PostgreSQL 源码解读（132）- MVCC#16（vacuum过程-lazy_vacuum_index函数#1）

本节简单介绍了postgresql手工执行vacuum的处理流程,主要分析了ExecVacuum->vacuum->vacuum_rel->heap_vacuum_rel->lazy_scan_heap->lazy_vacuum_index函数的实现逻辑,该函数清理index relation。

一、数据结构

宏定义
Vacuum和Analyze命令选项

typedef enum VacuumOption
{
    VACOPT_VACUUM = 1 << 0,     
    VACOPT_ANALYZE = 1 << 1,    
    VACOPT_VERBOSE = 1 << 2,    
    VACOPT_FREEZE = 1 << 3,     
    VACOPT_FULL = 1 << 4,       
    VACOPT_SKIP_LOCKED = 1 << 5,    
    VACOPT_SKIPTOAST = 1 << 6,  
    VACOPT_DISABLE_PAGE_SKIPPING = 1 << 7   
} VacuumOption;

IndexVacuumInfo
传递给ambulkdelete/amvacuumcleanup的输入参数结构体

typedef struct IndexVacuumInfo
{
    //index relation
    Relation    index;          
    //是否只是ANALYZE(没有实际的vacuum)
    bool        analyze_only;   
    //如为T,则num_heap_tuples是一个估算值
    bool        estimated_count;    
    //进度信息的日志等级
    int         message_level;  
    //在堆中仍存在的元组数
    double      num_heap_tuples;    
    //访问策略
    BufferAccessStrategy strategy;  
} IndexVacuumInfo;

IndexBulkDeleteResult
ambulkdelete/amvacuumcleanup返回的统计信息结构体

typedef struct IndexBulkDeleteResult
{
    //index中剩下的pages
    BlockNumber num_pages;      
    //在vacuum期间清除的元组数
    BlockNumber pages_removed;  
    //num_index_tuples是一个估算值?
    bool        estimated_count;    
    //剩余的元组数
    double      num_index_tuples;   
    //在vacuum期间清除的元组数
    double      tuples_removed; 
    //索引中未使用的pages
    BlockNumber pages_deleted;  
    //可重用的pages
    BlockNumber pages_free;     
} IndexBulkDeleteResult;

二、源码解读

lazy_vacuum_index
lazy_vacuum_index清理index relation,删除指向在vacrelstats->dead_tuples元组的索引条目,更新运行时统计信息.
主要逻辑如下:
1.初始化IndexVacuumInfo结构体变量
2.调用index_bulk_delete函数
3.报告进展

static void
lazy_vacuum_index(Relation indrel,
                  IndexBulkDeleteResult **stats,
                  LVRelStats *vacrelstats)
{
    IndexVacuumInfo ivinfo;
    PGRUsage    ru0;
    pg_rusage_init(&ru0);
    ivinfo.index = indrel;
    ivinfo.analyze_only = false;
    ivinfo.estimated_count = true;
    ivinfo.message_level = elevel;
    
    //这里只能提供num_heap_tuples的近似值
    ivinfo.num_heap_tuples = vacrelstats->old_live_tuples;
    ivinfo.strategy = vac_strategy;
    
    //执行批量删除
    *stats = index_bulk_delete(&ivinfo, *stats,
                               lazy_tid_reaped, (void *) vacrelstats);
    ereport(elevel,
            (errmsg("scanned index \"%s\" to remove %d row versions",
                    RelationGetRelationName(indrel),
                    vacrelstats->num_dead_tuples),
             errdetail_internal("%s", pg_rusage_show(&ru0))));
}

lazy_vacuum_index->index_bulk_delete
index_bulk_delete批量删除索引项,回调函数会给出main-heap元组是否将被删除,返回值是已预分配内存的统计信息结构体.

IndexBulkDeleteResult *
index_bulk_delete(IndexVacuumInfo *info,
                  IndexBulkDeleteResult *stats,
                  IndexBulkDeleteCallback callback,
                  void *callback_state)
{
    //获取relation
    Relation    indexRelation = info->index;
    RELATION_CHECKS;
    CHECK_REL_PROCEDURE(ambulkdelete);
    //ambulkdelete指向的实际函数是btbulkdelete
    return indexRelation->rd_indam->ambulkdelete(info, stats,
                                                 callback, callback_state);
}

lazy_vacuum_index->index_bulk_delete->…btbulkdelete
Index Relation的rd_amroutine->ambulkdelete,实际是btbulkdelete函数

IndexBulkDeleteResult *
btbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state)
{
    //relation
    Relation    rel = info->index;
    BTCycleId   cycleid;
    
    //如果是第一次调用,则分配内存,否则重用已存在的结构体
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
    
    
    //建立vacuum循环ID,用于本次扫描
    //PG_ENSURE_ERROR_CLEANUP确保在发生故障时清理共享内存
    PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    {
        TransactionId oldestBtpoXact;//事务ID
        //开始vacuum
        cycleid = _bt_start_vacuum(rel);
        //指向BTree vacuum扫描
        btvacuumscan(info, stats, callback, callback_state, cycleid,
                     &oldestBtpoXact);
        
        _bt_update_meta_cleanup_info(info->index, oldestBtpoXact,
                                     info->num_heap_tuples);
    }
    PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    _bt_end_vacuum(rel);
    //返回统计信息
    return stats;
}

三、跟踪分析

测试脚本 : 删除数据,执行vacuum

10:08:46 (xdb@[local]:5432)testdb=# delete from t1 where id < 1200;
DELETE 100
11:26:03 (xdb@[local]:5432)testdb=# checkpoint;
CHECKPOINT
11:26:04 (xdb@[local]:5432)testdb=# 
11:25:55 (xdb@[local]:5432)testdb=# vacuum t1;

启动gdb,设置断点

(gdb) b lazy_vacuum_index
Breakpoint 1 at 0x6bea40: file vacuumlazy.c, line 1689.
...
Breakpoint 1, lazy_vacuum_index (indrel=0x7f7334825050, stats=0x2aaffb8, vacrelstats=0x2aaf958) at vacuumlazy.c:1689
1689        pg_rusage_init(&ru0);
(gdb)

输入参数

(gdb) p *indrel
$6 = {rd_node = {spcNode = 1663, dbNode = 16402, relNode = 50823}, 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 = 0x7f733491ad20, rd_att = 0x7f733491a9b8, rd_id = 50823, 
  rd_lockInfo = {lockRelId = {relId = 50823, dbId = 16402}}, 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 = 0x7f733491a8d8, rd_indextuple = 0x7f733491a8a0, 
  rd_amhandler = 330, rd_indexcxt = 0x2a05340, rd_amroutine = 0x2a05480, rd_opfamily = 0x2a05598, rd_opcintype = 0x2a055b8, 
  rd_support = 0x2a055d8, rd_supportinfo = 0x2a05600, rd_indoption = 0x2a05738, rd_indexprs = 0x0, rd_indpred = 0x0, 
  rd_exclops = 0x0, rd_exclprocs = 0x0, rd_exclstrats = 0x0, rd_amcache = 0x0, rd_indcollation = 0x2a05718, 
  rd_fdwroutine = 0x0, rd_toastoid = 0, pgstat_info = 0x2a5e198}
(gdb) p *indrel->rd_rel
$9 = {relname = {data = "idx_t1_id", '\000' <repeats 54 times>}, relnamespace = 2200, reltype = 0, reloftype = 0, 
  relowner = 10, relam = 403, relfilenode = 50823, reltablespace = 0, relpages = 60, reltuples = 8901, relallvisible = 0, 
  reltoastrelid = 0, relhasindex = false, relisshared = false, relpersistence = 112 'p', relkind = 105 'i', relnatts = 1, 
  relchecks = 0, relhasoids = false, relhasrules = false, relhastriggers = false, relhassubclass = false, 
  relrowsecurity = false, relforcerowsecurity = false, relispopulated = true, relreplident = 110 'n', 
  relispartition = false, relrewrite = 0, relfrozenxid = 0, relminmxid = 0}
(gdb) p *stats
$7 = (IndexBulkDeleteResult *) 0x0
(gdb) p *vacrelstats
$8 = {hasindex = true, old_rel_pages = 124, rel_pages = 124, scanned_pages = 59, pinskipped_pages = 0, 
  frozenskipped_pages = 1, tupcount_pages = 59, old_live_tuples = 12686, new_rel_tuples = 14444, new_live_tuples = 14444, 
  new_dead_tuples = 0, pages_removed = 0, tuples_deleted = 100, nonempty_pages = 124, num_dead_tuples = 100, 
  max_dead_tuples = 36084, dead_tuples = 0x2ab8820, num_index_scans = 0, latestRemovedXid = 397076, 
  lock_waiter_detected = false}
(gdb)

初始化IndexVacuumInfo结构体

(gdb) n
1691        ivinfo.index = indrel;
(gdb) 
1692        ivinfo.analyze_only = false;
(gdb) 
1693        ivinfo.estimated_count = true;
(gdb) 
1694        ivinfo.message_level = elevel;
(gdb) 
1696        ivinfo.num_heap_tuples = vacrelstats->old_live_tuples;
(gdb) 
1697        ivinfo.strategy = vac_strategy;
(gdb)

调用index_bulk_delete,进入该函数

1700        *stats = index_bulk_delete(&ivinfo, *stats,
(gdb) step
index_bulk_delete (info=0x7fff39c5d620, stats=0x0, callback=0x6bf507 <lazy_tid_reaped>, callback_state=0x2aaf958)
    at indexam.c:748
748     Relation    indexRelation = info->index;
(gdb)

输入参数
info -> IndexVacuumInfo结构体
stats为NULL
回调函数为lazy_tid_reaped
回调函数状态结构体为callback_state

(gdb) p *info
$10 = {index = 0x7f7334825050, analyze_only = false, estimated_count = true, message_level = 13, num_heap_tuples = 12686, 
  strategy = 0x2a9d478}
(gdb) 
(gdb) p *callback_state
Attempt to dereference a generic pointer.
(gdb) 
(gdb) p *info->strategy
$11 = {btype = BAS_VACUUM, ring_size = 32, current = 4, current_was_in_ring = false, buffers = 0x2a9d488}
(gdb)

调用indexRelation->rd_amroutine->ambulkdelete,该函数实际指向的是btbulkdelete

(gdb) n
750     RELATION_CHECKS;
(gdb) 
751     CHECK_REL_PROCEDURE(ambulkdelete);
(gdb) 
753     return indexRelation->rd_amroutine->ambulkdelete(info, stats,
(gdb) p indexRelation->rd_amroutine
$12 = (struct IndexAmRoutine *) 0x2a05480
(gdb) p *indexRelation->rd_amroutine
$13 = {type = T_IndexAmRoutine, amstrategies = 5, amsupport = 3, amcanorder = true, amcanorderbyop = false, 
  amcanbackward = true, amcanunique = true, amcanmulticol = true, amoptionalkey = true, amsearcharray = true, 
  amsearchnulls = true, amstorage = false, amclusterable = true, ampredlocks = true, amcanparallel = true, 
  amcaninclude = true, amkeytype = 0, ambuild = 0x5123f0 <btbuild>, ambuildempty = 0x507e6b <btbuildempty>, 
  aminsert = 0x507f11 <btinsert>, ambulkdelete = 0x5096b6 <btbulkdelete>, amvacuumcleanup = 0x509845 <btvacuumcleanup>, 
  amcanreturn = 0x50a21f <btcanreturn>, amcostestimate = 0x9c5356 <btcostestimate>, amoptions = 0x511cd4 <btoptions>, 
  amproperty = 0x511cfe <btproperty>, amvalidate = 0x51522b <btvalidate>, ambeginscan = 0x5082f7 <btbeginscan>, 
  amrescan = 0x508492 <btrescan>, amgettuple = 0x507f90 <btgettuple>, amgetbitmap = 0x50819e <btgetbitmap>, 
  amendscan = 0x508838 <btendscan>, ammarkpos = 0x508b28 <btmarkpos>, amrestrpos = 0x508d20 <btrestrpos>, 
  amestimateparallelscan = 0x5090e6 <btestimateparallelscan>, aminitparallelscan = 0x5090f1 <btinitparallelscan>, 
  amparallelrescan = 0x50913f <btparallelrescan>}

进入btbulkdelete

(gdb) step
btbulkdelete (info=0x7fff39c5d620, stats=0x0, callback=0x6bf507 <lazy_tid_reaped>, callback_state=0x2aaf958) at nbtree.c:857
857     Relation    rel = info->index;
(gdb)

输入参数参见上述函数输入参数,类似
获取relation,初始化统计信息

857     Relation    rel = info->index;
(gdb) n
861     if (stats == NULL)
(gdb) 
862         stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
(gdb) 
866     PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
(gdb)

获取cycleid

(gdb) n
870         cycleid = _bt_start_vacuum(rel);
(gdb) 
872         btvacuumscan(info, stats, callback, callback_state, cycleid,
(gdb) p cycleid
$14 = 1702
(gdb)

调用btvacuumscan,返回统计信息

(gdb) n
880         _bt_update_meta_cleanup_info(info->index, oldestBtpoXact,
(gdb) 
883     PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
(gdb) 
884     _bt_end_vacuum(rel);
(gdb) 
886     return stats;
(gdb) p *stats
$15 = {num_pages = 60, pages_removed = 0, estimated_count = false, num_index_tuples = 8801, tuples_removed = 100, 
  pages_deleted = 6, pages_free = 6}
(gdb)

DONE!

btvacuumscan下节再行介绍

四、参考资料

PG Source Code