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這篇文章主要介紹Postgres中UPDATE更新語句怎么用，文中介紹的非常詳細，具有一定的參考價值，感興趣的小伙伴們一定要看完！
本文主要描述SQL中UPDATE語句的源碼分析，代碼為PG13.3版本。
以update dtea set id = 1;這條最簡單的Update語句進行源碼分析(dtea不是分區表，不考慮并行等，沒有建立任何索引)，幫助我們理解update的大致流程。
SQL流程如下：
parser(語法解析，生成語法解析樹UpdateStmt，檢查是否有語法層面的錯誤)
analyze(語義分析， UpdateStmt轉為查詢樹Query， 會查系統表檢查有無語義方面的錯誤)
rewrite(規則重寫, 根據規則rules重寫查詢樹Query， 根據事先存儲在系統表中的規則進行重寫，沒有的話不進行重寫，另外加一句，視圖的實現是根據規則系統實現的，也是在這里需要進行處理)
optimizer(優化器：邏輯優化、物理優化、生成執行計劃， 由Query生成對應的執行計劃PlannedStmt， 基于代價的優化器，由最佳路徑Path生成最佳執行計劃Plan)
executor(執行器，會有各種算子，依據執行計劃進行處理，火山模型，一次一元組)
storage(存儲引擎)。中間還有事務處理。事務處理部分的代碼這里不再進行分析，免得將問題復雜化。存儲引擎那部分也不進行分析，重點關注解析、優化、執行這三部分。
對應的代碼：
exec_simple_query(const char *query_string) // ------- 解析器部分-------------- --> pg_parse_query(query_string); //生成語法解析樹 --> pg_analyze_and_rewrite(parsetree, query_string,NULL, 0, NULL); // 生成查詢樹Query --> parse_analyze(parsetree, query_string, paramTypes, numParams,queryEnv); // 語義分析 --> pg_rewrite_query(query); // 規則重寫 // --------優化器---------- --> pg_plan_queries() //-------- 執行器---------- --> PortalStart(portal, NULL, 0, InvalidSnapshot); --> PortalRun(portal,FETCH_ALL,true,true,receiver,receiver,&qc); // 執行器執行 --> PortalDrop(portal, false);
關鍵數據結構：UpdateStmt、RangeVar、ResTarget:
/* Update Statement */
typedef struct UpdateStmt
{
NodeTag type;
RangeVar *relation; /* relation to update */
List *targetList; /* the target list (of ResTarget) */ // 對應語句中的set id = 0;信息在這里
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
{
NodeTag type;
char *catalogname; /* the catalog (database) name, or NULL */
char *schemaname; /* the schema name, or NULL */
char *relname; /* the relation/sequence name */
bool inh; /* expand rel by inheritance? recursively act
* on children? */
char relpersistence; /* see RELPERSISTENCE_* in pg_class.h */
Alias *alias; /* table alias & optional column aliases */
int location; /* token location, or -1 if unknown */
} RangeVar;
// set id = 0; 經transformTargetList() -> transformTargetEntry，會轉為TargetEntry
typedef struct ResTarget
{
NodeTag type;
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表達式節點存在這里
int location; /* token location, or -1 if unknown */
} ResTarget;用戶輸入的update語句update dtea set id = 1由字符串會轉為可由數據庫理解的內部數據結構語法解析樹UpdateStmt。執行邏輯在pg_parse_query(query_string);中，需要理解flex與bison。
gram.y中Update語法的定義：
/*****************************************************************************
* QUERY:
* UpdateStmt (UPDATE)
*****************************************************************************/
//結合這條語句分析 update dtea set id = 0;
UpdateStmt: opt_with_clause UPDATE relation_expr_opt_alias
SET 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 { $$ = list_concat($1,$3); }
;
// 對應的是 set id = 0
set_clause: // id = 0
set_target '=' a_expr
{
$1->val = (Node *) $3;
$$ = 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); }
;生成了UpdateStmt后， 會經由parse_analyze語義分析，生成查詢樹Query，以供后續優化器生成執行計劃。主要代碼在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); // 實際由UpdateStmt轉為Query的處理函數
具體的我們看一下transformUpdateStmt函數實現：
/* 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); // 處理SQL語句中的 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;
}這里面要重點關注一下transformTargetList，會將抽象語法樹中的ResTarget轉為查詢器的TargetEntry。
typedef struct TargetEntry
{
Expr xpr;
Expr *expr; /* expression to evaluate */
AttrNumber resno; /* attribute number (see notes above) */
char *resname; /* name of the column (could be NULL) */
Index ressortgroupref; /* nonzero if referenced by a sort/group clause */
Oid resorigtbl; /* OID of column's source table */
AttrNumber resorigcol; /* column's number in source table */
bool resjunk; /* set to true to eliminate the attribute from final target list */
} TargetEntry;對于其內部處理可參考源碼
src/backend/parser中的相關處理，這里不再細述。需要重點閱讀一下README，PG源碼中所有的README都是非常好的資料，一定要認真讀。
這塊的內容很多，主要的邏輯是先進行邏輯優化，比如子查詢、子鏈接、常量表達式、選擇下推等等的處理，因為我們要分析的這條語句十分簡單，所以邏輯優化的這部分都沒有涉及到。物理優化，涉及到選擇率，代價估計，索引掃描還是順序掃描，選擇那種連接方式，應用動態規劃呢還是基因算法，選擇nestloop-join、merge-join還是hash-join等。因為我們這個表沒有建索引，更新單表也不涉及到多表連接，所以物理優化這塊涉及的也不多。路徑生成，生成最佳路徑，再由最佳路徑生成執行計劃。
在路徑生成這塊，最基礎的是對表的掃描方式，比如順序掃描、索引掃描，再往上是連接方式，采用那種連接方式，再往上是比如排序、Limit等路徑......，由底向上生成路徑。我們要分析的語句很簡單，沒有其他處理，就順序掃描再更新就可以了。
這里先不考慮并行執行計劃。我們先看一下其執行計劃結果：
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)
下面我們分析一下其執行計劃的生成流程：
// 由查詢樹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); // 涉及到很多邏輯優化的內容，很多不列出 --> pull_up_sublinks(root); --> pull_up_subqueries(root); // 這里只列出幾個重要的邏輯優化內容，其他的不再列出...... // 如果是update/delete分區表繼承表則走inheritance_planner(),其他情況走grouping_planner() --> inheritance_planner() // update/delete分區表繼承表的情況 --> grouping_planner() --> grouping_planner() // 非分區表、繼承表的情況 --> preprocess_targetlist(root); // update雖然只更新一列，但是插入一條新元組的時候，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); // 展開分區表到PlannerInfo中的相關字段中 --> 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); // 如果是分區表或者繼承走這里，否則走下面 --> set_rel_size(root, childrel, childRTindex, childRTE); // 處理子分區表 --> set_plain_rel_size(root, rel, rte); --> set_plain_rel_size() // 如果不是分區表或者繼承 --> 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); // 生成各分區表的訪問路徑 --> make_rel_from_joinlist(root, joinlist);// 動態規劃還是基因規劃 --> standard_join_search() // 動態規劃 --> geqo() // 基因規劃與動態規劃二選一 --> 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); // 后續處理，由Plan ---> PlannedStmt
核心數據結構：PlannedStmt、PlannerInfo、RelOptInfo（存儲訪問路徑及其代價）、Path
Path：所有的路徑都繼承自Path，所以這個比較重要。
typedef struct Path
{
NodeTag type;
NodeTag pathtype; /* 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 */
bool parallel_aware; /* engage parallel-aware logic? */
bool parallel_safe; /* OK to use as part of parallel plan? */
int parallel_workers; /* desired # of workers; 0 = not parallel */
/* estimated size/costs for path (see costsize.c for more info) */
double rows; /* estimated number of result tuples */
Cost startup_cost; /* cost expended before fetching any tuples */
Cost total_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
{
Path path; // 可以看到ModifyTablePath繼承自Path
CmdType operation; /* INSERT, UPDATE, or DELETE */
bool canSetTag; /* do we set the command tag/es_processed? */
Index nominalRelation; /* Parent RT index for use of EXPLAIN */
Index rootRelation; /* Root RT index, if target is partitioned */
bool partColsUpdated; /* 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 */
int epqParam; /* ID of Param for EvalPlanQual re-eval */
} ModifyTablePath;生成update執行路徑，最終都是要生成ModifyTablePath，本例中路徑生成過程：Path-->ProjectionPath-->ModifyTablePath，也就是先順序掃描表，再修改表。后面由路徑生成執行計劃。
/* 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);
double total_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;
}現在我們生成了最優的update路徑，需要由路徑生成執行計劃：
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); // 實際實現是在這里
/** 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;
}
// 由最佳路徑生成最佳執行計劃
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;
}最終的執行計劃是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
{
Plan plan;
CmdType operation; /* INSERT, UPDATE, or DELETE */
bool canSetTag; /* do we set the command tag/es_processed? */
Index nominalRelation; /* Parent RT index for use of EXPLAIN */
Index rootRelation; /* Root RT index, if target is partitioned */
bool partColsUpdated; /* some part key in hierarchy updated */
List *resultRelations; /* integer list of RT indexes */
int resultRelIndex; /* index of first resultRel in plan's list */
int rootResultRelIndex; /* 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) */
int epqParam; /* 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 */
Index exclRelRTI; /* RTI of the EXCLUDED pseudo relation */
List *exclRelTlist; /* tlist of the EXCLUDED pseudo relation */
} ModifyTable;根據上面的執行計劃，去執行。主要是各種算子的實現，其中要理解執行器的運行原理，主要是火山模型，一次一元組。我們看一下其調用過程。
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(); // 創建EState
--> estate->es_output_cid = GetCurrentCommandId(true); // 獲得cid，后面更新的時候要用
--> InitPlan(queryDesc, eflags);
--> ExecInitNode(plan, estate, eflags);
--> ExecInitModifyTable() // 初始化ModifyTableState
--> ExecutorRun(queryDesc, ForwardScanDirection, 0L, true);
--> standard_ExecutorRun()
--> ExecutePlan()
--> ExecProcNode(planstate); // 一次一元組 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);關鍵數據結構：
// ModifyTableState information
typedef struct ModifyTableState
{
PlanState ps; /* its first field is NodeTag */
CmdType operation; /* INSERT, UPDATE, or DELETE */
bool canSetTag; /* do we set the command tag/es_processed? */
bool mt_done; /* are we done? */
PlanState **mt_plans; /* subplans (one per target rel) */
int mt_nplans; /* number of plans in the array */
int mt_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 */
bool fireBSTriggers; /* 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;核心執行算子實現：
/* ----------------------------------------------------------------
* 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;
CmdType operation = 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++; // 分區表的update，每個分區分布對應一個subplan，當執行完一個分區再執行下一個分區
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;
}
else
break;
}
/* 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)
{
char relkind;
Datum datum;
bool isNull;
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 = 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;
}
else
Assert(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;
}我們看一下具體執行Update的實現
```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;
bool partition_constraint_failed;
bool update_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)
{
bool tuple_deleted;
TupleTableSlot *ret_slot;
TupleTableSlot *orig_slot = slot;
TupleTableSlot *epqslot = NULL;
PartitionTupleRouting *proute = mtstate->mt_partition_tuple_routing;
int map_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儲引擎的部分了，我們重點看一下其對外的接口輸入參數。重點是這4個參數：
relation - table to be modified (caller must hold suitable lock) （要更新的那個表）
otid - TID of old tuple to be replaced （要更新的元組ID，對應的是老的元組，更新后相當于是插入一條新元組，老元組的tid值要更新為新的tid值）
slot - newly constructed tuple data to store （新元組的值）
cid - update command ID (used for visibility test, and stored into cmax/cmin if successful) （cid值，事務相關） 執行器層面的更新算子是建立在存儲引擎提供的底層table_tuple_update接口之上的。是我們編寫ExecUpdate以及ExecModifyTable的基礎。
/*
* 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_Result
table_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);
}這一塊主要是要理解PG中update語句并不是原地更新元組，而是插入一條新元組。因為PG實現MVCC與Mysql，Oracle的實現方式有所不同，并不是通過undo日志實現的，相當于把undo日志記錄到了原有的表中，并不是單獨存放在一個地方。具體的不再細述，內容太多了，以后再分析事務部分。
好了，內容很多，分析源碼的時候，涉及到的知識點以及邏輯是非常多的，我們最好每次分析只抓一個主干，不然每個都分析，最后就會比較亂。就先分析到這里吧。
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