PostgreSQL中vacuum过程分析
本篇内容主要讲解“PostgreSQL中vacuum过程分析”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“PostgreSQL中vacuum过程分析”吧!
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一、数据结构
宏定义
Vacuum和Analyze命令选项
/* ----------------------
* Vacuum and Analyze Statements
* Vacuum和Analyze命令选项
*
* Even though these are nominally two statements, it's convenient to use
* just one node type for both. Note that at least one of VACOPT_VACUUM
* and VACOPT_ANALYZE must be set in options.
* 虽然在这里有两种不同的语句,但只需要使用统一的Node类型即可.
* 注意至少VACOPT_VACUUM/VACOPT_ANALYZE在选项中设置.
* ----------------------
*/
typedef enum VacuumOption
{
VACOPT_VACUUM = 1 << 0, /* do VACUUM */
VACOPT_ANALYZE = 1 << 1, /* do ANALYZE */
VACOPT_VERBOSE = 1 << 2, /* print progress info */
VACOPT_FREEZE = 1 << 3, /* FREEZE option */
VACOPT_FULL = 1 << 4, /* FULL (non-concurrent) vacuum */
VACOPT_SKIP_LOCKED = 1 << 5, /* skip if cannot get lock */
VACOPT_SKIPTOAST = 1 << 6, /* don't process the TOAST table, if any */
VACOPT_DISABLE_PAGE_SKIPPING = 1 << 7 /* don't skip any pages */
} VacuumOption;VacuumStmt
存储vacuum命令的option&Relation链表
typedef struct VacuumStmt
{
NodeTag type;//Tag
//VacuumOption位标记
int options; /* OR of VacuumOption flags */
//VacuumRelation链表,如为NIL-->所有Relation.
List *rels; /* list of VacuumRelation, or NIL for all */
} VacuumStmt;VacuumParams
vacuum命令参数
/*
* Parameters customizing behavior of VACUUM and ANALYZE.
* 客户端调用VACUUM/ANALYZE时的定制化参数
*/
typedef struct VacuumParams
{
//最小freeze age,-1表示使用默认
int freeze_min_age; /* min freeze age, -1 to use default */
//扫描整个table的freeze age
int freeze_table_age; /* age at which to scan whole table */
//最小的multixact freeze age,-1表示默认
int multixact_freeze_min_age; /* min multixact freeze age, -1 to
* use default */
//扫描全表的freeze age,-1表示默认
int multixact_freeze_table_age; /* multixact age at which to scan
* whole table */
//是否强制wraparound?
bool is_wraparound; /* force a for-wraparound vacuum */
//以毫秒为单位的最小执行阈值
int log_min_duration; /* minimum execution threshold in ms at
* which verbose logs are activated, -1
* to use default */
} VacuumParams;VacuumRelation
VACUUM/ANALYZE命令的目标表信息
/*
* Info about a single target table of VACUUM/ANALYZE.
* VACUUM/ANALYZE命令的目标表信息.
*
* If the OID field is set, it always identifies the table to process.
* Then the relation field can be NULL; if it isn't, it's used only to report
* failure to open/lock the relation.
* 如设置了OID字段,该值通常是将要处理的数据表.
* 那么关系字段可以为空;如果不是，则仅用于报告未能打开/锁定关系。
*/
typedef struct VacuumRelation
{
NodeTag type;
RangeVar *relation; /* table name to process, or NULL */
Oid oid; /* table's OID; InvalidOid if not looked up */
List *va_cols; /* list of column names, or NIL for all */
} VacuumRelation;二、源码解读
vacuum是VACUUM/ANALYZE命令的内部处理入口.
逻辑比较简单:
1.配置vacuum处理的相关参数,如命令类型等
2.执行相关检查
3.构造vacuum处理上下文
4.构造vacuum需处理的relation链表
5.循环遍历relation链表
5.1 获取relation
5.2 执行vacuum_rel
6.收尾工作
/*
* Internal entry point for VACUUM and ANALYZE commands.
* VACUUM/ANALYZE命令的内部处理入口
*
* options is a bitmask of VacuumOption flags, indicating what to do.
* options是VacuumOption选项标记位,指示应该做什么.
*
* relations, if not NIL, is a list of VacuumRelation to process; otherwise,
* we process all relevant tables in the database. For each VacuumRelation,
* if a valid OID is supplied, the table with that OID is what to process;
* otherwise, the VacuumRelation's RangeVar indicates what to process.
* relations,如果不是空指针NIL,那么存储了待处理的VacuumRelation结构体链表.
* 如为NIL,将处理数据库中的所有相关数据表.
* 对每一个VacuumRelation,如提供了有效OID,该OID对应table就会被处理,
* 否则,VacuumRelation的RangeVar指示了如何处理.
*
* params contains a set of parameters that can be used to customize the
* behavior.
* params是客户端定制的参数集合.
*
* bstrategy is normally given as NULL, but in autovacuum it can be passed
* in to use the same buffer strategy object across multiple vacuum() calls.
* bstrategy通常是NULL,但在autovacuum中,
* 该参数可用于指示在多个vacuum()调用中使用同样的缓存strategy object
*
* isTopLevel should be passed down from ProcessUtility.
* isTopLevel通过ProcessUtility向下传递
*
* It is the caller's responsibility that all parameters are allocated in a
* memory context that will not disappear at transaction commit.
* 调用者应确保所有的参数在同一个内存上下文分配内存,而不会在事务commit时突然消失.
*/
void
vacuum(int options, List *relations, VacuumParams *params,
BufferAccessStrategy bstrategy, bool isTopLevel)
{
static bool in_vacuum = false;//是否在vacuum
const char *stmttype;//语句类型,vacuum?analyze?
volatile bool in_outer_xact,
use_own_xacts;
Assert(params != NULL);
stmttype = (options & VACOPT_VACUUM) ? "VACUUM" : "ANALYZE";
/*
* We cannot run VACUUM inside a user transaction block; if we were inside
* a transaction, then our commit- and start-transaction-command calls
* would not have the intended effect! There are numerous other subtle
* dependencies on this, too.
* 不能在用户事务块中运行VACUUM,如果我们在事务块中,
* 那么处理过程中的commit-和start-transaction-command调用不会有正确的效果.
* 而且还有许多其他微妙的依赖关系。
*
* ANALYZE (without VACUUM) can run either way.
* ANALYZE(不带VACUUM)则没有此问题.
*/
if (options & VACOPT_VACUUM)
{
PreventInTransactionBlock(isTopLevel, stmttype);
in_outer_xact = false;
}
else
in_outer_xact = IsInTransactionBlock(isTopLevel);
/*
* Due to static variables vac_context, anl_context and vac_strategy,
* vacuum() is not reentrant. This matters when VACUUM FULL or ANALYZE
* calls a hostile index expression that itself calls ANALYZE.
* 鉴于vac_context, anl_context and vac_strategy这是变量都是静态变量,
* 因此vacuum()函数是不能重入的(状态已出现变化).
* 在VACUUM FULL或者ANALYZE调用了hostile index expression,
* 而此逻辑又调用了ANALYZE时会出现此情况,务必注意.
*/
if (in_vacuum)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("%s cannot be executed from VACUUM or ANALYZE",
stmttype)));
/*
* Sanity check DISABLE_PAGE_SKIPPING option.
* 检查
*/
if ((options & VACOPT_FULL) != 0 &&
(options & VACOPT_DISABLE_PAGE_SKIPPING) != 0)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("VACUUM option DISABLE_PAGE_SKIPPING cannot be used with FULL")));
/*
* Send info about dead objects to the statistics collector, unless we are
* in autovacuum --- autovacuum.c does this for itself.
* 发送dead objects的统计信息给收集器,除非我们在autovacuum中
* -- autovacuum.c会自己做这个事情.
*/
if ((options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
pgstat_vacuum_stat();
/*
* Create special memory context for cross-transaction storage.
* 跨事务存储,需要创建特别的内存上下文.
*
* Since it is a child of PortalContext, it will go away eventually even
* if we suffer an error; there's no need for special abort cleanup logic.
* 因为这是PortalContext的子对象,即使我们犯了错误，它最终也会消失;不需要特殊的中止清理逻辑。
*/
vac_context = AllocSetContextCreate(PortalContext,
"Vacuum",
ALLOCSET_DEFAULT_SIZES);
/*
* If caller didn't give us a buffer strategy object, make one in the
* cross-transaction memory context.
* 如果调用者没有提供buffer strategy object,
* 在跨事务的内存上下文中创建一个.
*/
if (bstrategy == NULL)
{
MemoryContext old_context = MemoryContextSwitchTo(vac_context);
bstrategy = GetAccessStrategy(BAS_VACUUM);
MemoryContextSwitchTo(old_context);
}
vac_strategy = bstrategy;
/*
* Build list of relation(s) to process, putting any new data in
* vac_context for safekeeping.
* 构建要处理的关系列表，将所有新数据放入vac_context中以进行安全(位于vacuum上下文中)保存。
*/
if (relations != NIL)
{
List *newrels = NIL;
ListCell *lc;
foreach(lc, relations)
{
VacuumRelation *vrel = lfirst_node(VacuumRelation, lc);
List *sublist;
MemoryContext old_context;
sublist = expand_vacuum_rel(vrel, options);
old_context = MemoryContextSwitchTo(vac_context);
newrels = list_concat(newrels, sublist);
MemoryContextSwitchTo(old_context);
}
relations = newrels;
}
else
relations = get_all_vacuum_rels(options);
/*
* Decide whether we need to start/commit our own transactions.
* 确定是否需要start/commit自己的事务
*
* For VACUUM (with or without ANALYZE): always do so, so that we can
* release locks as soon as possible. (We could possibly use the outer
* transaction for a one-table VACUUM, but handling TOAST tables would be
* problematic.)
* 对于VACUUM(包含或不包含ANALYZE):通常需要这样处理,以便我们可以尽可能快的释放锁.
* (对于一张表的VACUUM,我们可能使用外层事务,但处理TOAST表是会有问题)
*
* For ANALYZE (no VACUUM): if inside a transaction block, we cannot
* start/commit our own transactions. Also, there's no need to do so if
* only processing one relation. For multiple relations when not within a
* transaction block, and also in an autovacuum worker, use own
* transactions so we can release locks sooner.
* 对于ANALYZE(没有VACUUM选项):如果在事务块中,我们不能start/commit自己的事务.
* 同时,如果只需要处理一个relation,则不需要这样处理.
* 对于不在一个事务块中的多个relations/在autovacuum worker中,
* 使用自己的事务以便更快的释放锁.
*/
if (options & VACOPT_VACUUM)
use_own_xacts = true;
else
{
Assert(options & VACOPT_ANALYZE);
if (IsAutoVacuumWorkerProcess())
use_own_xacts = true;
else if (in_outer_xact)
use_own_xacts = false;
else if (list_length(relations) > 1)
use_own_xacts = true;
else
use_own_xacts = false;
}
/*
* vacuum_rel expects to be entered with no transaction active; it will
* start and commit its own transaction. But we are called by an SQL
* command, and so we are executing inside a transaction already. We
* commit the transaction started in PostgresMain() here, and start
* another one before exiting to match the commit waiting for us back in
* PostgresMain().
* 在进入vacuum_rel前,不希望存在事务活动.该函数会启动和提交自己的事务.
* 但由于我们是通过SQL命令调用的,因此我们已处于事务中执行.
* 在这里我们提交在PostgresMain()中启动的事务,
* 并在退出之前启动另一个，以匹配在PostgresMain()中等待我们的提交。
*/
if (use_own_xacts)
{
Assert(!in_outer_xact);
/* ActiveSnapshot is not set by autovacuum */
//autovacuum不会设置ActiveSnapshot
if (ActiveSnapshotSet())
PopActiveSnapshot();
/* matches the StartTransaction in PostgresMain() */
//匹配PostgresMain()中的StartTransaction
CommitTransactionCommand();
}
/* Turn vacuum cost accounting on or off, and set/clear in_vacuum */
//设置vacuum成本计数on/off,并set/clear in_vacuum参数
PG_TRY();
{
ListCell *cur;
in_vacuum = true;
VacuumCostActive = (VacuumCostDelay > 0);
VacuumCostBalance = 0;
VacuumPageHit = 0;
VacuumPageMiss = 0;
VacuumPageDirty = 0;
/*
* Loop to process each selected relation.
* 循环处理每一个已选中的relation.
*/
foreach(cur, relations)
{
VacuumRelation *vrel = lfirst_node(VacuumRelation, cur);
if (options & VACOPT_VACUUM)
{
//执行vacuum处理
if (!vacuum_rel(vrel->oid, vrel->relation, options, params))
continue;
}
if (options & VACOPT_ANALYZE)
{
/*
* If using separate xacts, start one for analyze. Otherwise,
* we can use the outer transaction.
* 如果使用独立的xacts,为analyze启动一个何事务.
* 否则,我们可以使用外层事务.
*/
if (use_own_xacts)
{
//使用自己的事务
StartTransactionCommand();
/* functions in indexes may want a snapshot set */
//快照压栈
PushActiveSnapshot(GetTransactionSnapshot());
}
//分析relation
analyze_rel(vrel->oid, vrel->relation, options, params,
vrel->va_cols, in_outer_xact, vac_strategy);
if (use_own_xacts)
{
//使用自己的事务,出栈
PopActiveSnapshot();
//提交事务
CommitTransactionCommand();
}
}
}
}
PG_CATCH();
{
in_vacuum = false;
VacuumCostActive = false;
PG_RE_THROW();
}
PG_END_TRY();
in_vacuum = false;
VacuumCostActive = false;
/*
* Finish up processing.
* 完成处理过程
*/
if (use_own_xacts)
{
/* here, we are not in a transaction */
//在这里,没有处于事务中
/*
* This matches the CommitTransaction waiting for us in
* PostgresMain().
* 匹配在PostgresMain()函数中等待我们的CommitTransaction.
*/
StartTransactionCommand();
}
if ((options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
{
/*
* Update pg_database.datfrozenxid, and truncate pg_xact if possible.
* (autovacuum.c does this for itself.)
* 更新pg_database.datfrozenxid,如可能截断pg_xact.
* (autovacuum.c不会处理这事情)
*/
vac_update_datfrozenxid();
}
/*
* Clean up working storage --- note we must do this after
* StartTransactionCommand, else we might be trying to delete the active
* context!
* 清除工作存储 --- 注意必须在StartTransactionCommand命令后执行清除过程,
* 否则我们可能会尝试删除活动的上下文.
*/
MemoryContextDelete(vac_context);
vac_context = NULL;
}三、跟踪分析
测试脚本
17:19:28 (xdb@[local]:5432)testdb=# vacuum t1;
启动gdb,设置断点
(gdb) b vacuum Breakpoint 1 at 0x6b9b8c: file vacuum.c, line 175. (gdb) c Continuing. Breakpoint 1, vacuum (options=1, relations=0x2294988, params=0x7fff403d8880, bstrategy=0x0, isTopLevel=true) at vacuum.c:175 175 Assert(params != NULL); (gdb)
输入参数
options=1 —> VACOPT_VACUUM
relations=0x2294988,relation链表,里面只有一个item,即t1
params=0x7fff403d8880,默认参数
bstrategy=NULL,
isTopLevel=T,为顶层事务
(gdb) p *params
$2 = {freeze_min_age = -1, freeze_table_age = -1, multixact_freeze_min_age = -1, multixact_freeze_table_age = -1,
is_wraparound = false, log_min_duration = -1}
(gdb)变量赋值并执行相关判断
(gdb) n 177 stmttype = (options & VACOPT_VACUUM) ? "VACUUM" : "ANALYZE"; (gdb) 187 if (options & VACOPT_VACUUM) (gdb) 189 PreventInTransactionBlock(isTopLevel, stmttype); (gdb) 190 in_outer_xact = false; (gdb) 200 if (in_vacuum) (gdb) 209 if ((options & VACOPT_FULL) != 0 && (gdb)
统计信息
219 if ((options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess()) (gdb) 220 pgstat_vacuum_stat(); (gdb)
创建并设置内存上下文
(gdb) n 228 vac_context = AllocSetContextCreate(PortalContext, (gdb) 236 if (bstrategy == NULL) (gdb) 238 MemoryContext old_context = MemoryContextSwitchTo(vac_context); (gdb) 240 bstrategy = GetAccessStrategy(BAS_VACUUM); (gdb) 241 MemoryContextSwitchTo(old_context); (gdb) 243 vac_strategy = bstrategy; (gdb) 249 if (relations != NIL) (gdb)
构造VacuumRelation链表
(gdb)
251 List *newrels = NIL;
(gdb)
254 foreach(lc, relations)
(gdb)
256 VacuumRelation *vrel = lfirst_node(VacuumRelation, lc);
(gdb)
260 sublist = expand_vacuum_rel(vrel);
(gdb) p *vrel
$3 = {type = T_VacuumRelation, relation = 0x22948d0, oid = 0, va_cols = 0x0}
(gdb) p *vrel->relation
$4 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x22948b0 "t1", inh = true,
relpersistence = 112 'p', alias = 0x0, location = 7}
(gdb)
(gdb) n
261 old_context = MemoryContextSwitchTo(vac_context);
(gdb)
262 newrels = list_concat(newrels, sublist);
(gdb)
263 MemoryContextSwitchTo(old_context);
(gdb)
254 foreach(lc, relations)
(gdb)
265 relations = newrels;
(gdb)使用自主事务
284 if (options & VACOPT_VACUUM) (gdb) 285 use_own_xacts = true; (gdb) 307 if (use_own_xacts) (gdb) 307 if (use_own_xacts) (gdb) 309 Assert(!in_outer_xact); (gdb) 312 if (ActiveSnapshotSet()) (gdb) 313 PopActiveSnapshot(); (gdb) 316 CommitTransactionCommand(); (gdb) 320 PG_TRY(); (gdb)
开始执行,设置vacuum成本计数on/off,并set/clear in_vacuum参数
(gdb) 324 in_vacuum = true; (gdb) 325 VacuumCostActive = (VacuumCostDelay > 0); (gdb) 326 VacuumCostBalance = 0; (gdb) 327 VacuumPageHit = 0; (gdb) 328 VacuumPageMiss = 0; (gdb) 329 VacuumPageDirty = 0; (gdb)
循环relation,调用vacuum_rel
334 foreach(cur, relations) (gdb) 336 VacuumRelation *vrel = lfirst_node(VacuumRelation, cur); (gdb) 338 if (options & VACOPT_VACUUM) (gdb) 340 if (!vacuum_rel(vrel->oid, vrel->relation, options, params)) (gdb) 344 if (options & VACOPT_ANALYZE) (gdb) 334 foreach(cur, relations) (gdb) 374 PG_END_TRY(); (gdb)
执行收尾工作
(gdb) 376 in_vacuum = false; (gdb) 377 VacuumCostActive = false; (gdb) 382 if (use_own_xacts) (gdb) 390 StartTransactionCommand(); (gdb) 393 if ((options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess()) (gdb) 399 vac_update_datfrozenxid(); (gdb) 407 MemoryContextDelete(vac_context); (gdb) 408 vac_context = NULL; (gdb)
完成调用
409 } (gdb) ExecVacuum (vacstmt=0x22949c0, isTopLevel=true) at vacuum.c:142 142 } (gdb)
到此，相信大家对“PostgreSQL中vacuum过程分析”有了更深的了解，不妨来实际操作一番吧！这里是创新互联网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
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