb进行跟踪分析：
-- 这次启动事务
testdb=# begin;
BEGIN
testdb=# select pg_backend_pid();
pg_backend_pid
----------------
1556
(1 row)
testdb=# insert into t_insert values(11,'11','11','11');
（挂起）
启动gdb：
[root@localhost ~]# gdb -p 1556
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-100.el7
Copyright (C) 2013 Free Software Foundation, Inc.
...
(gdb) b heap_insert
Breakpoint 1 at 0x4c343c: file heapam.c, line 2444.
#输入参数：
(gdb) p *relation
$1 = {rd_node = {spcNode = 1663, dbNode = 16477, relNode = 26731}, rd_smgr = 0x0, rd_refcnt = 1, rd_backend = -1, rd_islocaltemp = false, rd_isnailed = false, rd_isvalid = true,
rd_indexvalid = 0 '\000', rd_statvalid = false, rd_createSubid = 0, rd_newRelfilenodeSubid = 0, rd_rel = 0x7f5fdd1771f0, rd_att = 0x7f5fdd177300, rd_id = 26731, rd_lockInfo = {lockRelId = {
relId = 26731, dbId = 16477}}, rd_rules = 0x0, rd_rulescxt = 0x0, trigdesc = 0x0, rd_rsdesc = 0x0, rd_fkeylist = 0x0, rd_fkeyvalid = false, rd_partkeycxt = 0x0, rd_partkey = 0x0, rd_pdcxt = 0x0,
rd_partdesc = 0x0, rd_partcheck = 0x0, rd_indexlist = 0x0, rd_oidindex = 0, rd_pkindex = 0, rd_replidindex = 0, rd_statlist = 0x0, rd_indexattr = 0x0, rd_projindexattr = 0x0, rd_keyattr = 0x0,
rd_pkattr = 0x0, rd_idattr = 0x0, rd_projidx = 0x0, rd_pubactions = 0x0, rd_options = 0x0, rd_index = 0x0, rd_indextuple = 0x0, rd_amhandler = 0, rd_indexcxt = 0x0, rd_amroutine = 0x0,
rd_opfamily = 0x0, rd_opcintype = 0x0, rd_support = 0x0, rd_supportinfo = 0x0, rd_indoption = 0x0, rd_indexprs = 0x0, rd_indpred = 0x0, rd_exclops = 0x0, rd_exclprocs = 0x0, rd_exclstrats = 0x0,
rd_amcache = 0x0, rd_indcollation = 0x0, rd_fdwroutine = 0x0, rd_toastoid = 0, pgstat_info = 0x146f9b8}
(gdb) p *tup
$2 = {t_len = 61, t_self = {ip_blkid = {bi_hi = 65535, bi_lo = 65535}, ip_posid = 0}, t_tableOid = 26731, t_data = 0x14b19f8}
(gdb) p *(tup->t_data)
$3 = {t_choice = {t_heap = {t_xmin = 244, t_xmax = 4294967295, t_field3 = {t_cid = 2249, t_xvac = 2249}}, t_datum = {datum_len_ = 244, datum_typmod = -1, datum_typeid = 2249}}, t_ctid = {ip_blkid = {
bi_hi = 65535, bi_lo = 65535}, ip_posid = 0}, t_infomask2 = 4, t_infomask = 2, t_hoff = 24 '\030', t_bits = 0x14b1a0f ""}
(gdb) p *(tup->t_data->t_bits)
$4 = 0 '\000'
(gdb) p cid
$5 = 0
(gdb) p options
$6 = 0
(gdb) p bistate
$7 = (BulkInsertState) 0x0
(gdb) next
2447 Buffer vmbuffer = InvalidBuffer;
(gdb) p xid
$8 = 1612859
(gdb) next
2448 bool all_visible_cleared = false;
(gdb)
2457 heaptup = heap_prepare_insert(relation, tup, xid, cid, options);
(gdb)
2463 buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
(gdb) p *heaptup
$9 = {t_len = 61, t_self = {ip_blkid = {bi_hi = 65535, bi_lo = 65535}, ip_posid = 0}, t_tableOid = 26731, t_data = 0x14b19f8}
(gdb) next
2482 CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);
(gdb) p buffer
$10 = 185
(gdb) next
2485 START_CRIT_SECTION();
(gdb)
2488 (options & HEAP_INSERT_SPECULATIVE) != 0);
(gdb)
2487 RelationPutHeapTuple(relation, buffer, heaptup,
(gdb)
2490 if (PageIsAllVisible(BufferGetPage(buffer)))
(gdb)
2510 MarkBufferDirty(buffer);
(gdb) p buffer
$11 = 185
(gdb) next
2513 if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
(gdb)
2518 Page page = BufferGetPage(buffer);
(gdb)
2519 uint8 info = XLOG_HEAP_INSERT;
(gdb) p *page
$12 = 1 '\001'
(gdb) p *(PageHeader)page
$13 = {pd_lsn = {xlogid = 1, xrecoff = 3677481952}, pd_checksum = 0, pd_flags = 0, pd_lower = 64, pd_upper = 7552, pd_special = 8192, pd_pagesize_version = 8196, pd_prune_xid = 0,
pd_linp = 0x7f5fc5409318}
(gdb) next
2520 int bufflags = 0;
(gdb)
2526 if (RelationIsAccessibleInLogicalDecoding(relation))
(gdb)
2534 if (ItemPointerGetOffsetNumber(&(heaptup->t_self)) == FirstOffsetNumber &&
(gdb)
2541 xlrec.offnum = ItemPointerGetOffsetNumber(&heaptup->t_self);
(gdb)
2542 xlrec.flags = 0;
(gdb)
2543 if (all_visible_cleared)
(gdb)
2545 if (options & HEAP_INSERT_SPECULATIVE)
(gdb)
2554 if (RelationIsLogicallyLogged(relation))
(gdb)
2560 XLogBeginInsert();
(gdb)
2561 XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);
(gdb) p xlrec
$14 = {offnum = 10, flags = 0 '\000'}
(gdb) next
2563 xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
(gdb)
2564 xlhdr.t_infomask = heaptup->t_data->t_infomask;
(gdb)
2565 xlhdr.t_hoff = heaptup->t_data->t_hoff;
(gdb)
2572 XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);
(gdb)
2573 XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
(gdb)
2577 heaptup->t_len - SizeofHeapTupleHeader);
(gdb)
2575 XLogRegisterBufData(0,
(gdb)
2576 (char *) heaptup->t_data + SizeofHeapTupleHeader,
(gdb)
2575 XLogRegisterBufData(0,
(gdb)
2580 XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);
(gdb)
2582 recptr = XLogInsert(RM_HEAP_ID, info);
(gdb)
2584 PageSetLSN(page, recptr);
(gdb)
2587 END_CRIT_SECTION();
(gdb)
2589 UnlockReleaseBuffer(buffer);
(gdb)
2590 if (vmbuffer != InvalidBuffer)
(gdb)
2599 CacheInvalidateHeapTuple(relation, heaptup, NULL);
(gdb)
2602 pgstat_count_heap_insert(relation, 1);
(gdb)
2608 if (heaptup != tup)
(gdb)
2614 return HeapTupleGetOid(tup);
(gdb) p *tup
$15 = {t_len = 61, t_self = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 10}, t_tableOid = 26731, t_data = 0x14b19f8}
(gdb) p *(tup->t_data)
$16 = {t_choice = {t_heap = {t_xmin = 1612859, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 1612859, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {
bi_hi = 65535, bi_lo = 65535}, ip_posid = 0}, t_infomask2 = 4, t_infomask = 2050, t_hoff = 24 '\030', t_bits = 0x14b1a0f ""}
(gdb)
(gdb) n
2615 }
(gdb) n
#done!
ExecInsert (mtstate=0x14b0c10, slot=0x14b1250, planSlot=0x14b1250, estate=0x14b08c0, canSetTag=true) at nodeModifyTable.c:534
534 if (resultRelInfo->ri_NumIndices > 0)
四、小结
1、简单的反面是复杂：插入一行数据，涉及缓冲区管理（在PG中还需要考虑死锁）、日志处理等一系列的细节，原理/理论是简单的，但要在工程上实现得漂亮，不容易！程序猿们，加油吧！
2、NOSQL是“简单”的，RDBMS是“复杂”的：NoSQL不需要考虑事务，简化了日志处理，实现逻辑相对简单；RDBMS需要考虑A/B/C/D...，权衡了各种利弊，值得深入学习。
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