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一、数据结构
全局/静态变量
/*
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
* quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
* 当前已注册的快照.
* 按照xmin堆排序，这样我们可以快速找到xmin最小的一个，从而可以设置MyPgXact->xmin。
*/
static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
void *arg);
static pairingheap RegisteredSnapshots = {&xmin_cmp, NULL, NULL};
/* first GetTransactionSnapshot call in a transaction? */
bool FirstSnapshotSet = false;
/*
* Remember the serializable transaction snapshot, if any. We cannot trust
* FirstSnapshotSet in combination with IsolationUsesXactSnapshot(), because
* GUC may be reset before us, changing the value of IsolationUsesXactSnapshot.
* 如存在则记下serializable事务快照.
* 我们不能信任与IsolationUsesXactSnapshot()结合使用的FirstSnapshotSet，
* 因为GUC可能会在我们之前重置，改变IsolationUsesXactSnapshot的值。
*/
static Snapshot FirstXactSnapshot = NULL;
/*
* CurrentSnapshot points to the only snapshot taken in transaction-snapshot
* mode, and to the latest one taken in a read-committed transaction.
* SecondarySnapshot is a snapshot that's always up-to-date as of the current
* instant, even in transaction-snapshot mode. It should only be used for
* special-purpose code (say, RI checking.) CatalogSnapshot points to an
* MVCC snapshot intended to be used for catalog scans; we must invalidate it
* whenever a system catalog change occurs.
* CurrentSnapshot指向在transaction-snapshot模式下获取的唯一快照/在read-committed事务中获取的最新快照。
* SecondarySnapshot是即使在transaction-snapshot模式下，也总是最新的快照。它应该只用于特殊用途码(例如，RI检查)。
* CatalogSnapshot指向打算用于catalog扫描的MVCC快照;
* 无论何时发生system catalog更改，我们都必须马上使其失效。
*
* These SnapshotData structs are static to simplify memory allocation
* (see the hack in GetSnapshotData to avoid repeated malloc/free).
* 这些SnapshotData结构体是静态的便于简化内存分配.
* (可以回过头来看GetSnapshotData函数如何避免重复的malloc/free)
*/
static SnapshotData CurrentSnapshotData = {HeapTupleSatisfiesMVCC};
static SnapshotData SecondarySnapshotData = {HeapTupleSatisfiesMVCC};
SnapshotData CatalogSnapshotData = {HeapTupleSatisfiesMVCC};
/* Pointers to valid snapshots */
//指向有效的快照
static Snapshot CurrentSnapshot = NULL;
static Snapshot SecondarySnapshot = NULL;
static Snapshot CatalogSnapshot = NULL;
static Snapshot HistoricSnapshot = NULL;
/*
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
* 这些变量通过函数GetSnapshotData更新.
* 为了便于TransactionIdIsInProgress，以这种方式初始化它们:
* 即使在引导模式下，我们也不希望表示BootstrapTransactionId正在进行中。
*
* RecentGlobalXmin and RecentGlobalDataXmin are initialized to
* InvalidTransactionId, to ensure that no one tries to use a stale
* value. Readers should ensure that it has been set to something else
* before using it.
* RecentGlobalXmin和RecentGlobalDataXmin初始化为InvalidTransactionId,
* 以确保没有人尝试使用过时的值。
* 在使用它之前，读取进程应确保它已经被设置为其他值。
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
TransactionId RecentGlobalXmin = InvalidTransactionId;
TransactionId RecentGlobalDataXmin = InvalidTransactionId;
/* (table, ctid) => (cmin, cmax) mapping during timetravel */
static HTAB *tuplecid_data = NULL;
MyPgXact
当前的事务信息.
/*
* Flags for PGXACT->vacuumFlags
* PGXACT->vacuumFlags标记
*
* Note: If you modify these flags, you need to modify PROCARRAY_XXX flags
* in src/include/storage/procarray.h.
* 注意:如果修改了这些标记,需要更新src/include/storage/procarray.h中的PROCARRAY_XXX标记
*
* PROC_RESERVED may later be assigned for use in vacuumFlags, but its value is
* used for PROCARRAY_SLOTS_XMIN in procarray.h, so GetOldestXmin won't be able
* to match and ignore processes with this flag set.
* PROC_RESERVED可能在接下来分配给vacuumFlags使用,
* 但是它在procarray.h中用于标识PROCARRAY_SLOTS_XMIN,
* 因此GetOldestXmin不能匹配和忽略使用此标记的进程.
*/
//是否auto vacuum worker?
#define PROC_IS_AUTOVACUUM 0x01 /* is it an autovac worker? */
//正在运行lazy vacuum
#define PROC_IN_VACUUM 0x02 /* currently running lazy vacuum */
//正在运行analyze
#define PROC_IN_ANALYZE 0x04 /* currently running analyze */
//只能通过auto vacuum设置
#define PROC_VACUUM_FOR_WRAPAROUND 0x08 /* set by autovac only */
//在事务外部正在执行逻辑解码
#define PROC_IN_LOGICAL_DECODING 0x10 /* currently doing logical
* decoding outside xact */
//保留用于procarray
#define PROC_RESERVED 0x20 /* reserved for procarray */
/* flags reset at EOXact */
//在EOXact时用于重置标记的MASK
#define PROC_VACUUM_STATE_MASK /
(PROC_IN_VACUUM | PROC_IN_ANALYZE | PROC_VACUUM_FOR_WRAPAROUND)
/*
* Prior to PostgreSQL 9.2, the fields below were stored as part of the
* PGPROC. However, benchmarking revealed that packing these particular
* members into a separate array as tightly as possible sped up GetSnapshotData
* considerably on systems with many CPU cores, by reducing the number of
* cache lines needing to be fetched. Thus, think very carefully before adding
* anything else here.
*/
typedef struct PGXACT
{
//当前的顶层事务ID(非子事务)
//出于优化的目的,只读事务并不会分配事务号(xid = 0)
TransactionId xid; /* id of top-level transaction currently being
* executed by this proc, if running and XID
* is assigned; else InvalidTransactionId */
//在启动事务时,当前正在执行的最小事务号XID,但不包括LAZY VACUUM
//vacuum不能清除删除事务号xid >= xmin的元组
TransactionId xmin; /* minimal running XID as it was when we were
* starting our xact, excluding LAZY VACUUM:
* vacuum must not remove tuples deleted by
* xid >= xmin ! */
//vacuum相关的标记
uint8 vacuumFlags; /* vacuum-related flags, see above */
bool overflowed;
bool delayChkpt; /* true if this proc delays checkpoint start;
* previously called InCommit */
uint8 nxids;
} PGXACT;
extern PGDLLIMPORT struct PGXACT *MyPgXact;
Snapshot
SnapshotData结构体指针,SnapshotData结构体可表达的信息囊括了所有可能的快照.
有以下几种不同类型的快照:
1.常规的MVCC快照
2.在恢复期间的MVCC快照(处于Hot-Standby模式)
3.在逻辑解码过程中使用的历史MVCC快照
4.作为参数传递给HeapTupleSatisfiesDirty()函数的快照
5.作为参数传递给HeapTupleSatisfiesNonVacuumable()函数的快照
6.用于在没有成员访问情况下SatisfiesAny、Toast和Self的快照
//SnapshotData结构体指针
typedef struct SnapshotData *Snapshot;
//无效的快照
#define InvalidSnapshot ((Snapshot) NULL)
/*
* We use SnapshotData structures to represent both "regular" (MVCC)
* snapshots and "special" snapshots that have non-MVCC semantics.
* The specific semantics of a snapshot are encoded by the "satisfies"
* function.
* 我们使用SnapshotData结构体表示"regular" (MVCC) snapshots和具有非MVCC语义的"special" snapshots。
*/
//测试函数
typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
Snapshot snapshot, Buffer buffer);
//常见的有:
//HeapTupleSatisfiesMVCC：判断元组对某一快照版本是否有效
//HeapTupleSatisfiesUpdate：判断元组是否可更新(同时更新同一个元组)
//HeapTupleSatisfiesDirty：判断当前元组是否存在脏数据
//HeapTupleSatisfiesSelf：判断tuple对自身信息是否有效
//HeapTupleSatisfiesToast：判断是否TOAST表
//HeapTupleSatisfiesVacuum：判断元组是否能被VACUUM删除
//HeapTupleSatisfiesAny：所有元组都可见
//HeapTupleSatisfiesHistoricMVCC：用于CATALOG 表
/*
* Struct representing all kind of possible snapshots.
* 该结构体可表达的信息囊括了所有可能的快照.
*
* There are several different kinds of snapshots:
* * Normal MVCC snapshots
* * MVCC snapshots taken during recovery (in Hot-Standby mode)
* * Historic MVCC snapshots used during logical decoding
* * snapshots passed to HeapTupleSatisfiesDirty()
* * snapshots passed to HeapTupleSatisfiesNonVacuumable()
* * snapshots used for SatisfiesAny, Toast, Self where no members are
* accessed.
* 有以下几种不同类型的快照:
* * 常规的MVCC快照
* * 在恢复期间的MVCC快照(处于Hot-Standby模式)
* * 在逻辑解码过程中使用的历史MVCC快照
* * 作为参数传递给HeapTupleSatisfiesDirty()函数的快照
* * 作为参数传递给HeapTupleSatisfiesNonVacuumable()函数的快照
* * 用于在没有成员访问情况下SatisfiesAny、Toast和Self的快照
*
* TODO: It's probably a good idea to split this struct using a NodeTag
* similar to how parser and executor nodes are handled, with one type for
* each different kind of snapshot to avoid overloading the meaning of
* individual fields.
* TODO: 使用类似于parser/executor nodes的处理,使用NodeTag来拆分结构体会是一个好的做法,
* 使用OO(面向对象继承)的方法.
*/
typedef struct SnapshotData
{
//测试tuple是否可见的函数
SnapshotSatisfiesFunc satisfies; /* tuple test function */
/*
* The remaining fields are used only for MVCC snapshots, and are normally
* just zeroes in special snapshots. (But xmin and xmax are used
* specially by HeapTupleSatisfiesDirty, and xmin is used specially by
* HeapTupleSatisfiesNonVacuumable.)
* 余下的字段仅用于MVCC快照，在特殊快照中通常为0。
* (xmin和xmax可用于HeapTupleSatisfiesDirty，xmin可用于HeapTupleSatisfiesNonVacuumable)
*
* An MVCC snapshot can never see the effects of XIDs >= xmax. It can see
* the effects of all older XIDs except those listed in the snapshot. xmin
* is stored as an optimization to avoid needing to search the XID arrays
* for most tuples.
* XIDs >= xmax的事务,对该快照是不可见的(没有任何影响).
* 对该快照可见的是小于xmax,但不在snapshot列表中的XIDs.
* 记录xmin是出于优化的目的,避免为大多数tuples搜索XID数组.
*/
//XID ∈ [2,min)是可见的
TransactionId xmin; /* all XID < xmin are visible to me */
//XID ∈ [xmax,∞)是不可见的
TransactionId xmax; /* all XID >= xmax are invisible to me */
/*
* For normal MVCC snapshot this contains the all xact IDs that are in
* progress, unless the snapshot was taken during recovery in which case
* it's empty. For historic MVCC snapshots, the meaning is inverted, i.e.
* it contains *committed* transactions between xmin and xmax.
* 对于普通的MVCC快照,xip存储了所有正在进行中的XIDs,除非在恢复期间产生的快照(这时候数组为空)
* 对于历史MVCC快照,意义相反，即它包含xmin和xmax之间的*已提交*事务。
*
* note: all ids in xip[] satisfy xmin <= xip[i] < xmax
* 注意: 所有在xip数组中的XIDs满足xmin <= xip[i] < xmax
*/
TransactionId *xip;
//xip数组中的元素个数
uint32 xcnt; /* # of xact ids in xip[] */
/*
* For non-historic MVCC snapshots, this contains subxact IDs that are in
* progress (and other transactions that are in progress if taken during
* recovery). For historic snapshot it contains *all* xids assigned to the
* replayed transaction, including the toplevel xid.
* 对于非历史MVCC快照,下面这些域含有活动的subxact IDs.
* (以及在恢复过程中状态为进行中的事务).
* 对于历史MVCC快照,这些域字段含有*所有*用于回放事务的快照,包括顶层事务XIDs.
*
* note: all ids in subxip[] are >= xmin, but we don't bother filtering
* out any that are >= xmax
* 注意:sbuxip数组中的元素均≥ xmin,但我们不需要过滤掉任何>= xmax的项
*/
TransactionId *subxip;
//subxip数组元素个数
int32 subxcnt; /* # of xact ids in subxip[] */
//是否溢出?
bool suboverflowed; /* has the subxip array overflowed? */
//在Recovery期间的快照?
bool takenDuringRecovery; /* recovery-shaped snapshot? */
//如为静态快照,则该值为F
bool copied; /* false if it's a static snapshot */
//在自身的事务中,CID < curcid是可见的
CommandId curcid; /* in my xact, CID < curcid are visible */
/*
* An extra return value for HeapTupleSatisfiesDirty, not used in MVCC
* snapshots.
* HeapTupleSatisfiesDirty返回的值,在MVCC快照中无用
*/
uint32 speculativeToken;
/*
* Book-keeping information, used by the snapshot manager
* 用于快照管理器的Book-keeping信息
*/
//在ActiveSnapshot栈中的引用计数
uint32 active_count; /* refcount on ActiveSnapshot stack */
//在RegisteredSnapshots中的引用计数
uint32 regd_count; /* refcount on RegisteredSnapshots */
//RegisteredSnapshots堆中的链接
pairingheap_node ph_node; /* link in the RegisteredSnapshots heap */
//快照"拍摄"时间戳
TimestampTz whenTaken; /* timestamp when snapshot was taken */
//拍照时WAL stream中的位置
XLogRecPtr lsn; /* position in the WAL stream when taken */
} SnapshotData;
ShmemVariableCache
VariableCache是共享内存中的一种数据结构，用于跟踪OID和XID分配状态。
ShmemVariableCache是VariableCache结构体指针.
/*
* VariableCache is a data structure in shared memory that is used to track
* OID and XID assignment state. For largely historical reasons, there is
* just one struct with different fields that are protected by different
* LWLocks.
* VariableCache是共享内存中的一种数据结构，用于跟踪OID和XID分配状态。
* 由于历史原因，这个结构体有不同的字段,由不同的LWLocks保护。
*
* Note: xidWrapLimit and oldestXidDB are not "active" values, but are
* used just to generate useful messages when xidWarnLimit or xidStopLimit
* are exceeded.
* 注意:xidWrapLimit和oldestXidDB是不"活跃"的值,在xidWarnLimit或xidStopLimit
* 超出限制时用于产生有用的信息.
*/
typedef struct VariableCacheData
{
/*
* These fields are protected by OidGenLock.
* 这些域字段通过OidGenLock字段保护
*/
//下一个待分配的OID
Oid nextOid; /* next OID to assign */
//在必须执行XLOG work前可用OIDs
uint32 oidCount; /* OIDs available before must do XLOG work */
/*
* These fields are protected by XidGenLock.
* 这些字段通过XidGenLock锁保护.
*/
//下一个待分配的事务ID
TransactionId nextXid; /* next XID to assign */
//集群范围内最小datfrozenxid
TransactionId oldestXid; /* cluster-wide minimum datfrozenxid */
//在该XID开始强制执行autovacuum
TransactionId xidVacLimit; /* start forcing autovacuums here */
//在该XID开始提出警告
TransactionId xidWarnLimit; /* start complaining here */
//在该XID开外,拒绝生成下一个XID
TransactionId xidStopLimit; /* refuse to advance nextXid beyond here */
//"世界末日"XID,需回卷
TransactionId xidWrapLimit; /* where the world ends */
//持有最小datfrozenxid的DB
Oid oldestXidDB; /* database with minimum datfrozenxid */
/*
* These fields are protected by CommitTsLock
* 这些字段通过CommitTsLock锁保护
*/
TransactionId oldestCommitTsXid;
TransactionId newestCommitTsXid;
/*
* These fields are protected by ProcArrayLock.
* 这些字段通过ProcArrayLock锁保护
*/
TransactionId latestCompletedXid; /* newest XID that has committed or
* aborted */
/*
* These fields are protected by CLogTruncationLock
* 这些字段通过CLogTruncationLock锁保护
*/
//clog中最古老的XID
TransactionId oldestClogXid; /* oldest it's safe to look up in clog */
} VariableCacheData;
//结构体指针
typedef VariableCacheData *VariableCache;
/* pointer to "variable cache" in shared memory (set up by shmem.c) */
//共享内存中的指针(通过shmem.c设置)
VariableCache ShmemVariableCache = NULL;
二、源码解读
GetSnapshotData函数返回快照信息.
重点是构造xmin : xmax : xip_list,其实现逻辑简单总结如下:
1.获取xmax = ShmemVariableCache->latestCompletedXid + 1;
2.遍历全局procArray数组,构建快照信息
2.1 获取进程相应的事务信息pgxact
2.2 获取进程事务ID(pgxact->xid),取最小的xid作为xmin(不包括0)
2.3 把xid放入快照->xip数组中(不包括本进程所在的事务id)
/*
* GetSnapshotData -- returns information about running transactions.
* GetSnapshotData -- 返回关于正在运行中的事务的相关信息
*
* The returned snapshot includes xmin (lowest still-running xact ID),
* xmax (highest completed xact ID + 1), and a list of running xact IDs
* in the range xmin <= xid < xmax. It is used as follows:
* All xact IDs < xmin are considered finished.
* All xact IDs >= xmax are considered still running.
* For an xact ID xmin <= xid < xmax, consult list to see whether
* it is considered running or not.
* This ensures that the set of transactions seen as "running" by the
* current xact will not change after it takes the snapshot.
* 返回的snapshot包括xmin(最小的正在运行的事务ID),xmax(已完结事务ID + 1),
* 以及在xmin <= xid < xmax之间正在运行的事务IDs.
* 意义如下:
* 事务IDs < xmin是已确定完成的事务.
* 事务IDs >= xmax是正在运行的事务.
* 对于XID ∈ [xmin,xmax)的事务,需查阅列表确认是否正在运行中
*
* All running top-level XIDs are included in the snapshot, except for lazy
* VACUUM processes. We also try to include running subtransaction XIDs,
* but since PGPROC has only a limited cache area for subxact XIDs, full
* information may not be available. If we find any overflowed subxid arrays,
* we have to mark the snapshot's subxid data as overflowed, and extra work
* *may* need to be done to determine what's running (see XidInMVCCSnapshot()
* in tqual.c).
* 所有正在运行的顶层XIDs包含在快照中,除了lazy VACUUM进程.
* 我们尝试包含所有正在运行的子事务XIDs,但由于PGPROC只有有限的缓存,包含所有的子事务信息暂未实现.
* 如果我们搜索溢出的子事务数组,我们必须标记快照的subxid数据为溢出,
* 而且需要执行额外的工作以确定哪些在运行(查看tqual.c中的XidInMVCCSnapshot()函数)
*
* We also update the following backend-global variables:
* TransactionXmin: the oldest xmin of any snapshot in use in the
* current transaction (this is the same as MyPgXact->xmin).
* RecentXmin: the xmin computed for the most recent snapshot. XIDs
* older than this are known not running any more.
* RecentGlobalXmin: the global xmin (oldest TransactionXmin across all
* running transactions, except those running LAZY VACUUM). This is
* the same computation done by
* GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM).
* RecentGlobalDataXmin: the global xmin for non-catalog tables
* >= RecentGlobalXmin
* 我们同时更新了以下后台全局变量:
* TransactionXmin: 当前事务中在所有仍在使用的快照中最旧的xmin(与MyPgXact->xmin一致).
* RecentXmin: 最近快照的xmin.小于xmin的事务已知已完结.
* RecentGlobalXmin:全局的xmin(除了正在运行的LAZY VACUUM,跨越所有正在运行事务的最旧的TransactionXmin),
* 这是使用同样的规则,通过GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM)处理.
* RecentGlobalDataXmin:非catalog数据表的全局xmin,该值>= RecentGlobalXmin.
*
* Note: this function should probably not be called with an argument that's
* not statically allocated (see xip allocation below).
* 注意:不应该使用非静态分配的参数调用这个函数(参见下面的xip分配)。
*/
Snapshot
GetSnapshotData(Snapshot snapshot)
{
ProcArrayStruct *arrayP = procArray;//进程数组
TransactionId xmin;//xmin
TransactionId xmax;//xmax
TransactionId globalxmin;//全局xmin
int index;
int count = 0;
int subcount = 0;
bool suboverflowed = false;
TransactionId replication_slot_xmin = InvalidTransactionId;
TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
Assert(snapshot != NULL);
/*
* Allocating space for maxProcs xids is usually overkill; numProcs would
* be sufficient. But it seems better to do the malloc while not holding
* the lock, so we can't look at numProcs. Likewise, we allocate much
* more subxip storage than is probably needed.
* 为maxProcs xids分配空间通常是多余的;numProcs就足够了。
* 但是在不持有锁的情况下执行malloc似乎更好，因此我们不能查看numProcs。
* 同样地，我们分配的子xip存储可能比实际需要的多得多。
*
* This does open a possibility for avoiding repeated malloc/free: since
* maxProcs does not change at runtime, we can simply reuse the previous
* xip arrays if any. (This relies on the fact that all callers pass
* static SnapshotData structs.)
* 这确实为避免重复的malloc/free创造了一种可能性:因为maxProcs在运行时不会改变，
* 如果有的话，我们可以简单地重用前面的xip数组。
* (这依赖于所有调用者都传递静态快照数据结构这一事实。)
*/
if (snapshot->xip == NULL)
{
/*
* First call for this snapshot. Snapshot is same size whether or not
* we are in recovery, see later comments.
* 首次调用.快照的大小不管是在常规还是在恢复状态都是一样的,看稍后的注释.
*/
snapshot->xip = (TransactionId *)
malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId));
if (snapshot->xip == NULL)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
Assert(snapshot->subxip == NULL);
snapshot->subxip = (TransactionId *)
malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
if (snapshot->subxip == NULL)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
}
/*
* It is sufficient to get shared lock on ProcArrayLock, even if we are
* going to set MyPgXact->xmin.
* 即使我们要设置MyPgXact->xmin,也需要获取锁,在ProcArrayLock上获得共享锁就足够了.
*
*/
LWLockAcquire(ProcArrayLock, LW_SHARED);
/* xmax is always latestCompletedXid + 1 */
//xmax = latestCompletedXid + 1
//已完结事务号 + 1
xmax = ShmemVariableCache->latestCompletedXid;
Assert(TransactionIdIsNormal(xmax));
TransactionIdAdvance(xmax);// + 1
/* initialize xmin calculation with xmax */
//初始化xmin为xmax
globalxmin = xmin = xmax;
//是否处于恢复过程中?
snapshot->takenDuringRecovery = RecoveryInProgress();
if (!snapshot->takenDuringRecovery)
{
//不是,正常运行中
int *pgprocnos = arrayP->pgprocnos;//进程数
int numProcs;
/*
* Spin over procArray checking xid, xmin, and subxids. The goal is
* to gather all active xids, find the lowest xmin, and try to record
* subxids.
* Spin Over procArray,检查xid/xmin和subxids.
* 目标是搜集所有活动的xids,找到最小的xmin,并尝试记录subxids.
*/
numProcs = arrayP->numProcs;
for (index = 0; index < numProcs; index++)//遍历procArray数组
{
int pgprocno = pgprocnos[index];//allPgXact[]索引
PGXACT *pgxact = &allPgXact[pgprocno];//获取PGXACT
TransactionId xid;//事务id
/*
* Skip over backends doing logical decoding which manages xmin
* separately (check below) and ones running LAZY VACUUM.
* 跳过正在执行逻辑解码(单独管理xmin)和执行LAZY VACUUM的进程.
*
*/
if (pgxact->vacuumFlags &
(PROC_IN_LOGICAL_DECODING | PROC_IN_VACUUM))
continue;
/* Update globalxmin to be the smallest valid xmin */
//更新globalxmin为最小有效的xmin
xid = UINT32_ACCESS_ONCE(pgxact->xmin);//获取进程事务的xmin
if (TransactionIdIsNormal(xid) &&
NormalTransactionIdPrecedes(xid, globalxmin))
globalxmin = xid;
/* Fetch xid just once - see GetNewTransactionId */
//只提取一次xid -- 查看函数GetNewTransactionId
xid = UINT32_ACCESS_ONCE(pgxact->xid);
/*
* If the transaction has no XID assigned, we can skip it; it
* won't have sub-XIDs either. If the XID is >= xmax, we can also
* skip it; such transactions will be treated as running anyway
* (and any sub-XIDs will also be >= xmax).
* 如果事务未分配XID事务号,跳过此事务.该事务也不会含有子事务.
* 如果XID >= xmax,我们也可以跳过,这些事务可被处理为正在运行的思维.
* (这些事务的子事务XID也同样会 >= xmax)
*/
if (!TransactionIdIsNormal(xid)
|| !NormalTransactionIdPrecedes(xid, xmax))
continue;
/*
* We don't include our own XIDs (if any) in the snapshot, but we
* must include them in xmin.
* 在快照中,不会包含自己的XIDs,但必须体现在xmin中
*/
if (NormalTransactionIdPrecedes(xid, xmin))
//xid 小于 xmin,设置为xid
xmin = xid;
if (pgxact == MyPgXact)
continue;//跳过本事务
/* Add XID to snapshot. */
//添加XID到快照中
snapshot->xip[count++] = xid;
/*
* Save subtransaction XIDs if possible (if we've already
* overflowed, there's no point). Note that the subxact XIDs must
* be later than their parent, so no need to check them against
* xmin. We could filter against xmax, but it seems better not to
* do that much work while holding the ProcArrayLock.
* 如可能,保存子事务XIDs(如果已经溢出,那就没法了).
* 注意子事务XIDs必须在他们的父事务之后发生,因此无需检查xmin.
* 我们可以利用xmax进行过滤，但是在持有锁ProcArrayLock时最好不要做那么多的工作。
*
* The other backend can add more subxids concurrently, but cannot
* remove any. Hence it's important to fetch nxids just once.
* Should be safe to use memcpy, though. (We needn't worry about
* missing any xids added concurrently, because they must postdate
* xmax.)
* 其他后台进程可能并发增加子事务ID,但不能清除.
* 因此,只取一次nxids很重要.不过,使用memcpy是安全的.
* (不需要担心遗漏并发增加xids,因为他们在xmax之后)
*
* Again, our own XIDs are not included in the snapshot.
* 再次,我们自己的XIDs不需要包含在快照中
*/
if (!suboverflowed)
{
if (pgxact->overflowed)
suboverflowed = true;
else
{
int nxids = pgxact->nxids;
if (nxids > 0)
{
PGPROC *proc = &allProcs[pgprocno];
pg_read_barrier(); /* pairs with GetNewTransactionId */
memcpy(snapshot->subxip + subcount,
(void *) proc->subxids.xids,
nxids * sizeof(TransactionId));
subcount += nxids;
}
}
}
}
}
else
{
/*
* We're in hot standby, so get XIDs from KnownAssignedXids.
* 处于hot standby中,通过KnownAssignedXids获取XIDs.
*
* We store all xids directly into subxip[]. Here's why:
* 直接存储所有的xids到subxip[]中,这是因为:
*
* In recovery we don't know which xids are top-level and which are
* subxacts, a design choice that greatly simplifies xid processing.
* 在恢复过程中,我们不需要知道哪些xids是顶层事务,哪些是子事务,
* 这可以极大的简化xid处理过程.
*
* It seems like we would want to try to put xids into xip[] only, but
* that is fairly small. We would either need to make that bigger or
* to increase the rate at which we WAL-log xid assignment; neither is
* an appealing choice.
* 似乎我们只想把xid放到xip[]中，但xip数组是相当小的。
* 我们要么需要扩展，要么提高WAL-log xid分派的速度;
* 但这两个选择都不吸引人。
*
* We could try to store xids into xip[] first and then into subxip[]
* if there are too many xids. That only works if the snapshot doesn't
* overflow because we do not search subxip[] in that case. A simpler
* way is to just store all xids in the subxact array because this is
* by far the bigger array. We just leave the xip array empty.
* 如果xid太多的话,我们尝试先将xid存储到xip[]中，然后再在subxip[]中存储。
* 这只在快照没有溢出的情况下有效，因为在这种情况下我们不搜索subxip[]。
* 一种更简单的方法是将所有xid存储在subxact数组中，因为这个数组要大得多。
* 让xip数组为空。
*
* Either way we need to change the way XidInMVCCSnapshot() works
* depending upon when the snapshot was taken, or change normal
* snapshot processing so it matches.
* 无论哪种方式，我们都需要根据快照的拍摄时间更改XidInMVCCSnapshot()的工作方式，
* 或者更改正常的快照处理，使其匹配。
*
* Note: It is possible for recovery to end before we finish taking
* the snapshot, and for newly assigned transaction ids to be added to
* the ProcArray. xmax cannot change while we hold ProcArrayLock, so
* those newly added transaction ids would be filtered away, so we
* need not be concerned about them.
* 注意:在我们完成快照之前，恢复可能会结束，
* 并且新分配的事务id可能会添加到ProcArray中。
* 当我们持有锁ProcArrayLock时，xmax无法更改，
* 因此那些新添加的事务id将被过滤掉，因此无需担心。
*/
subcount = KnownAssignedXidsGetAndSetXmin(snapshot->subxip, &xmin,
xmax);
if (TransactionIdPrecedesOrEquals(xmin, procArray->lastOverflowedXid))
suboverflowed = true;
}
/*
* Fetch into local variable while ProcArrayLock is held - the
* LWLockRelease below is a barrier, ensuring this happens inside the
* lock.
* 持有ProcArrayLock锁时,提前到本地变量中,
* 下面的LWLockRelease是一个屏障，确保这发生在锁内部。
*/
replication_slot_xmin = procArray->replication_slot_xmin;
replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
if (!TransactionIdIsValid(MyPgXact->xmin))
MyPgXact->xmin = TransactionXmin = xmin;
LWLockRelease(ProcArrayLock);
/*
* Update globalxmin to include actual process xids. This is a slightly
* different way of computing it than GetOldestXmin uses, but should give
* the same result.
* 更新globalxmin已包含实际的进程xids.
* 这是一种与GetOldestXmin使用的计算方法略有不同的方法，但是应该会得到相同的结果。
*/
if (TransactionIdPrecedes(xmin, globalxmin))
globalxmin = xmin;
/* Update global variables too */
//更新全局变量
RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age;
if (!TransactionIdIsNormal(RecentGlobalXmin))
RecentGlobalXmin = FirstNormalTransactionId;
/* Check whether there's a replication slot requiring an older xmin. */
//检查是否存在正在请求更旧xmin的复制slot
if (TransactionIdIsValid(replication_slot_xmin) &&
NormalTransactionIdPrecedes(replication_slot_xmin, RecentGlobalXmin))
RecentGlobalXmin = replication_slot_xmin;
/* Non-catalog tables can be vacuumed if older than this xid */
//比该xid小的非catalog表可被vacuum进程清除
RecentGlobalDataXmin = RecentGlobalXmin;
/*
* Check whether there's a replication slot requiring an older catalog
* xmin.
* 检查是否存在正确请求更旧catalog xmin的复制slot
*/
if (TransactionIdIsNormal(replication_slot_catalog_xmin) &&
NormalTransactionIdPrecedes(replication_slot_catalog_xmin, RecentGlobalXmin))
RecentGlobalXmin = replication_slot_catalog_xmin;
RecentXmin = xmin;
snapshot->xmin = xmin;
snapshot->xmax = xmax;
snapshot->xcnt = count;
snapshot->subxcnt = subcount;
snapshot->suboverflowed = suboverflowed;
//当前命令id
snapshot->curcid = GetCurrentCommandId(false);
/*
* This is a new snapshot, so set both refcounts are zero, and mark it as
* not copied in persistent memory.
* 这是一个新的快照,因此设置refcounts为0,并标记其未在持久化内存中拷贝.
*/
snapshot->active_count = 0;
snapshot->regd_count = 0;
snapshot->copied = false;
if (old_snapshot_threshold < 0)
{
/*
* If not using "snapshot too old" feature, fill related fields with
* dummy values that don't require any locking.
* 如启用"snapshot too old"特性,使用虚拟值填充相关的字段,这里不需要锁.
*/
snapshot->lsn = InvalidXLogRecPtr;
snapshot->whenTaken = 0;
}
else
{
/*
* Capture the current time and WAL stream location in case this
* snapshot becomes old enough to need to fall back on the special
* "old snapshot" logic.
* 捕获当前时间和WAL流位置，以防快照变得足够旧时需要使用特殊的“old snapshot”逻辑。
*/
snapshot->lsn = GetXLogInsertRecPtr();
snapshot->whenTaken = GetSnapshotCurrentTimestamp();
MaintainOldSnapshotTimeMapping(snapshot->whenTaken, xmin);
}
//返回快照
return snapshot;
}
三、跟踪分析
执行简单查询,可触发获取快照逻辑.
16:35:08 (xdb@[local]:5432)testdb=# begin; BEGIN 16:35:13 (xdb@[local]:5432)testdb=#* select 1;
启动gdb,设置断点
(gdb) b GetSnapshotData Breakpoint 1 at 0x89aef3: file procarray.c, line 1519. (gdb) c Continuing. Breakpoint 1, GetSnapshotData (snapshot=0xf9be60 <CurrentSnapshotData>) at procarray.c:1519 1519 ProcArrayStruct *arrayP = procArray; (gdb)
输入参数snapshot,实质是全局变量CurrentSnapshotData
(gdb) p *snapshot
$1 = {satisfies = 0xa9310d <HeapTupleSatisfiesMVCC>, xmin = 2354, xmax = 2358, xip = 0x24c7e40, xcnt = 1,
subxip = 0x251dfa0, subxcnt = 0, suboverflowed = false, takenDuringRecovery = false, copied = false, curcid = 0,
speculativeToken = 0, active_count = 0, regd_count = 0, ph_node = {first_child = 0x0, next_sibling = 0x0,
prev_or_parent = 0x0}, whenTaken = 0, lsn = 0}
查看共享内存(ShmemVariableCache)中的信息.
nextXID = 2358,下一个待分配的事务ID = 2358.
(gdb) p *ShmemVariableCache
$2 = {nextOid = 42605, oidCount = 8183, nextXid = 2358, oldestXid = 561, xidVacLimit = 200000561,
xidWarnLimit = 2136484208, xidStopLimit = 2146484208, xidWrapLimit = 2147484208, oldestXidDB = 16400,
oldestCommitTsXid = 0, newestCommitTsXid = 0, latestCompletedXid = 2357, oldestClogXid = 561}
(gdb)
获取全局进程数组procArray,赋值->arrayP.
初始化相关变量.
(gdb) n 1524 int count = 0; (gdb) n 1525 int subcount = 0; (gdb) 1526 bool suboverflowed = false; (gdb) 1527 volatile TransactionId replication_slot_xmin = InvalidTransactionId; (gdb) 1528 volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId; (gdb) 1530 Assert(snapshot != NULL); (gdb) 1543 if (snapshot->xip == NULL) (gdb)
查看进程数组信息和allPgXact[]数组编号(arrayP->pgprocnos数组).
allPgXact定义:static PGXACT *allPgXact;
(gdb) p *arrayP
$3 = {numProcs = 5, maxProcs = 112, maxKnownAssignedXids = 7280, numKnownAssignedXids = 0, tailKnownAssignedXids = 0,
headKnownAssignedXids = 0, known_assigned_xids_lck = 0 '/000', lastOverflowedXid = 0, replication_slot_xmin = 0,
replication_slot_catalog_xmin = 0, pgprocnos = 0x7f8765d9a3a8}
(gdb) p arrayP->pgprocnos[0]
$4 = 97
(gdb) p arrayP->pgprocnos[1]
$5 = 98
(gdb) p arrayP->pgprocnos[2]
$6 = 99
(gdb) p arrayP->pgprocnos[3]
$7 = 103
(gdb) p arrayP->pgprocnos[4]
$9 = 111
加锁,获取/修改相关信息
(gdb) 1568 LWLockAcquire(ProcArrayLock, LW_SHARED);
计算xmax
(gdb) n 1571 xmax = ShmemVariableCache->latestCompletedXid; (gdb) 1572 Assert(TransactionIdIsNormal(xmax)); (gdb) p xmax $10 = 2357 (gdb) n 1573 TransactionIdAdvance(xmax); (gdb) 1576 globalxmin = xmin = xmax; (gdb) 1578 snapshot->takenDuringRecovery = RecoveryInProgress(); (gdb) p xmax $11 = 2358
判断是否处于恢复状态,当前不是恢复状态,进入相应的处理逻辑
(gdb) n 1580 if (!snapshot->takenDuringRecovery) (gdb) p snapshot->takenDuringRecovery $13 = false (gdb) n 1582 int *pgprocnos = arrayP->pgprocnos; (gdb)
获取进程数和PGXACT索引数组,准备遍历
(gdb) n 1590 numProcs = arrayP->numProcs; (gdb) 1591 for (index = 0; index < numProcs; index++) (gdb) (gdb) p *pgprocnos $14 = 97 (gdb) p numProcs $15 = 5 (gdb)
获取pgxact信息
(gdb) n
1593 int pgprocno = pgprocnos[index];
(gdb)
1594 volatile PGXACT *pgxact = &allPgXact[pgprocno];
(gdb)
1601 if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
(gdb)
1605 if (pgxact->vacuumFlags & PROC_IN_VACUUM)
(gdb)
1609 xid = pgxact->xmin; /* fetch just once */
(gdb) p *pgxact
$16 = {xid = 0, xmin = 0, vacuumFlags = 0 '/000', overflowed = false, delayChkpt = false, nxids = 0 '/000'}
(gdb)
不是正常的xid,下一个pgxact
(gdb) n 1610 if (TransactionIdIsNormal(xid) && (gdb) 1615 xid = pgxact->xid; (gdb) 1623 if (!TransactionIdIsNormal(xid) (gdb) p xid $17 = 0 (gdb) n 1625 continue; (gdb)
下一个xid = 2355,正常的事务ID
(gdb)
1591 for (index = 0; index < numProcs; index++)
(gdb)
1593 int pgprocno = pgprocnos[index];
(gdb)
1594 volatile PGXACT *pgxact = &allPgXact[pgprocno];
(gdb)
1601 if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
(gdb) p *pgxact
$18 = {xid = 2355, xmin = 0, vacuumFlags = 0 '/000', overflowed = false, delayChkpt = false, nxids = 0 '/000'}
(gdb)
进行处理
(gdb) n 1605 if (pgxact->vacuumFlags & PROC_IN_VACUUM) (gdb) 1609 xid = pgxact->xmin; /* fetch just once */ (gdb) 1610 if (TransactionIdIsNormal(xid) && (gdb) 1615 xid = pgxact->xid; (gdb) 1623 if (!TransactionIdIsNormal(xid) (gdb) 1624 || !NormalTransactionIdPrecedes(xid, xmax)) (gdb) 1631 if (NormalTransactionIdPrecedes(xid, xmin)) (gdb) p xid $19 = 2355 (gdb) p xmin $20 = 2358 (gdb) n 1632 xmin = xid; (gdb) 1633 if (pgxact == MyPgXact) (gdb)
这是同一个xact,处理下一个xact
(gdb) 1633 if (pgxact == MyPgXact) (gdb) p pgxact $21 = (volatile PGXACT *) 0x7f8765d9a218 (gdb) p MyPgXact $22 = (struct PGXACT *) 0x7f8765d9a218 (gdb) n 1634 continue; (gdb)
下一个是2354
...
(gdb) p *pgxact
$23 = {xid = 2354, xmin = 0, vacuumFlags = 0 '/000', overflowed = false, delayChkpt = false, nxids = 0 '/000'}
(gdb)
xmin调整为2354
1631 if (NormalTransactionIdPrecedes(xid, xmin)) (gdb) 1632 xmin = xid; (gdb) 1633 if (pgxact == MyPgXact) (gdb) p xmin $24 = 2354 (gdb)
写入到xip_list中
1637 snapshot->xip[count++] = xid; (gdb) 1654 if (!suboverflowed) (gdb) (gdb) p count $25 = 1
继续循环,完成5个pgxact的遍历
1591 for (index = 0; index < numProcs; index++) (gdb) 1715 replication_slot_xmin = procArray->replication_slot_xmin; (gdb)
无复制信息
(gdb) 1715 replication_slot_xmin = procArray->replication_slot_xmin; (gdb) p procArray->replication_slot_xmin $28 = 0 (gdb) n 1716 replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin; (gdb) 1718 if (!TransactionIdIsValid(MyPgXact->xmin))
调整本进程的事务信息
(gdb) n 1719 MyPgXact->xmin = TransactionXmin = xmin; (gdb) p MyPgXact->xmin $29 = 0 (gdb) n
释放锁
1721 LWLockRelease(ProcArrayLock); (gdb) 1728 if (TransactionIdPrecedes(xmin, globalxmin)) (gdb)
调整全局xmin
(gdb) p xmin $30 = 2354 (gdb) p globalxmin $31 = 2358 (gdb) n 1729 globalxmin = xmin; (gdb)
更新其他信息
(gdb) 1732 RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age; (gdb) p RecentGlobalXmin $32 = 2354 (gdb) p vacuum_defer_cleanup_age $33 = 0 (gdb) n 1733 if (!TransactionIdIsNormal(RecentGlobalXmin)) (gdb) 1737 if (TransactionIdIsValid(replication_slot_xmin) && (gdb) 1742 RecentGlobalDataXmin = RecentGlobalXmin; (gdb) p RecentGlobalXmin $34 = 2354 (gdb) n 1748 if (TransactionIdIsNormal(replication_slot_catalog_xmin) && (gdb)
填充snapshot域字段信息
(gdb) 1752 RecentXmin = xmin; (gdb) 1754 snapshot->xmin = xmin; (gdb) 1755 snapshot->xmax = xmax; (gdb) 1756 snapshot->xcnt = count; (gdb) 1757 snapshot->subxcnt = subcount; (gdb) 1758 snapshot->suboverflowed = suboverflowed; (gdb) 1760 snapshot->curcid = GetCurrentCommandId(false); (gdb) 1766 snapshot->active_count = 0; (gdb) 1767 snapshot->regd_count = 0; (gdb) 1768 snapshot->copied = false; (gdb) 1770 if (old_snapshot_threshold < 0) (gdb) 1776 snapshot->lsn = InvalidXLogRecPtr; (gdb) 1777 snapshot->whenTaken = 0; (gdb) 1791 return snapshot; (gdb)
返回snapshot
(gdb) p snapshot
$35 = (Snapshot) 0xf9be60 <CurrentSnapshotData>
(gdb) p *snapshot
$36 = {satisfies = 0xa9310d <HeapTupleSatisfiesMVCC>, xmin = 2354, xmax = 2358, xip = 0x24c7e40, xcnt = 1,
subxip = 0x251dfa0, subxcnt = 0, suboverflowed = false, takenDuringRecovery = false, copied = false, curcid = 0,
speculativeToken = 0, active_count = 0, regd_count = 0, ph_node = {first_child = 0x0, next_sibling = 0x0,
prev_or_parent = 0x0}, whenTaken = 0, lsn = 0}
(gdb)
注意:snapshot->satisfies函数在初始化该全局变量已设置为HeapTupleSatisfiesMVCC.
感谢各位的阅读，以上就是“PostgreSQL中GetSnapshotData的处理过程是什么”的内容了，经过本文的学习后，相信大家对PostgreSQL中GetSnapshotData的处理过程是什么这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是亿速云，小编将为大家推送更多相关知识点的文章，欢迎关注！
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