简介
PostgreSQL查询优化器执行过程
- 语法分析：生成查询树
- 语义检查：对SQL表达的语义进行检查
- 查询优化
- 视图重写
- 逻辑优化：子查询优化，条件化简，等价谓词重写，连接消除，得到逻辑计划
- 物理优化：基于代价优化，得到物理计划。PostgreSQL主要采用动态规划和遗传算法
- 非SPJ优化：主要针对分组，排序，去重等操作
- 查询计划执行
在PostgreSQL中，语法树并不是一棵树状结构的，把关系平面化到一个链表里面。因为，PostgreSQL认为，在这个阶段不清楚表之间如何链接。
重要数据结构
查询语法树
typedef struct Query{ //上面还有节点类型，是否存在相关子句 List *cteList; /* WITH 子句 */ List *rtable; /* list of range table entries */ FromExpr *jointree; /* table join tree (FROM and WHERE clauses) */ List *targetList; /* target list (of TargetEntry) */ List *returningList; /* return-values list (of TargetEntry) */ List *groupClause; /* a list of SortGroupClause's */ Node *havingQual; /* qualifications applied to groups */ List *windowClause; /* 窗口函数子句链表 */ List *distinctClause; /* a list of SortGroupClause's */ List *sortClause; /* a list of SortGroupClause's */ Node *limitOffset; /* limit的offset子句 */ Node *limitCount; /* limit的个数*/ Node *setOperations; /* 是否为多个SQL UNION/INTERSECT/EXCEPT query */} Query; 范围表(优化前)
表示被查询的对象，可以是一张表，一个From子句中的子查询，一个连接操作的结果
typedef struct RangeTblEntry{ //普通表 Oid relid; /* OID of the relation */ char relkind; /* relation kind (see pg_class.relkind) */ struct TableSampleClause *tablesample; /* sampling info, or NULL */ //子查询 Query *subquery; /* the sub-query */ bool security_barrier; /* is from security_barrier view?如果是视图展开的子查询，PostgreSQL不做优化 */ //连接类型 JoinType jointype; /* type of join */ List *joinaliasvars; /* list of alias-var expansions */} RangeTblEntry; 关系优化信息(优化过程中)
对应PlannerInfo结构体的两个成员(simple_rel_array和join_rel_list)，是优化阶段的操作对象，具有查询优化的相关信息
typedef struct RelOptInfo{ /* all relations included in this RelOptInfo */ Relids relids; /* set of base relids (rangetable indexes) */ /* 估算结果的行数 */ double rows; /* materialization information */ List *pathlist; /* 存放所有可能的路径 */ List *ppilist; /* ParamPathInfos used in pathlist */ List *partial_pathlist; /* partial Paths */ /* 局部最优不一定后面最优，上一层的3个可能的最优结果 */ struct Path *cheapest_startup_path; struct Path *cheapest_total_path; struct Path *cheapest_unique_path; List *cheapest_parameterized_paths; //本关系为单表或者join /* used by various scans and joins: */ List *baserestrictinfo; /* RestrictInfo structures (if base * rel) */ QualCost baserestrictcost; /* cost of evaluating the above */ List *joininfo; /* RestrictInfo structures for join clauses * involving this rel */ bool has_eclass_joins; /* T means joininfo is incomplete */ } RelOptInfo; 计划节点信息
全局查询优化计划的相关信息，存放在PlannerInfo结构体
typedef struct PlannerInfo{ Query *parse; /* 开始时的查询计划树 */ PlannerGlobal *glob; /* global info for current planner run */ Index query_level; /* 本计划所处的层数*/ struct PlannerInfo *parent_root; /* NULL at outermost Query */ struct RelOptInfo **simple_rel_array; /* 所有基本表信息 */ int simple_rel_array_size; /* allocated size of array */ RangeTblEntry **simple_rte_array; /* rangetable as an array */ //考虑过连接后生成的新关系 List *join_rel_list; /* list of join-relation RelOptInfos */ struct HTAB *join_rel_hash; /* optional hashtable for join relations */ List **join_rel_level; /*结果关系*/ int join_cur_level; /* index of list being extended */ } PlannerInfo; 计划节点
代表根据最有路径，生成的物理计划(Plan)
typedef struct Plan{ /* * estimated execution costs for plan (see costsize.c for more info) */ Cost startup_cost; /* cost expended before fetching any tuples */ Cost total_cost; /* total cost (assuming all tuples fetched) */ /* * 估计的元组数和元组宽度 */ double plan_rows; int plan_width; /* * Common structural data for all Plan types. */ int plan_node_id; /* unique across entire final plan tree */ List *targetlist; /* target list to be computed at this node */ List *qual; /* implicitly-ANDed qual conditions */ struct Plan *lefttree; /* input plan tree(s) */ struct Plan *righttree; List *initPlan; /* Init Plan nodes (un-correlated expr * subselects) */} Plan; PlannedStmt
优化器结果，保存查询执行计划，范围表相关信息
SelectStmt
语法分析结果
typedef struct SelectStmt{ List *distinctClause; /* distinct子句*/ IntoClause *intoClause; /* target for SELECT INTO */ List *targetList; /* 投影列子句 */ List *fromClause; /* From子句，包括join */ Node *whereClause; /* where子句 */ List *groupClause; /* GROUP BY clauses */ Node *havingClause; /* HAVING conditional-expression */ List *windowClause; /* WINDOW window_name AS (...), ... */ List *sortClause; /* sort clause (a list of SortBy's) */ Node *limitOffset; /* # of result tuples to skip */ Node *limitCount; /* # of result tuples to return */ List *lockingClause; /* FOR UPDATE (list of LockingClause's) */} SelectStmt; 结构体关系
- PlannerInfo是逻辑优化的主要产物，拥有查询树(Query)，关系优化信息，约束条件
- 路径是物理优化阶段的主要产物，拥有排序键和连接节点
- PlannerInfo和路径混杂在一起
- 查询执行计划Plan是所有路径的最小代价生成的
代码入口
planner：主入口函数
PlannedStmt *planner(Query *parse, int cursorOptions, ParamListInfo boundParams){ PlannedStmt *result; if (planner_hook) result = (*planner_hook) (parse, cursorOptions, boundParams); else result = standard_planner(parse, cursorOptions, boundParams); return result;} 被主函数调用
standard_planner——标准的查询优化器入口
standard_planner只是查询优化器的外壳，通过调用subquery_planner完成查询优化，通过调用set_plan_references完成清理辅助工作
PlannedStmt *standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams){ PlannedStmt *result;//结果 PlannerGlobal *glob;//查询优化一些所有子查询需要的公共信息 double tuple_fraction;// PlannerInfo *root; RelOptInfo *final_rel; Path *best_path; Plan *top_plan; ListCell *lp, *lr;··· /* primary planning entry point (may recurse for subqueries) */ root = subquery_planner(glob, parse, NULL, false, tuple_fraction); /* Select best Path and turn it into a Plan */ final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL); best_path = get_cheapest_fractional_path(final_rel, tuple_fraction); top_plan = create_plan(root, best_path);··· /* final cleanup of the plan */ Assert(glob->finalrtable == NIL); Assert(glob->finalrowmarks == NIL); Assert(glob->resultRelations == NIL); top_plan = set_plan_references(root, top_plan); /* ... and the subplans (both regular subplans and initplans) */ Assert(list_length(glob->subplans) == list_length(glob->subroots)); forboth(lp, glob->subplans, lr, glob->subroots) { Plan *subplan = (Plan *) lfirst(lp); PlannerInfo *subroot = (PlannerInfo *) lfirst(lr); lfirst(lp) = set_plan_references(subroot, subplan); }··· /* build the PlannedStmt result */ result = makeNode(PlannedStmt); result->commandType = parse->commandType; result->queryId = parse->queryId; //parse结果 result->planTree = top_plan; //查询计划 result->rtable = glob->finalrtable; //范围表 result->resultRelations = glob->resultRelations; // return result;} - subquery_planner返回逻辑优化和物理优化结果root(PlannerInfo *)
- create_plan根据最优路径，和PlannerInfo生成物理执行计划Plan
- set_plan_references对执行计划部分调整和清理
subquery_planner生成(子)查询执行计划的函数
subquery_planner分为两步。第一步是逻辑优化；第二步是物理优化
//传入glob，parse,typle_fraction，parent_root最开始为NULLPlannerInfo *subquery_planner(PlannerGlobal *glob, Query *parse, PlannerInfo *parent_root, bool hasRecursion, double tuple_fraction){ PlannerInfo *root; List *newWithCheckOptions; List *newHaving; bool hasOuterJoins; RelOptInfo *final_rel; ListCell *l; /* 为当前子查询创建PlannerInfo */ root = makeNode(PlannerInfo); root->parse = parse; root->glob = glob; root->query_level = parent_root ? parent_root->query_level + 1 : 1; root->parent_root = parent_root; root->hasRecursion = hasRecursion; if (hasRecursion) root->wt_param_id = SS_assign_special_param(root); else root->wt_param_id = -1; root->non_recursive_path = NULL; /* * Look for ANY and EXISTS SubLinks in WHERE and JOIN/ON clauses, and try * to transform them into joins. Note that this step does not descend * into subqueries; if we pull up any subqueries below, their SubLinks are * processed just before pulling them up. *子连接：在where和join子句含有ANY和EXISTS */ if (parse->hasSubLinks) pull_up_sublinks(root); //上拉子查询 pull_up_subqueries(root); //子查询合并 if (parse->setOperations) flatten_simple_union_all(root); //上拉子查询后处理继承关系 preprocess_rowmarks(root); expand_inherited_tables(root); //条件化简 preprocess_expression //合并having子句到where子句，如果having里面含有聚集函数，易失函数，子查询不能合并 //消除外连接 if (hasOuterJoins) reduce_outer_joins(root); /* * Do the main planning. If we have an inherited target relation, that * needs special processing, else go straight to grouping_planner. */ if (parse->resultRelation && rt_fetch(parse->resultRelation, parse->rtable)->inh) //含有继承关系的物理优化 inheritance_planner(root); else //物理优化 grouping_planner(root, false, tuple_fraction); set_cheapest(final_rel); return root;} - 逻辑优化
- 处理CTE表达式（ss_process_ctes）
- 上拉子连接
- 上拉子查询
- Union all处理：flatten_simple_union_all
- 处理for update(row lock)：preprocess_rowmark
- 继承表处理(expand_inherited_tables)
- 处理目标列(prepocess_expression)
- 处理withCheckOptions：prepocess_expression
- 处理return 表达式，window子句，limit off子句：prepocess_expression
- 合并having到where子句
- 消除外连接
- 物理优化：生成本成查询PlanInfo的三条最优路径，返回给上层
整体流程
- subquery_planner
- 处理CTE表达式
- 上拉子链接(去除in, some ,exist)
- 上拉子查询
- 预处理表达式(and/or， 计算明显的结果， 处理不能上拉的子连接)
- 消除外连接
- grouping_planer
- 处理集合(生成子查询)
- 处理非集合，调整order, group, target list之句中的顺序
- query_planner
- 构建基本表的RelOptInfo
- 选择下推
- 投影下推
- 推导隐含表达式
- 生成pathkey
- make_one_rel(通过数据直方图计算选择率)
- 处理单表最优的查询方式
- 处理两表join最优的方式(判定是否special join)
- 动态规划或者遗传算法构建多表Join
- 获取cheatest_path，再加上其他子句