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NULL
PlannerInfo *
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