Postgres中UPDATE更新语句源码分析 Postgres中UPDATE更新语句源码分析

PG中UPDATE源码分析
整体流程分析
解析部分——生成语法解析树UpdateStmt
解析部分——生成查询树Query
优化器——生成执行计划
执行器
事务
总结

PG中UPDATE源码分析本文主要描述SQL中UPDATE语句的源码分析，代码为PG13.3版本。

整体流程分析以

update dtea set id = 1;

这条最简单的Update语句进行源码分析(dtea不是分区表，不考虑并行等，没有建立任何索引)，帮助我们理解update的大致流程。
SQL流程如下：

parser(语法解析，生成语法解析树UpdateStmt，检查是否有语法层面的错误)
analyze(语义分析， UpdateStmt转为查询树Query，会查系统表检查有无语义方面的错误)
rewrite(规则重写, 根据规则rules重写查询树Query，根据事先存储在系统表中的规则进行重写，没有的话不进行重写，另外加一句，视图的实现是根据规则系统实现的，也是在这里需要进行处理)
optimizer(优化器：逻辑优化、物理优化、生成执行计划，由Query生成对应的执行计划PlannedStmt，基于代价的优化器，由最佳路径Path生成最佳执行计划Plan)
executor(执行器，会有各种算子，依据执行计划进行处理，火山模型，一次一元组)
storage(存储引擎)。中间还有事务处理。事务处理部分的代码这里不再进行分析，免得将问题复杂化。存储引擎那部分也不进行分析，重点关注解析、优化、执行这三部分。

对应的代码：

exec_simple_query(const char *query_string)// ------- 解析器部分----------------> pg_parse_query(query_string); //生成语法解析树--> pg_analyze_and_rewrite(parsetree, query_string,NULL, 0, NULL); // 生成查询树Query--> parse_analyze(parsetree, query_string, paramTypes, numParams,queryEnv); // 语义分析--> pg_rewrite_query(query); // 规则重写// --------优化器------------> pg_plan_queries()//-------- 执行器------------> PortalStart(portal, NULL, 0, InvalidSnapshot); --> PortalRun(portal,FETCH_ALL,true,true,receiver,receiver,&qc); // 执行器执行--> PortalDrop(portal, false);

解析部分——生成语法解析树UpdateStmt 关键数据结构：UpdateStmt、RangeVar、ResTarget:

/* Update Statement*/typedef struct UpdateStmt{ NodeTagtype; RangeVar*relation; /* relation to update */ List*targetList; /* the target list (of ResTarget) */ // 对应语句中的set id = 0; 信息在这里 Node*whereClause; /* qualifications */ List*fromClause; /* optional from clause for more tables */ List*returningList; /* list of expressions to return */ WithClause *withClause; /* WITH clause */} UpdateStmt; // dtea 表typedef struct RangeVar{ NodeTagtype; char*catalogname; /* the catalog (database) name, or NULL */ char*schemaname; /* the schema name, or NULL */ char*relname; /* the relation/sequence name */ boolinh; /* expand rel by inheritance? recursively act* on children? */ charrelpersistence; /* see RELPERSISTENCE_* in pg_class.h */ Alias*alias; /* table alias & optional column aliases */ intlocation; /* token location, or -1 if unknown */} RangeVar; // set id = 0; 经transformTargetList() -> transformTargetEntry，会转为TargetEntrytypedef struct ResTarget{ NodeTagtype; char*name; /* column name or NULL */// id column List*indirection; /* subscripts, field names, and '*', or NIL */ Node*val; /* the value expression to compute or assign */// = 1表达式节点存在这里 intlocation; /* token location, or -1 if unknown */} ResTarget;

用户输入的update语句update dtea set id = 1由字符串会转为可由数据库理解的内部数据结构语法解析树UpdateStmt。执行逻辑在

pg_parse_query(query_string);

中，需要理解flex与bison。
gram.y中Update语法的定义：

/***************************************************************************** *QUERY: *UpdateStmt (UPDATE) *****************************************************************************///结合这条语句分析 update dtea set id = 0; UpdateStmt: opt_with_clause UPDATE relation_expr_opt_aliasSET set_clause_list from_clause where_or_current_clause returning_clause{UpdateStmt *n = makeNode(UpdateStmt); n->relation = $3; n->targetList = $5; n->fromClause = $6; n->whereClause = $7; n->returningList = $8; n->withClause = $1; $$ = (Node *)n; }; set_clause_list:set_clause{ $$ = $1; }| set_clause_list ',' set_clause { $$ = list_concat($1,$3); }; // 对应的是 set id = 0set_clause:// id=0set_target '=' a_expr{$1->val = (Node *) $3; $$ = list_make1($1); }| '(' set_target_list ')' '=' a_expr{int ncolumns = list_length($2); int i = 1; ListCell *col_cell; foreach(col_cell, $2) /* Create a MultiAssignRef source for each target */{ResTarget *res_col = (ResTarget *) lfirst(col_cell); MultiAssignRef *r = makeNode(MultiAssignRef); r->source = (Node *) $5; r->colno = i; r->ncolumns = ncolumns; res_col->val = (Node *) r; i++; }$$ = $2; }; set_target:ColId opt_indirection{$$ = makeNode(ResTarget); $$->name = $1; $$->indirection = check_indirection($2, yyscanner); $$->val = NULL; /* upper production sets this */$$->location = @1; }; set_target_list:set_target{ $$ = list_make1($1); }| set_target_list ',' set_target{ $$ = lappend($1,$3); };

解析部分——生成查询树Query 生成了UpdateStmt后，会经由parse_analyze语义分析，生成查询树Query，以供后续优化器生成执行计划。主要代码在src/backent/parser/analyze.c中

analyze.c : transform the raw parse tree into a query tree

parse_analyze()--> transformTopLevelStmt(pstate, parseTree); --> transformOptionalSelectInto(pstate, parseTree->stmt); --> transformStmt(pstate, parseTree); // transforms an update statement--> transformUpdateStmt(pstate, (UpdateStmt *) parseTree); // 实际由UpdateStmt转为Query的处理函数

具体的我们看一下transformUpdateStmt函数实现：

/* transformUpdateStmt -transforms an update statement*/static Query *transformUpdateStmt(ParseState *pstate, UpdateStmt *stmt) { Query*qry = makeNode(Query); ParseNamespaceItem *nsitem; Node*qual; qry->commandType = CMD_UPDATE; pstate->p_is_insert = false; /* process the WITH clause independently of all else */ if (stmt->withClause) {qry->hasRecursive = stmt->withClause->recursive; qry->cteList = transformWithClause(pstate, stmt->withClause); qry->hasModifyingCTE = pstate->p_hasModifyingCTE; } qry->resultRelation = setTargetTable(pstate, stmt->relation, stmt->relation->inh, true, ACL_UPDATE); nsitem = pstate->p_target_nsitem; /* subqueries in FROM cannot access the result relation */ nsitem->p_lateral_only = true; nsitem->p_lateral_ok = false; /* the FROM clause is non-standard SQL syntax. We used to be able to do this with REPLACE in POSTQUEL so we keep the feature.*/ transformFromClause(pstate, stmt->fromClause); /* remaining clauses can reference the result relation normally */ nsitem->p_lateral_only = false; nsitem->p_lateral_ok = true; qual = transformWhereClause(pstate, stmt->whereClause,EXPR_KIND_WHERE, "WHERE"); qry->returningList = transformReturningList(pstate, stmt->returningList); /* Now we are done with SELECT-like processing, and can get on with* transforming the target list to match the UPDATE target columns.*/ qry->targetList = transformUpdateTargetList(pstate, stmt->targetList); // 处理SQL语句中的 set id =1 qry->rtable = pstate->p_rtable; qry->jointree = makeFromExpr(pstate->p_joinlist, qual); qry->hasTargetSRFs = pstate->p_hasTargetSRFs; qry->hasSubLinks = pstate->p_hasSubLinks; assign_query_collations(pstate, qry); return qry; }

这里面要重点关注一下transformTargetList，会将抽象语法树中的ResTarget转为查询器的TargetEntry。

typedef struct TargetEntry{ Exprxpr; Expr*expr; /* expression to evaluate */ AttrNumber resno; /* attribute number (see notes above) */ char*resname; /* name of the column (could be NULL) */ Indexressortgroupref; /* nonzero if referenced by a sort/group clause */ Oidresorigtbl; /* OID of column's source table */ AttrNumber resorigcol; /* column's number in source table */ boolresjunk; /* set to true to eliminate the attribute from final target list */} TargetEntry;

对于其内部处理可参考源码src/backend/parser中的相关处理，这里不再细述。需要重点阅读一下README，PG源码中所有的README都是非常好的资料，一定要认真读。

优化器——生成执行计划这块的内容很多，主要的逻辑是先进行逻辑优化，比如子查询、子链接、常量表达式、选择下推等等的处理，因为我们要分析的这条语句十分简单，所以逻辑优化的这部分都没有涉及到。物理优化，涉及到选择率，代价估计，索引扫描还是顺序扫描，选择那种连接方式，应用动态规划呢还是基因算法，选择nestloop-join、merge-join还是hash-join等。因为我们这个表没有建索引，更新单表也不涉及到多表连接，所以物理优化这块涉及的也不多。路径生成，生成最佳路径，再由最佳路径生成执行计划。
在路径生成这块，最基础的是对表的扫描方式，比如顺序扫描、索引扫描，再往上是连接方式，采用那种连接方式，再往上是比如排序、Limit等路径......，由底向上生成路径。我们要分析的语句很简单，没有其他处理，就顺序扫描再更新就可以了。
这里先不考虑并行执行计划。我们先看一下其执行计划结果：

postgres@postgres=# explain update dtea set id = 0; QUERY PLAN-------------------------------------------------------------- Update on dtea(cost=0.00..19.00 rows=900 width=68)->Seq Scan on dtea(cost=0.00..19.00 rows=900 width=68)(2 rows)

下面我们分析一下其执行计划的生成流程：

// 由查询树Query--> Path --> Plan (PlannedStmt)pg_plan_queries()--> pg_plan_query()--> planner()--> standard_planner(Query *parse, const char *query_string, int cursorOptions,ParamListInfo boundParams)// 由Query---> PlannerInfo--> subquery_planner(glob, parse, NULL,false, tuple_fraction); // 涉及到很多逻辑优化的内容，很多不列出--> pull_up_sublinks(root); --> pull_up_subqueries(root); // 这里只列出几个重要的逻辑优化内容，其他的不再列出......// 如果是update/delete分区表继承表则走inheritance_planner(),其他情况走grouping_planner()--> inheritance_planner()// update/delete分区表继承表的情况--> grouping_planner()--> grouping_planner() // 非分区表、继承表的情况--> preprocess_targetlist(root); // update虽然只更新一列，但是插入一条新元组的时候，需要知道其他列信息.--> rewriteTargetListUD(parse, target_rte, target_relation); --> expand_targetlist()--> query_planner(root, standard_qp_callback, &qp_extra); // 重要--> add_base_rels_to_query()--> deconstruct_jointree(root); --> add_other_rels_to_query(root); // 展开分区表到PlannerInfo中的相关字段中 --> expand_inherited_rtentry()--> expand_planner_arrays(root, num_live_parts); --> make_one_rel(root, joinlist); --> set_base_rel_sizes(root); --> set_rel_size(); --> set_append_rel_size(root, rel, rti, rte); // 如果是分区表或者继承走这里，否则走下面--> set_rel_size(root, childrel, childRTindex, childRTE); // 处理子分区表--> set_plain_rel_size(root, rel, rte); --> set_plain_rel_size()// 如果不是分区表或者继承--> set_baserel_size_estimates()--> set_base_rel_pathlists(root); --> set_rel_pathlist(root, rel, rti, root->simple_rte_array[rti]); --> set_append_rel_pathlist(root, rel, rti, rte); // 生成各分区表的访问路径--> make_rel_from_joinlist(root, joinlist); // 动态规划还是基因规划--> standard_join_search() // 动态规划--> geqo() // 基因规划与动态规划二选一--> apply_scanjoin_target_to_paths()--> create_modifytable_path()// 由PlannerInfo---> RelOptInfo --> fetch_upper_rel(root, UPPERREL_FINAL, NULL); // 由RelOptInfo---> Path--> get_cheapest_fractional_path(final_rel, tuple_fraction); // 由 PlannerInfo+Path---> Plan--> create_plan(root, best_path); // 后续处理，由Plan ---> PlannedStmt

核心数据结构：PlannedStmt、PlannerInfo、RelOptInfo（存储访问路径及其代价）、Path
Path：所有的路径都继承自Path，所以这个比较重要。

typedef struct Path{ NodeTagtype; NodeTagpathtype; /* tag identifying scan/join method */ RelOptInfo *parent; /* the relation this path can build */ PathTarget *pathtarget; /* list of Vars/Exprs, cost, width */ ParamPathInfo *param_info; /* parameterization info, or NULL if none */ boolparallel_aware; /* engage parallel-aware logic? */ boolparallel_safe; /* OK to use as part of parallel plan? */ intparallel_workers; /* desired # of workers; 0 = not parallel */ /* estimated size/costs for path (see costsize.c for more info) */ doublerows; /* estimated number of result tuples */ Coststartup_cost; /* cost expended before fetching any tuples */ Costtotal_cost; /* total cost (assuming all tuples fetched) */ List*pathkeys; /* sort ordering of path's output */ /* pathkeys is a List of PathKey nodes; see above */} Path; /* ModifyTablePath represents performing INSERT/UPDATE/DELETE modifications * We represent most things that will be in the ModifyTable plan node * literally, except we have child Path(s) not Plan(s).But analysis of the * OnConflictExpr is deferred to createplan.c, as is collection of FDW data. */typedef struct ModifyTablePath{ Pathpath; // 可以看到ModifyTablePath继承自Path CmdTypeoperation; /* INSERT, UPDATE, or DELETE */ boolcanSetTag; /* do we set the command tag/es_processed? */ IndexnominalRelation; /* Parent RT index for use of EXPLAIN */ IndexrootRelation; /* Root RT index, if target is partitioned */ boolpartColsUpdated; /* some part key in hierarchy updated */ List*resultRelations; /* integer list of RT indexes */ List*subpaths; /* Path(s) producing source data */ List*subroots; /* per-target-table PlannerInfos */ List*withCheckOptionLists; /* per-target-table WCO lists */ List*returningLists; /* per-target-table RETURNING tlists */ List*rowMarks; /* PlanRowMarks (non-locking only) */ OnConflictExpr *onconflict; /* ON CONFLICT clause, or NULL */ intepqParam; /* ID of Param for EvalPlanQual re-eval */} ModifyTablePath;

生成update执行路径，最终都是要生成ModifyTablePath，本例中路径生成过程：Path-->ProjectionPath-->ModifyTablePath，也就是先顺序扫描表，再修改表。后面由路径生成执行计划。

/* create_modifytable_path *Creates a pathnode that represents performing INSERT/UPDATE/DELETE mods * * 'rel' is the parent relation associated with the result * 'resultRelations' is an integer list of actual RT indexes of target rel(s) * 'subpaths' is a list of Path(s) producing source data (one per rel) * 'subroots' is a list of PlannerInfo structs (one per rel)*/ModifyTablePath *create_modifytable_path(PlannerInfo *root, RelOptInfo *rel,CmdType operation, bool canSetTag,Index nominalRelation, Index rootRelation,bool partColsUpdated,List *resultRelations, List *subpaths,List *subroots,List *withCheckOptionLists, List *returningLists,List *rowMarks, OnConflictExpr *onconflict,int epqParam){ ModifyTablePath *pathnode = makeNode(ModifyTablePath); doubletotal_size; ListCell*lc; Assert(list_length(resultRelations) == list_length(subpaths)); Assert(list_length(resultRelations) == list_length(subroots)); Assert(withCheckOptionLists == NIL || list_length(resultRelations) == list_length(withCheckOptionLists)); Assert(returningLists == NIL || list_length(resultRelations) == list_length(returningLists)); pathnode->path.pathtype = T_ModifyTable; pathnode->path.parent = rel; pathnode->path.pathtarget = rel->reltarget; /* pathtarget is not interesting, just make it minimally valid */ /* For now, assume we are above any joins, so no parameterization */ pathnode->path.param_info = NULL; pathnode->path.parallel_aware = false; pathnode->path.parallel_safe = false; pathnode->path.parallel_workers = 0; pathnode->path.pathkeys = NIL; /** Compute cost & rowcount as sum of subpath costs & rowcounts.** Currently, we don't charge anything extra for the actual table* modification work, nor for the WITH CHECK OPTIONS or RETURNING* expressions if any.It would only be window dressing, since* ModifyTable is always a top-level node and there is no way for the* costs to change any higher-level planning choices.But we might want* to make it look better sometime.*/ pathnode->path.startup_cost = 0; pathnode->path.total_cost = 0; pathnode->path.rows = 0; total_size = 0; foreach(lc, subpaths) {Path*subpath = (Path *) lfirst(lc); if (lc == list_head(subpaths)) /* first node? */pathnode->path.startup_cost = subpath->startup_cost; pathnode->path.total_cost += subpath->total_cost; pathnode->path.rows += subpath->rows; total_size += subpath->pathtarget->width * subpath->rows; } /* Set width to the average width of the subpath outputs.XXX this is* totally wrong: we should report zero if no RETURNING, else an average* of the RETURNING tlist widths.But it's what happened historically,* and improving it is a task for another day.*/ if (pathnode->path.rows > 0)total_size /= pathnode->path.rows; pathnode->path.pathtarget->width = rint(total_size); pathnode->operation = operation; pathnode->canSetTag = canSetTag; pathnode->nominalRelation = nominalRelation; pathnode->rootRelation = rootRelation; pathnode->partColsUpdated = partColsUpdated; pathnode->resultRelations = resultRelations; pathnode->subpaths = subpaths; pathnode->subroots = subroots; pathnode->withCheckOptionLists = withCheckOptionLists; pathnode->returningLists = returningLists; pathnode->rowMarks = rowMarks; pathnode->onconflict = onconflict; pathnode->epqParam = epqParam; return pathnode; }

现在我们生成了最优的update路径，需要由路径生成执行计划：

Plan *create_plan(PlannerInfo *root, Path *best_path){ Plan*plan; Assert(root->plan_params == NIL); /* plan_params should not be in use in current query level */ /* Initialize this module's workspace in PlannerInfo */ root->curOuterRels = NULL; root->curOuterParams = NIL; /* Recursively process the path tree, demanding the correct tlist result */ plan = create_plan_recurse(root, best_path, CP_EXACT_TLIST); // 实际实现是在这里 /** Make sure the topmost plan node's targetlist exposes the original* column names and other decorative info.Targetlists generated within* the planner don't bother with that stuff, but we must have it on the* top-level tlist seen at execution time.However, ModifyTable plan* nodes don't have a tlist matching the querytree targetlist.*/ if (!IsA(plan, ModifyTable))apply_tlist_labeling(plan->targetlist, root->processed_tlist); /** Attach any initPlans created in this query level to the topmost plan* node.(In principle the initplans could go in any plan node at or* above where they're referenced, but there seems no reason to put them* any lower than the topmost node for the query level.Also, see* comments for SS_finalize_plan before you try to change this.)*/ SS_attach_initplans(root, plan); /* Check we successfully assigned all NestLoopParams to plan nodes */ if (root->curOuterParams != NIL)elog(ERROR, "failed to assign all NestLoopParams to plan nodes"); /** Reset plan_params to ensure param IDs used for nestloop params are not re-used later*/ root->plan_params = NIL; return plan; }// 由最佳路径生成最佳执行计划static ModifyTable *create_modifytable_plan(PlannerInfo *root, ModifyTablePath *best_path){ ModifyTable *plan; List*subplans = NIL; ListCell*subpaths,*subroots; /* Build the plan for each input path */ forboth(subpaths, best_path->subpaths, subroots, best_path->subroots) {Path*subpath = (Path *) lfirst(subpaths); PlannerInfo *subroot = (PlannerInfo *) lfirst(subroots); Plan*subplan; /* In an inherited UPDATE/DELETE, reference the per-child modified* subroot while creating Plans from Paths for the child rel.This is* a kluge, but otherwise it's too hard to ensure that Plan creation* functions (particularly in FDWs) don't depend on the contents of* "root" matching what they saw at Path creation time.The main* downside is that creation functions for Plans that might appear* below a ModifyTable cannot expect to modify the contents of "root"* and have it "stick" for subsequent processing such as setrefs.c.* That's not great, but it seems better than the alternative.*/subplan = create_plan_recurse(subroot, subpath, CP_EXACT_TLIST); /* Transfer resname/resjunk labeling, too, to keep executor happy */apply_tlist_labeling(subplan->targetlist, subroot->processed_tlist); subplans = lappend(subplans, subplan); } plan = make_modifytable(root,best_path->operation,best_path->canSetTag,best_path->nominalRelation,best_path->rootRelation,best_path->partColsUpdated,best_path->resultRelations,subplans,best_path->subroots,best_path->withCheckOptionLists,best_path->returningLists,best_path->rowMarks,best_path->onconflict,best_path->epqParam); copy_generic_path_info(&plan->plan, &best_path->path); return plan; }

最终的执行计划是ModifyTable：

/* ---------------- *ModifyTable node - *Apply rows produced by subplan(s) to result table(s), *by inserting, updating, or deleting. * * If the originally named target table is a partitioned table, both * nominalRelation and rootRelation contain the RT index of the partition * root, which is not otherwise mentioned in the plan.Otherwise rootRelation * is zero.However, nominalRelation will always be set, as it's the rel that * EXPLAIN should claim is the INSERT/UPDATE/DELETE target. * * Note that rowMarks and epqParam are presumed to be valid for all the * subplan(s); they can't contain any info that varies across subplans. * ----------------*/typedef struct ModifyTable{ Planplan; CmdTypeoperation; /* INSERT, UPDATE, or DELETE */ boolcanSetTag; /* do we set the command tag/es_processed? */ IndexnominalRelation; /* Parent RT index for use of EXPLAIN */ IndexrootRelation; /* Root RT index, if target is partitioned */ boolpartColsUpdated; /* some part key in hierarchy updated */ List*resultRelations; /* integer list of RT indexes */ intresultRelIndex; /* index of first resultRel in plan's list */ introotResultRelIndex; /* index of the partitioned table root */ List*plans; /* plan(s) producing source data */ List*withCheckOptionLists; /* per-target-table WCO lists */ List*returningLists; /* per-target-table RETURNING tlists */ List*fdwPrivLists; /* per-target-table FDW private data lists */ Bitmapset*fdwDirectModifyPlans; /* indices of FDW DM plans */ List*rowMarks; /* PlanRowMarks (non-locking only) */ intepqParam; /* ID of Param for EvalPlanQual re-eval */ OnConflictAction onConflictAction; /* ON CONFLICT action */ List*arbiterIndexes; /* List of ON CONFLICT arbiter index OIDs*/ List*onConflictSet; /* SET for INSERT ON CONFLICT DO UPDATE */ Node*onConflictWhere; /* WHERE for ON CONFLICT UPDATE */ IndexexclRelRTI; /* RTI of the EXCLUDED pseudo relation */ List*exclRelTlist; /* tlist of the EXCLUDED pseudo relation */} ModifyTable;

执行器根据上面的执行计划，去执行。主要是各种算子的实现，其中要理解执行器的运行原理，主要是火山模型，一次一元组。我们看一下其调用过程。

CreatePortal("", true, true); PortalDefineQuery(portal,NULL,query_string,commandTag,plantree_list,NULL); PortalStart(portal, NULL, 0, InvalidSnapshot); PortalRun(portal,FETCH_ALL,true,true,receiver,receiver,&qc); --> PortalRunMulti() --> ProcessQuery()--> ExecutorStart(queryDesc, 0); --> standard_ExecutorStart()--> estate = CreateExecutorState(); // 创建EState--> estate->es_output_cid = GetCurrentCommandId(true); // 获得cid，后面更新的时候要用--> InitPlan(queryDesc, eflags); --> ExecInitNode(plan, estate, eflags); --> ExecInitModifyTable() // 初始化ModifyTableState--> ExecutorRun(queryDesc, ForwardScanDirection, 0L, true); --> standard_ExecutorRun()--> ExecutePlan()--> ExecProcNode(planstate); // 一次一元组火山模型--> node->ExecProcNode(node); --> ExecProcNodeFirst(PlanState *node)--> node->ExecProcNode(node); --> ExecModifyTable(PlanState *pstate)--> ExecUpdate()--> table_tuple_update(Relation rel, ......)--> rel->rd_tableam->tuple_update()--> heapam_tuple_update(Relation relation, ......)--> heap_update(relation, otid, tuple, cid, ......)--> ExecutorFinish(queryDesc); --> ExecutorEnd(queryDesc); PortalDrop(portal, false);

关键数据结构：

// ModifyTableState informationtypedef struct ModifyTableState{ PlanState ps; /* its first field is NodeTag */ CmdTypeoperation; /* INSERT, UPDATE, or DELETE */ boolcanSetTag; /* do we set the command tag/es_processed? */ boolmt_done; /* are we done? */ PlanState **mt_plans; /* subplans (one per target rel) */ intmt_nplans; /* number of plans in the array */ intmt_whichplan; /* which one is being executed (0..n-1) */ TupleTableSlot **mt_scans; /* input tuple corresponding to underlying* plans */ ResultRelInfo *resultRelInfo; /* per-subplan target relations */ ResultRelInfo *rootResultRelInfo; /* root target relation (partitioned* table root) */ List**mt_arowmarks; /* per-subplan ExecAuxRowMark lists */ EPQState mt_epqstate; /* for evaluating EvalPlanQual rechecks */ boolfireBSTriggers; /* do we need to fire stmt triggers? */ /* Slot for storing tuples in the root partitioned table's rowtype during* an UPDATE of a partitioned table. */ TupleTableSlot *mt_root_tuple_slot; struct PartitionTupleRouting *mt_partition_tuple_routing; /* Tuple-routing support info */ struct TransitionCaptureState *mt_transition_capture; /* controls transition table population for specified operation */ /* controls transition table population for INSERT...ON CONFLICT UPDATE */ struct TransitionCaptureState *mt_oc_transition_capture; /* Per plan map for tuple conversion from child to root */ TupleConversionMap **mt_per_subplan_tupconv_maps; } ModifyTableState;

核心执行算子实现：

/* ---------------------------------------------------------------- *ExecModifyTable * *Perform table modifications as required, and return RETURNING results *if needed. * ---------------------------------------------------------------- */static TupleTableSlot *ExecModifyTable(PlanState *pstate){ ModifyTableState *node = castNode(ModifyTableState, pstate); PartitionTupleRouting *proute = node->mt_partition_tuple_routing; EState*estate = node->ps.state; CmdTypeoperation = node->operation; ResultRelInfo *saved_resultRelInfo; ResultRelInfo *resultRelInfo; PlanState*subplanstate; JunkFilter *junkfilter; TupleTableSlot *slot; TupleTableSlot *planSlot; ItemPointer tupleid; ItemPointerData tuple_ctid; HeapTupleData oldtupdata; HeapTuple oldtuple; CHECK_FOR_INTERRUPTS(); /* This should NOT get called during EvalPlanQual; we should have passed a* subplan tree to EvalPlanQual, instead.Use a runtime test not just* Assert because this condition is easy to miss in testing. */ if (estate->es_epq_active != NULL)elog(ERROR, "ModifyTable should not be called during EvalPlanQual"); /* If we've already completed processing, don't try to do more.We need* this test because ExecPostprocessPlan might call us an extra time, and* our subplan's nodes aren't necessarily robust against being called* extra times.*/ if (node->mt_done)return NULL; /* On first call, fire BEFORE STATEMENT triggers before proceeding.*/ if (node->fireBSTriggers) {fireBSTriggers(node); node->fireBSTriggers = false; } /* Preload local variables */ resultRelInfo = node->resultRelInfo + node->mt_whichplan; subplanstate = node->mt_plans[node->mt_whichplan]; junkfilter = resultRelInfo->ri_junkFilter; /* es_result_relation_info must point to the currently active result relation while we are within this ModifyTable node.* Even though ModifyTable nodes can't be nested statically, they can be nested* dynamically (since our subplan could include a reference to a modifying* CTE).So we have to save and restore the caller's value.*/ saved_resultRelInfo = estate->es_result_relation_info; estate->es_result_relation_info = resultRelInfo; /* Fetch rows from subplan(s), and execute the required table modification for each row.*/ for (; ; ) {/* Reset the per-output-tuple exprcontext.This is needed because* triggers expect to use that context as workspace.It's a bit ugly* to do this below the top level of the plan, however.We might need to rethink this later.*/ResetPerTupleExprContext(estate); /* Reset per-tuple memory context used for processing on conflict and* returning clauses, to free any expression evaluation storage allocated in the previous cycle. */if (pstate->ps_ExprContext)ResetExprContext(pstate->ps_ExprContext); planSlot = ExecProcNode(subplanstate); if (TupIsNull(planSlot)){/* advance to next subplan if any */node->mt_whichplan++; // 分区表的update，每个分区分布对应一个subplan，当执行完一个分区再执行下一个分区if (node->mt_whichplan < node->mt_nplans){resultRelInfo++; subplanstate = node->mt_plans[node->mt_whichplan]; junkfilter = resultRelInfo->ri_junkFilter; estate->es_result_relation_info = resultRelInfo; EvalPlanQualSetPlan(&node->mt_epqstate, subplanstate->plan, node->mt_arowmarks[node->mt_whichplan]); /* Prepare to convert transition tuples from this child. */if (node->mt_transition_capture != NULL) {node->mt_transition_capture->tcs_map = tupconv_map_for_subplan(node, node->mt_whichplan); }if (node->mt_oc_transition_capture != NULL) {node->mt_oc_transition_capture->tcs_map = tupconv_map_for_subplan(node, node->mt_whichplan); }continue; }elsebreak; }/* Ensure input tuple is the right format for the target relation.*/if (node->mt_scans[node->mt_whichplan]->tts_ops != planSlot->tts_ops) {ExecCopySlot(node->mt_scans[node->mt_whichplan], planSlot); planSlot = node->mt_scans[node->mt_whichplan]; }/* If resultRelInfo->ri_usesFdwDirectModify is true, all we need to do here is compute the RETURNING expressions.*/if (resultRelInfo->ri_usesFdwDirectModify){Assert(resultRelInfo->ri_projectReturning); slot = ExecProcessReturning(resultRelInfo->ri_projectReturning, RelationGetRelid(resultRelInfo->ri_RelationDesc), NULL, planSlot); estate->es_result_relation_info = saved_resultRelInfo; return slot; }EvalPlanQualSetSlot(&node->mt_epqstate, planSlot); slot = planSlot; tupleid = NULL; oldtuple = NULL; if (junkfilter != NULL){/* extract the 'ctid' or 'wholerow' junk attribute.*/if (operation == CMD_UPDATE || operation == CMD_DELETE){charrelkind; Datumdatum; boolisNull; relkind = resultRelInfo->ri_RelationDesc->rd_rel->relkind; if (relkind == RELKIND_RELATION || relkind == RELKIND_MATVIEW){datum = ExecGetJunkAttribute(slot,junkfilter->jf_junkAttNo,&isNull); /* shouldn't ever get a null result... */if (isNull)elog(ERROR, "ctid is NULL"); tupleid = (ItemPointer) DatumGetPointer(datum); tuple_ctid = *tupleid; /* be sure we don't free ctid!! */tupleid = &tuple_ctid; }/* Use the wholerow attribute, when available, to reconstruct the old relation tuple.*/else if (AttributeNumberIsValid(junkfilter->jf_junkAttNo)){datum = ExecGetJunkAttribute(slot,junkfilter->jf_junkAttNo,&isNull); /* shouldn't ever get a null result... */if (isNull)elog(ERROR, "wholerow is NULL"); oldtupdata.t_data = https://www.it610.com/article/DatumGetHeapTupleHeader(datum); oldtupdata.t_len = HeapTupleHeaderGetDatumLength(oldtupdata.t_data); ItemPointerSetInvalid(&(oldtupdata.t_self)); /* Historically, view triggers see invalid t_tableOid. */oldtupdata.t_tableOid = (relkind == RELKIND_VIEW) ? InvalidOid : RelationGetRelid(resultRelInfo->ri_RelationDesc); oldtuple = &oldtupdata; }elseAssert(relkind == RELKIND_FOREIGN_TABLE); }/* apply the junkfilter if needed. */if (operation != CMD_DELETE)slot = ExecFilterJunk(junkfilter, slot); }switch (operation){case CMD_INSERT:if (proute)/* Prepare for tuple routing if needed. */slot = ExecPrepareTupleRouting(node, estate, proute, resultRelInfo, slot); slot = ExecInsert(node, slot, planSlot, NULL, estate->es_result_relation_info, estate, node->canSetTag); if (proute)/* Revert ExecPrepareTupleRouting's state change. */estate->es_result_relation_info = resultRelInfo; break; case CMD_UPDATE:slot = ExecUpdate(node, tupleid, oldtuple, slot, planSlot,&node->mt_epqstate, estate, node->canSetTag); break; case CMD_DELETE:slot = ExecDelete(node, tupleid, oldtuple, planSlot,&node->mt_epqstate, estate,true, node->canSetTag, false /* changingPart */ , NULL, NULL); break; default:elog(ERROR, "unknown operation"); break; }/* If we got a RETURNING result, return it to caller.We'll continue the work on next call.*/if (slot) {estate->es_result_relation_info = saved_resultRelInfo; return slot; } } estate->es_result_relation_info = saved_resultRelInfo; /* Restore es_result_relation_info before exiting */ fireASTriggers(node); /* We're done, but fire AFTER STATEMENT triggers before exiting.*/ node->mt_done = true; return NULL; }

我们看一下具体执行Update的实现

```c++/* ---------------------------------------------------------------- *ExecUpdate * *note: we can't run UPDATE queries with transactions off because UPDATEs are actually INSERTs and our *scan will mistakenly loop forever, updating the tuple it just inserted..This should be fixed but until it *is, we don't want to get stuck in an infinite loop which corrupts your database.. * *When updating a table, tupleid identifies the tuple to update and oldtuple is NULL. * *Returns RETURNING result if any, otherwise NULL. * ----------------------------------------------------------------*/static TupleTableSlot *ExecUpdate(ModifyTableState *mtstate,ItemPointer tupleid,HeapTuple oldtuple,TupleTableSlot *slot,TupleTableSlot *planSlot,EPQState *epqstate,EState *estate,bool canSetTag){ ResultRelInfo *resultRelInfo; Relation resultRelationDesc; TM_Result result; TM_FailureData tmfd; List*recheckIndexes = NIL; TupleConversionMap *saved_tcs_map = NULL; /* abort the operation if not running transactions*/ if (IsBootstrapProcessingMode())elog(ERROR, "cannot UPDATE during bootstrap"); ExecMaterializeSlot(slot); /* get information on the (current) result relation*/ resultRelInfo = estate->es_result_relation_info; resultRelationDesc = resultRelInfo->ri_RelationDesc; /* BEFORE ROW UPDATE Triggers */ if (resultRelInfo->ri_TrigDesc && resultRelInfo->ri_TrigDesc->trig_update_before_row) {if (!ExecBRUpdateTriggers(estate, epqstate, resultRelInfo, tupleid, oldtuple, slot))return NULL; /* "do nothing" */ } /* INSTEAD OF ROW UPDATE Triggers */ if (resultRelInfo->ri_TrigDesc && resultRelInfo->ri_TrigDesc->trig_update_instead_row) {if (!ExecIRUpdateTriggers(estate, resultRelInfo, oldtuple, slot))return NULL; /* "do nothing" */ } else if (resultRelInfo->ri_FdwRoutine) {/* Compute stored generated columns*/if (resultRelationDesc->rd_att->constr && resultRelationDesc->rd_att->constr->has_generated_stored)ExecComputeStoredGenerated(estate, slot, CMD_UPDATE); /* update in foreign table: let the FDW do it*/slot = resultRelInfo->ri_FdwRoutine->ExecForeignUpdate(estate, resultRelInfo, slot, planSlot); if (slot == NULL)/* "do nothing" */return NULL; /* AFTER ROW Triggers or RETURNING expressions might reference the* tableoid column, so (re-)initialize tts_tableOid before evaluating them. */slot->tts_tableOid = RelationGetRelid(resultRelationDesc); } else {LockTupleMode lockmode; boolpartition_constraint_failed; boolupdate_indexes; /* Constraints might reference the tableoid column, so (re-)initialize* tts_tableOid before evaluating them.*/slot->tts_tableOid = RelationGetRelid(resultRelationDesc); /* Compute stored generated columns*/if (resultRelationDesc->rd_att->constr && resultRelationDesc->rd_att->constr->has_generated_stored)ExecComputeStoredGenerated(estate, slot, CMD_UPDATE); /** Check any RLS UPDATE WITH CHECK policies** If we generate a new candidate tuple after EvalPlanQual testing, we* must loop back here and recheck any RLS policies and constraints.* (We don't need to redo triggers, however.If there are any BEFORE* triggers then trigger.c will have done table_tuple_lock to lock the* correct tuple, so there's no need to do them again.) */lreplace:; /* ensure slot is independent, consider e.g. EPQ */ExecMaterializeSlot(slot); /* If partition constraint fails, this row might get moved to another* partition, in which case we should check the RLS CHECK policy just* before inserting into the new partition, rather than doing it here.* This is because a trigger on that partition might again change the* row.So skip the WCO checks if the partition constraint fails. */partition_constraint_failed = resultRelInfo->ri_PartitionCheck && !ExecPartitionCheck(resultRelInfo, slot, estate, false); if (!partition_constraint_failed && resultRelInfo->ri_WithCheckOptions != NIL){/* ExecWithCheckOptions() will skip any WCOs which are not of the kind we are looking for at this point. */ExecWithCheckOptions(WCO_RLS_UPDATE_CHECK, resultRelInfo, slot, estate); }/* If a partition check failed, try to move the row into the right partition.*/if (partition_constraint_failed){booltuple_deleted; TupleTableSlot *ret_slot; TupleTableSlot *orig_slot = slot; TupleTableSlot *epqslot = NULL; PartitionTupleRouting *proute = mtstate->mt_partition_tuple_routing; intmap_index; TupleConversionMap *tupconv_map; /* Disallow an INSERT ON CONFLICT DO UPDATE that causes the* original row to migrate to a different partition.Maybe this* can be implemented some day, but it seems a fringe feature with* little redeeming value.*/if (((ModifyTable *) mtstate->ps.plan)->onConflictAction == ONCONFLICT_UPDATE)ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("invalid ON UPDATE specification"),errdetail("The result tuple would appear in a different partition than the original tuple."))); /* When an UPDATE is run on a leaf partition, we will not have* partition tuple routing set up. In that case, fail with* partition constraint violation error.*/if (proute == NULL)ExecPartitionCheckEmitError(resultRelInfo, slot, estate); /* Row movement, part 1.Delete the tuple, but skip RETURNING* processing. We want to return rows from INSERT.*/ExecDelete(mtstate, tupleid, oldtuple, planSlot, epqstate, estate, false, false /* canSetTag */ , true /* changingPart */ , &tuple_deleted, &epqslot); /* For some reason if DELETE didn't happen (e.g. trigger prevented* it, or it was already deleted by self, or it was concurrently* deleted by another transaction), then we should skip the insert* as well; otherwise, an UPDATE could cause an increase in the* total number of rows across all partitions, which is clearly wrong.** For a normal UPDATE, the case where the tuple has been the* subject of a concurrent UPDATE or DELETE would be handled by* the EvalPlanQual machinery, but for an UPDATE that we've* translated into a DELETE from this partition and an INSERT into* some other partition, that's not available, because CTID chains* can't span relation boundaries.We mimic the semantics to a* limited extent by skipping the INSERT if the DELETE fails to* find a tuple. This ensures that two concurrent attempts to* UPDATE the same tuple at the same time can't turn one tuple* into two, and that an UPDATE of a just-deleted tuple can't resurrect it.*/if (!tuple_deleted){/** epqslot will be typically NULL.But when ExecDelete()* finds that another transaction has concurrently updated the* same row, it re-fetches the row, skips the delete, and* epqslot is set to the re-fetched tuple slot. In that case,* we need to do all the checks again.*/if (TupIsNull(epqslot))return NULL; else{slot = ExecFilterJunk(resultRelInfo->ri_junkFilter, epqslot); goto lreplace; }}/* Updates set the transition capture map only when a new subplan* is chosen.But for inserts, it is set for each row. So after* INSERT, we need to revert back to the map created for UPDATE; * otherwise the next UPDATE will incorrectly use the one created* for INSERT.So first save the one created for UPDATE. */if (mtstate->mt_transition_capture)saved_tcs_map = mtstate->mt_transition_capture->tcs_map; /* resultRelInfo is one of the per-subplan resultRelInfos.So we* should convert the tuple into root's tuple descriptor, since* ExecInsert() starts the search from root.The tuple conversion* map list is in the order of mtstate->resultRelInfo[], so to* retrieve the one for this resultRel, we need to know the* position of the resultRel in mtstate->resultRelInfo[]. */map_index = resultRelInfo - mtstate->resultRelInfo; Assert(map_index >= 0 && map_index < mtstate->mt_nplans); tupconv_map = tupconv_map_for_subplan(mtstate, map_index); if (tupconv_map != NULL)slot = execute_attr_map_slot(tupconv_map->attrMap, slot, mtstate->mt_root_tuple_slot); /* Prepare for tuple routing, making it look like we're inserting into the root. */Assert(mtstate->rootResultRelInfo != NULL); slot = ExecPrepareTupleRouting(mtstate, estate, proute, mtstate->rootResultRelInfo, slot); ret_slot = ExecInsert(mtstate, slot, planSlot,orig_slot, resultRelInfo,estate, canSetTag); /* Revert ExecPrepareTupleRouting's node change. */estate->es_result_relation_info = resultRelInfo; if (mtstate->mt_transition_capture){mtstate->mt_transition_capture->tcs_original_insert_tuple = NULL; mtstate->mt_transition_capture->tcs_map = saved_tcs_map; }return ret_slot; }/* Check the constraints of the tuple.We've already checked the* partition constraint above; however, we must still ensure the tuple* passes all other constraints, so we will call ExecConstraints() and* have it validate all remaining checks.*/if (resultRelationDesc->rd_att->constr)ExecConstraints(resultRelInfo, slot, estate); /* replace the heap tuple** Note: if es_crosscheck_snapshot isn't InvalidSnapshot, we check* that the row to be updated is visible to that snapshot, and throw a* can't-serialize error if not. This is a special-case behavior* needed for referential integrity updates in transaction-snapshot mode transactions. */result = table_tuple_update(resultRelationDesc, tupleid, slot, estate->es_output_cid,estate->es_snapshot, estate->es_crosscheck_snapshot, true /* wait for commit */ ,&tmfd, &lockmode, &update_indexes); switch (result){case TM_SelfModified:/* The target tuple was already updated or deleted by the* current command, or by a later command in the current* transaction.The former case is possible in a join UPDATE* where multiple tuples join to the same target tuple. This* is pretty questionable, but Postgres has always allowed it:* we just execute the first update action and ignore* additional update attempts.** The latter case arises if the tuple is modified by a* command in a BEFORE trigger, or perhaps by a command in a* volatile function used in the query.In such situations we* should not ignore the update, but it is equally unsafe to* proceed.We don't want to discard the original UPDATE* while keeping the triggered actions based on it; and we* have no principled way to merge this update with the* previous ones.So throwing an error is the only safe* course.** If a trigger actually intends this type of interaction, it* can re-execute the UPDATE (assuming it can figure out how)* and then return NULL to cancel the outer update.*/if (tmfd.cmax != estate->es_output_cid)ereport(ERROR,(errcode(ERRCODE_TRIGGERED_DATA_CHANGE_VIOLATION),errmsg("tuple to be updated was already modified by an operation triggered by the current command"),errhint("Consider using an AFTER trigger instead of a BEFORE trigger to propagate changes to other rows."))); /* Else, already updated by self; nothing to do */return NULL; case TM_Ok:break; case TM_Updated:{TupleTableSlot *inputslot; TupleTableSlot *epqslot; if (IsolationUsesXactSnapshot())ereport(ERROR,(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),errmsg("could not serialize access due to concurrent update"))); /* Already know that we're going to need to do EPQ, so fetch tuple directly into the right slot. */inputslot = EvalPlanQualSlot(epqstate, resultRelationDesc,resultRelInfo->ri_RangeTableIndex); result = table_tuple_lock(resultRelationDesc, tupleid, estate->es_snapshot,inputslot, estate->es_output_cid, lockmode, LockWaitBlock, TUPLE_LOCK_FLAG_FIND_LAST_VERSION,&tmfd); switch (result){case TM_Ok:Assert(tmfd.traversed); epqslot = EvalPlanQual(epqstate, resultRelationDesc, resultRelInfo->ri_RangeTableIndex, inputslot); if (TupIsNull(epqslot))/* Tuple not passing quals anymore, exiting... */return NULL; slot = ExecFilterJunk(resultRelInfo->ri_junkFilter, epqslot); goto lreplace; case TM_Deleted:/* tuple already deleted; nothing to do */return NULL; case TM_SelfModified:/** This can be reached when following an update chain from a tuple updated by another session,* reaching a tuple that was already updated in this transaction. If previously modified by* this command, ignore the redundant update, otherwise error out.** See also TM_SelfModified response to table_tuple_update() above.*/if (tmfd.cmax != estate->es_output_cid)ereport(ERROR,(errcode(ERRCODE_TRIGGERED_DATA_CHANGE_VIOLATION),errmsg("tuple to be updated was already modified by an operation triggered by the current command"),errhint("Consider using an AFTER trigger instead of a BEFORE trigger to propagate changes to other rows."))); return NULL; default:/* see table_tuple_lock call in ExecDelete() */elog(ERROR, "unexpected table_tuple_lock status: %u", result); return NULL; }}break; case TM_Deleted:if (IsolationUsesXactSnapshot())ereport(ERROR,(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),errmsg("could not serialize access due to concurrent delete"))); /* tuple already deleted; nothing to do */return NULL; default:elog(ERROR, "unrecognized table_tuple_update status: %u",result); return NULL; }/* insert index entries for tuple if necessary */if (resultRelInfo->ri_NumIndices > 0 && update_indexes)recheckIndexes = ExecInsertIndexTuples(slot, estate, false, NULL, NIL); } if (canSetTag)(estate->es_processed)++; /* AFTER ROW UPDATE Triggers */ ExecARUpdateTriggers(estate, resultRelInfo, tupleid, oldtuple, slot,recheckIndexes,mtstate->operation == CMD_INSERT ?mtstate->mt_oc_transition_capture : mtstate->mt_transition_capture); list_free(recheckIndexes); /* Check any WITH CHECK OPTION constraints from parent views.We are* required to do this after testing all constraints and uniqueness* violations per the SQL spec, so we do it after actually updating the* record in the heap and all indexes.** ExecWithCheckOptions() will skip any WCOs which are not of the kind we* are looking for at this point. */ if (resultRelInfo->ri_WithCheckOptions != NIL)ExecWithCheckOptions(WCO_VIEW_CHECK, resultRelInfo, slot, estate); if (resultRelInfo->ri_projectReturning) /* Process RETURNING if present */return ExecProcessReturning(resultRelInfo->ri_projectReturning,RelationGetRelid(resultRelationDesc),slot, planSlot); return NULL; }

再往下就是涉及到存储引擎的部分了，我们重点看一下其对外的接口输入参数。重点是这4个参数：

relation - table to be modified (caller must hold suitable lock) （要更新的那个表）
otid - TID of old tuple to be replaced （要更新的元组ID，对应的是老的元组，更新后相当于是插入一条新元组，老元组的tid值要更新为新的tid值）
slot - newly constructed tuple data to store （新元组的值）
cid - update command ID (used for visibility test, and stored into cmax/cmin if successful) （cid值，事务相关）执行器层面的更新算子是建立在存储引擎提供的底层table_tuple_update接口之上的。是我们编写ExecUpdate以及ExecModifyTable的基础。

/* * Update a tuple. * Input parameters: * relation - table to be modified (caller must hold suitable lock) * otid - TID of old tuple to be replaced * slot - newly constructed tuple data to store * cid - update command ID (used for visibility test, and stored into cmax/cmin if successful) * crosscheck - if not InvalidSnapshot, also check old tuple against this * wait - true if should wait for any conflicting update to commit/abort * Output parameters: * tmfd - filled in failure cases (see below) * lockmode - filled with lock mode acquired on tuple *update_indexes - in success cases this is set to true if new index entries are required for this tuple * * Normal, successful return value is TM_Ok, which means we did actually update it. */static inline TM_Resulttable_tuple_update(Relation rel, ItemPointer otid, TupleTableSlot *slot, CommandId cid, Snapshot snapshot, Snapshot crosscheck, bool wait, TM_FailureData *tmfd, LockTupleMode *lockmode, bool *update_indexes){ return rel->rd_tableam->tuple_update(rel, otid, slot, cid, snapshot, crosscheck, wait, tmfd, lockmode, update_indexes); }

事务这一块主要是要理解PG中update语句并不是原地更新元组，而是插入一条新元组。因为PG实现MVCC与Mysql，Oracle的实现方式有所不同，并不是通过undo日志实现的，相当于把undo日志记录到了原有的表中，并不是单独存放在一个地方。具体的不再细述，内容太多了，以后再分析事务部分。
好了，内容很多，分析源码的时候，涉及到的知识点以及逻辑是非常多的，我们最好每次分析只抓一个主干，不然每个都分析，最后就会比较乱。就先分析到这里吧。

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