PostgreSQL源码解读（41）-查询语句#26（query_planner函数#4）
上一小节介绍了函数query_planner中子函数add_base_rels_to_query的实现逻辑,本节继续介绍其中的子函数build_base_rel_tlists/find_placeholders_in_jointree/find_lateral_references,这几个子函数的目的仍然是完善RelOptInfo结构体信息。
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query_planner代码片段:
//...
/*
* Examine the targetlist and join tree, adding entries to baserel
* targetlists for all referenced Vars, and generating PlaceHolderInfo
* entries for all referenced PlaceHolderVars. Restrict and join clauses
* are added to appropriate lists belonging to the mentioned relations. We
* also build EquivalenceClasses for provably equivalent expressions. The
* SpecialJoinInfo list is also built to hold information about join order
* restrictions. Finally, we form a target joinlist for make_one_rel() to
* work from.
*/
build_base_rel_tlists(root, tlist);//构建"base rels"的投影列
find_placeholders_in_jointree(root);//处理jointree中的PHI
find_lateral_references(root);//处理jointree中Lateral依赖
//...
一、重要的数据结构
PlannerInfo
PlannerInfo贯穿整个构建查询计划的全过程.
build_base_rel_tlists、find_placeholders_in_jointree和find_lateral_references函数完善了PlannerInfo->placeholder_list链表.
/*----------
* PlannerInfo
* Per-query information for planning/optimization
*
* This struct is conventionally called "root" in all the planner routines.
* It holds links to all of the planner's working state, in addition to the
* original Query. Note that at present the planner extensively modifies
* the passed-in Query data structure; someday that should stop.
*----------
*/
struct AppendRelInfo;
typedef struct PlannerInfo
{
NodeTag type;//Node标识
Query *parse; /* 查询树,the Query being planned */
PlannerGlobal *glob; /* 当前的planner全局信息,global info for current planner run */
Index query_level; /* 查询层次,1标识最高层,1 at the outermost Query */
struct PlannerInfo *parent_root; /* 如为子计划,则这里存储父计划器指针,NULL标识最高层,NULL at outermost Query */
/*
* plan_params contains the expressions that this query level needs to
* make available to a lower query level that is currently being planned.
* outer_params contains the paramIds of PARAM_EXEC Params that outer
* query levels will make available to this query level.
*/
List *plan_params; /* list of PlannerParamItems, see below */
Bitmapset *outer_params;
/*
* simple_rel_array holds pointers to "base rels" and "other rels" (see
* comments for RelOptInfo for more info). It is indexed by rangetable
* index (so entry 0 is always wasted). Entries can be NULL when an RTE
* does not correspond to a base relation, such as a join RTE or an
* unreferenced view RTE; or if the RelOptInfo hasn't been made yet.
*/
/* RelOptInfo数组,存储"base rels",比如基表/子查询等.该数组与RTE的顺序一一对应,而且是从1开始,因此[0]无用 */
struct RelOptInfo **simple_rel_array; /* All 1-rel RelOptInfos */
int simple_rel_array_size; /* 数组大小,allocated size of array */
/*
* simple_rte_array is the same length as simple_rel_array and holds
* pointers to the associated rangetable entries. This lets us avoid
* rt_fetch(), which can be a bit slow once large inheritance sets have
* been expanded.
*/
RangeTblEntry **simple_rte_array; /* RTE数组,rangetable as an array */
/*
* append_rel_array is the same length as the above arrays, and holds
* pointers to the corresponding AppendRelInfo entry indexed by
* child_relid, or NULL if none. The array itself is not allocated if
* append_rel_list is empty.
*/
struct AppendRelInfo **append_rel_array;//先前已介绍,在处理集合操作如UNION ALL时使用
/*
* all_baserels is a Relids set of all base relids (but not "other"
* relids) in the query; that is, the Relids identifier of the final join
* we need to form. This is computed in make_one_rel, just before we
* start making Paths.
*/
Relids all_baserels;//"base rels"
/*
* nullable_baserels is a Relids set of base relids that are nullable by
* some outer join in the jointree; these are rels that are potentially
* nullable below the WHERE clause, SELECT targetlist, etc. This is
* computed in deconstruct_jointree.
*/
Relids nullable_baserels;//Nullable-side端的"base rels"
/*
* join_rel_list is a list of all join-relation RelOptInfos we have
* considered in this planning run. For small problems we just scan the
* list to do lookups, but when there are many join relations we build a
* hash table for faster lookups. The hash table is present and valid
* when join_rel_hash is not NULL. Note that we still maintain the list
* even when using the hash table for lookups; this simplifies life for
* GEQO.
*/
List *join_rel_list; /* 参与连接的Relation的RelOptInfo链表,list of join-relation RelOptInfos */
struct HTAB *join_rel_hash; /* 可加快链表访问的hash表,optional hashtable for join relations */
/*
* When doing a dynamic-programming-style join search, join_rel_level[k]
* is a list of all join-relation RelOptInfos of level k, and
* join_cur_level is the current level. New join-relation RelOptInfos are
* automatically added to the join_rel_level[join_cur_level] list.
* join_rel_level is NULL if not in use.
*/
List **join_rel_level; /* RelOptInfo指针链表数组,k层的join存储在[k]中,lists of join-relation RelOptInfos */
int join_cur_level; /* 当前的join层次,index of list being extended */
List *init_plans; /* 查询的初始化计划链表,init SubPlans for query */
List *cte_plan_ids; /* CTE子计划ID链表,per-CTE-item list of subplan IDs */
List *multiexpr_params; /* List of Lists of Params for MULTIEXPR
* subquery outputs */
List *eq_classes; /* 活动的等价类链表,list of active EquivalenceClasses */
List *canon_pathkeys; /* 规范化PathKey链表,list of "canonical" PathKeys */
List *left_join_clauses; /* 外连接约束条件链表(左),list of RestrictInfos for mergejoinable
* outer join clauses w/nonnullable var on
* left */
List *right_join_clauses; /* 外连接约束条件链表(右),list of RestrictInfos for mergejoinable
* outer join clauses w/nonnullable var on
* right */
List *full_join_clauses; /* 全连接约束条件链表,list of RestrictInfos for mergejoinable
* full join clauses */
List *join_info_list; /* 特殊连接信息链表,list of SpecialJoinInfos */
List *append_rel_list; /* AppendRelInfo链表,list of AppendRelInfos */
List *rowMarks; /* list of PlanRowMarks */
List *placeholder_list; /* PHI链表,list of PlaceHolderInfos */
List *fkey_list; /* 外键信息链表,list of ForeignKeyOptInfos */
List *query_pathkeys; /* uery_planner()要求的PathKeys,desired pathkeys for query_planner() */
List *group_pathkeys; /* groupClause pathkeys, if any */
List *window_pathkeys; /* pathkeys of bottom window, if any */
List *distinct_pathkeys; /* distinctClause pathkeys, if any */
List *sort_pathkeys; /* sortClause pathkeys, if any */
List *part_schemes; /* 已规范化的分区Schema,Canonicalised partition schemes used in the
* query. */
List *initial_rels; /* 尝试连接的RelOptInfo链表,RelOptInfos we are now trying to join */
/* Use fetch_upper_rel() to get any particular upper rel */
List *upper_rels[UPPERREL_FINAL + 1]; /* 上层的RelOptInfo链表, upper-rel RelOptInfos */
/* Result tlists chosen by grouping_planner for upper-stage processing */
struct PathTarget *upper_targets[UPPERREL_FINAL + 1];//
/*
* grouping_planner passes back its final processed targetlist here, for
* use in relabeling the topmost tlist of the finished Plan.
*/
List *processed_tlist;//最后需处理的投影列
/* Fields filled during create_plan() for use in setrefs.c */
AttrNumber *grouping_map; /* for GroupingFunc fixup */
List *minmax_aggs; /* List of MinMaxAggInfos */
MemoryContext planner_cxt; /* 内存上下文,context holding PlannerInfo */
double total_table_pages; /* 所有的pages,# of pages in all tables of query */
double tuple_fraction; /* query_planner输入参数:元组处理比例,tuple_fraction passed to query_planner */
double limit_tuples; /* query_planner输入参数:limit_tuples passed to query_planner */
Index qual_security_level; /* 表达式的最新安全等级,minimum security_level for quals */
/* Note: qual_security_level is zero if there are no securityQuals */
InheritanceKind inhTargetKind; /* indicates if the target relation is an
* inheritance child or partition or a
* partitioned table */
bool hasJoinRTEs; /* 存在RTE_JOIN的RTE,true if any RTEs are RTE_JOIN kind */
bool hasLateralRTEs; /* 存在标记为LATERAL的RTE,true if any RTEs are marked LATERAL */
bool hasDeletedRTEs; /* 存在已在jointree删除的RTE,true if any RTE was deleted from jointree */
bool hasHavingQual; /* 存在Having子句,true if havingQual was non-null */
bool hasPseudoConstantQuals; /* true if any RestrictInfo has
* pseudoconstant = true */
bool hasRecursion; /* 递归语句,true if planning a recursive WITH item */
/* These fields are used only when hasRecursion is true: */
int wt_param_id; /* PARAM_EXEC ID for the work table */
struct Path *non_recursive_path; /* a path for non-recursive term */
/* These fields are workspace for createplan.c */
Relids curOuterRels; /* outer rels above current node */
List *curOuterParams; /* not-yet-assigned NestLoopParams */
/* optional private data for join_search_hook, e.g., GEQO */
void *join_search_private;
/* Does this query modify any partition key columns? */
bool partColsUpdated;
} PlannerInfo;
RelOptInfo
RelOptInfo结构体贯彻逻辑优化和物理优化过程的始终.
build_base_rel_tlists完善了结构体中attr_needed和reltarget变量,find_lateral_references函数完善了结构体中lateral_vars变量.
typedef struct RelOptInfo
{
NodeTag type;//节点标识
RelOptKind reloptkind;//RelOpt类型
/* all relations included in this RelOptInfo */
Relids relids; /*Relids(rtindex)集合 set of base relids (rangetable indexes) */
/* size estimates generated by planner */
double rows; /*结果元组的估算数量 estimated number of result tuples */
/* per-relation planner control flags */
bool consider_startup; /*是否考虑启动成本?是,需要保留启动成本低的路径 keep cheap-startup-cost paths? */
bool consider_param_startup; /*是否考虑参数化?的路径 ditto, for parameterized paths? */
bool consider_parallel; /*是否考虑并行处理路径 consider parallel paths? */
/* default result targetlist for Paths scanning this relation */
struct PathTarget *reltarget; /*扫描该Relation时默认的结果 list of Vars/Exprs, cost, width */
/* materialization information */
List *pathlist; /*访问路径链表 Path structures */
List *ppilist; /*路径链表中使用参数化路径进行 ParamPathInfos used in pathlist */
List *partial_pathlist; /* partial Paths */
struct Path *cheapest_startup_path;//代价最低的启动路径
struct Path *cheapest_total_path;//代价最低的整体路径
struct Path *cheapest_unique_path;//代价最低的获取唯一值的路径
List *cheapest_parameterized_paths;//代价最低的参数化?路径链表
/* parameterization information needed for both base rels and join rels */
/* (see also lateral_vars and lateral_referencers) */
Relids direct_lateral_relids; /*使用lateral语法,需依赖的Relids rels directly laterally referenced */
Relids lateral_relids; /* minimum parameterization of rel */
/* information about a base rel (not set for join rels!) */
//reloptkind=RELOPT_BASEREL时使用的数据结构
Index relid; /* Relation ID */
Oid reltablespace; /* 表空间 containing tablespace */
RTEKind rtekind; /* 基表?子查询?还是函数等等?RELATION, SUBQUERY, FUNCTION, etc */
AttrNumber min_attr; /* 最小的属性编号 smallest attrno of rel (often <0) */
AttrNumber max_attr; /* 最大的属性编号 largest attrno of rel */
Relids *attr_needed; /* 数组 array indexed [min_attr .. max_attr] */
int32 *attr_widths; /* 属性宽度 array indexed [min_attr .. max_attr] */
List *lateral_vars; /* 关系依赖的Vars/PHVs LATERAL Vars and PHVs referenced by rel */
Relids lateral_referencers; /*依赖该关系的Relids rels that reference me laterally */
List *indexlist; /* 该关系的IndexOptInfo链表 list of IndexOptInfo */
List *statlist; /* 统计信息链表 list of StatisticExtInfo */
BlockNumber pages; /* 块数 size estimates derived from pg_class */
double tuples; /* 元组数 */
double allvisfrac; /* ? */
PlannerInfo *subroot; /* 如为子查询,存储子查询的root if subquery */
List *subplan_params; /* 如为子查询,存储子查询的参数 if subquery */
int rel_parallel_workers; /* 并行执行,需要多少个workers? wanted number of parallel workers */
/* Information about foreign tables and foreign joins */
//FWD相关信息
Oid serverid; /* identifies server for the table or join */
Oid userid; /* identifies user to check access as */
bool useridiscurrent; /* join is only valid for current user */
/* use "struct FdwRoutine" to avoid including fdwapi.h here */
struct FdwRoutine *fdwroutine;
void *fdw_private;
/* cache space for remembering if we have proven this relation unique */
//已知的,可保证唯一的Relids链表
List *unique_for_rels; /* known unique for these other relid
* set(s) */
List *non_unique_for_rels; /* 已知的,不唯一的Relids链表 known not unique for these set(s) */
/* used by various scans and joins: */
List *baserestrictinfo; /* 如为基本关系,存储约束条件 RestrictInfo structures (if base rel) */
QualCost baserestrictcost; /* 解析约束表达式的成本? cost of evaluating the above */
Index baserestrict_min_security; /* 最低安全等级 min security_level found in
* baserestrictinfo */
List *joininfo; /* 连接语句的约束条件信息 RestrictInfo structures for join clauses
* involving this rel */
bool has_eclass_joins; /* 是否存在等价类连接? T means joininfo is incomplete */
/* used by partitionwise joins: */
bool consider_partitionwise_join; /* 分区? consider partitionwise
* join paths? (if
* partitioned rel) */
Relids top_parent_relids; /* Relids of topmost parents (if "other"
* rel) */
/* used for partitioned relations */
//分区表使用
PartitionScheme part_scheme; /* 分区的schema Partitioning scheme. */
int nparts; /* 分区数 number of partitions */
struct PartitionBoundInfoData *boundinfo; /* 分区边界信息 Partition bounds */
List *partition_qual; /* 分区约束 partition constraint */
struct RelOptInfo **part_rels; /* 分区的RelOptInfo数组 Array of RelOptInfos of partitions,
* stored in the same order of bounds */
List **partexprs; /* 非空分区键表达式 Non-nullable partition key expressions. */
List **nullable_partexprs; /* 可为空的分区键表达式 Nullable partition key expressions. */
List *partitioned_child_rels; /* RT Indexes链表 List of RT indexes. */
} RelOptInfo;
二、源码解读
基本概念
PlaceHolder
PlaceHolder即占位符,常用于减少SQL的parse过程提高性能.
如JDBC中常用的PreparedStatement:
String sql = "select * from t_dwxx where dwbh = ? and dwmc = ?";
PreparedStatement pstmt = connection.preparestatement(sql);
pstmt.setstring(1,'1001');
pstmt.setstring(2,'测试');
resultset rs = ps.executequery();
可以认为,其中的?所代表的是占位符.
在psql中,使用set命令定义变量,在SQL语句中使用占位符:
testdb=# \set v1 '\'1001\''
testdb=# select * from t_dwxx where dwbh = :v1;
dwmc | dwbh | dwdz
-----------+------+--------------------
X有限公司 | 1001 | 广东省广州市荔湾区
(1 row)
build_base_rel_tlists
/*
******************************** build_base_rel_tlists *****************
*/
/*
* build_base_rel_tlists
* Add targetlist entries for each var needed in the query's final tlist
* (and HAVING clause, if any) to the appropriate base relations.
*
* 把最终的投影列信息添加到合适的"base rels"中.
*
* We mark such vars as needed by "relation 0" to ensure that they will
* propagate up through all join plan steps.
*/
void
build_base_rel_tlists(PlannerInfo *root, List *final_tlist)
{
List *tlist_vars = pull_var_clause((Node *) final_tlist,
PVC_RECURSE_AGGREGATES |
PVC_RECURSE_WINDOWFUNCS |
PVC_INCLUDE_PLACEHOLDERS);//获取投影列
if (tlist_vars != NIL)
{
//添加到相应的Relation's targetlist(如不存在)
//标记其为连接或者最终输出所需要
add_vars_to_targetlist(root, tlist_vars, bms_make_singleton(0), true);
list_free(tlist_vars);
}
/*
* If there's a HAVING clause, we'll need the Vars it uses, too. Note
* that HAVING can contain Aggrefs but not WindowFuncs.
*/
if (root->parse->havingQual)//如存在Having子句,顶层的Having是在查询语句的后期才执行,需保留需要的Vars
{
List *having_vars = pull_var_clause(root->parse->havingQual,
PVC_RECURSE_AGGREGATES |
PVC_INCLUDE_PLACEHOLDERS);
if (having_vars != NIL)
{
add_vars_to_targetlist(root, having_vars,
bms_make_singleton(0), true);
list_free(having_vars);
}
}
}
/*
* add_vars_to_targetlist
* For each variable appearing in the list, add it to the owning
* relation's targetlist if not already present, and mark the variable
* as being needed for the indicated join (or for final output if
* where_needed includes "relation 0").
*
* The list may also contain PlaceHolderVars. These don't necessarily
* have a single owning relation; we keep their attr_needed info in
* root->placeholder_list instead. If create_new_ph is true, it's OK
* to create new PlaceHolderInfos; otherwise, the PlaceHolderInfos must
* already exist, and we should only update their ph_needed. (This should
* be true before deconstruct_jointree begins, and false after that.)
*/
void
add_vars_to_targetlist(PlannerInfo *root, List *vars,
Relids where_needed, bool create_new_ph)
{
ListCell *temp;
Assert(!bms_is_empty(where_needed));
foreach(temp, vars)
{
Node *node = (Node *) lfirst(temp);
if (IsA(node, Var))
{
Var *var = (Var *) node;//属性Var
RelOptInfo *rel = find_base_rel(root, var->varno);//找到相应的RelOptInfo
int attno = var->varattno;//属性编号
if (bms_is_subset(where_needed, rel->relids))//where_needed是否rel的子集?
continue;//是,继续循环
Assert(attno >= rel->min_attr && attno <= rel->max_attr);
attno -= rel->min_attr;
if (rel->attr_needed[attno] == NULL)
{
/* Variable not yet requested, so add to rel's targetlist */
/* XXX is copyObject necessary here? */
rel->reltarget->exprs = lappend(rel->reltarget->exprs,
copyObject(var));//TODO,添加到rel->reltarget->exprs 中
/* reltarget cost and width will be computed later */
}
rel->attr_needed[attno] = bms_add_members(rel->attr_needed[attno],
where_needed);//where_needed添加到bitmapset中
}
else if (IsA(node, PlaceHolderVar))
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
PlaceHolderInfo *phinfo = find_placeholder_info(root, phv,
create_new_ph);
phinfo->ph_needed = bms_add_members(phinfo->ph_needed,
where_needed);//添加PHV
}
else
elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
}
}
find_placeholders_in_jointree
/*
******************************** find_placeholders_in_jointree *****************
*/
/*
* find_placeholders_in_jointree
* Search the jointree for PlaceHolderVars, and build PlaceHolderInfos
*
* 搜索jointree中的PHV,并且构建PHInfos
*
* We don't need to look at the targetlist because build_base_rel_tlists()
* will already have made entries for any PHVs in the tlist.
*
* This is called before we begin deconstruct_jointree. Once we begin
* deconstruct_jointree, all active placeholders must be present in
* root->placeholder_list, because make_outerjoininfo and
* update_placeholder_eval_levels require this info to be available
* while we crawl up the join tree.
*/
void
find_placeholders_in_jointree(PlannerInfo *root)
{
/* We need do nothing if the query contains no PlaceHolderVars */
if (root->glob->lastPHId != 0)
{
/* Start recursion at top of jointree */
Assert(root->parse->jointree != NULL &&
IsA(root->parse->jointree, FromExpr));
find_placeholders_recurse(root, (Node *) root->parse->jointree);//递归处理
}
}
/*
* find_placeholders_recurse
* One recursion level of find_placeholders_in_jointree.
*
* jtnode is the current jointree node to examine.
*/
static void
find_placeholders_recurse(PlannerInfo *root, Node *jtnode)
{
if (jtnode == NULL)
return;
if (IsA(jtnode, RangeTblRef))//RTR
{
/* No quals to deal with here */
}
else if (IsA(jtnode, FromExpr))//FromExpr
{
FromExpr *f = (FromExpr *) jtnode;
ListCell *l;
/*
* First, recurse to handle child joins.
*/
foreach(l, f->fromlist)
{
find_placeholders_recurse(root, lfirst(l));
}
/*
* Now process the top-level quals.
*/
find_placeholders_in_expr(root, f->quals);//在表达式中搜索
}
else if (IsA(jtnode, JoinExpr))//JoinExpr
{
JoinExpr *j = (JoinExpr *) jtnode;
/*
* First, recurse to handle child joins.
*/
find_placeholders_recurse(root, j->larg);
find_placeholders_recurse(root, j->rarg);
/* Process the qual clauses */
find_placeholders_in_expr(root, j->quals);//在表达式中搜索
}
else
elog(ERROR, "unrecognized node type: %d",
(int) nodeTag(jtnode));
}
/*
* find_placeholders_in_expr
* Find all PlaceHolderVars in the given expression, and create
* PlaceHolderInfo entries for them.
*/
static void
find_placeholders_in_expr(PlannerInfo *root, Node *expr)
{
List *vars;
ListCell *vl;
/*
* pull_var_clause does more than we need here, but it'll do and it's
* convenient to use.
*/
vars = pull_var_clause(expr,
PVC_RECURSE_AGGREGATES |
PVC_RECURSE_WINDOWFUNCS |
PVC_INCLUDE_PLACEHOLDERS);//遍历Vars,得到PH链表
foreach(vl, vars)
{
PlaceHolderVar *phv = (PlaceHolderVar *) lfirst(vl);
/* Ignore any plain Vars */
if (!IsA(phv, PlaceHolderVar))
continue;
/* Create a PlaceHolderInfo entry if there's not one already */
(void) find_placeholder_info(root, phv, true);//创建PHInfo
}
list_free(vars);
}
/*
* find_placeholder_info
* Fetch the PlaceHolderInfo for the given PHV
*
* If the PlaceHolderInfo doesn't exist yet, create it if create_new_ph is
* true, else throw an error.
*
* This is separate from make_placeholder_expr because subquery pullup has
* to make PlaceHolderVars for expressions that might not be used at all in
* the upper query, or might not remain after const-expression simplification.
* We build PlaceHolderInfos only for PHVs that are still present in the
* simplified query passed to query_planner().
*
* Note: this should only be called after query_planner() has started. Also,
* create_new_ph must not be true after deconstruct_jointree begins, because
* make_outerjoininfo assumes that we already know about all placeholders.
*/
PlaceHolderInfo *
find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
bool create_new_ph)
{
PlaceHolderInfo *phinfo;//结果
Relids rels_used;
ListCell *lc;
/* if this ever isn't true, we'd need to be able to look in parent lists */
Assert(phv->phlevelsup == 0);
foreach(lc, root->placeholder_list)//已存在,返回
{
phinfo = (PlaceHolderInfo *) lfirst(lc);
if (phinfo->phid == phv->phid)
return phinfo;
}
/* Not found, so create it */
if (!create_new_ph)
elog(ERROR, "too late to create a new PlaceHolderInfo");
phinfo = makeNode(PlaceHolderInfo);//构建PHInfo
phinfo->phid = phv->phid;
phinfo->ph_var = copyObject(phv);
/*
* Any referenced rels that are outside the PHV's syntactic scope are
* LATERAL references, which should be included in ph_lateral but not in
* ph_eval_at. If no referenced rels are within the syntactic scope,
* force evaluation at the syntactic location.
*/
rels_used = pull_varnos((Node *) phv->phexpr);
phinfo->ph_lateral = bms_difference(rels_used, phv->phrels);
if (bms_is_empty(phinfo->ph_lateral))
phinfo->ph_lateral = NULL; /* make it exactly NULL if empty */
phinfo->ph_eval_at = bms_int_members(rels_used, phv->phrels);
/* If no contained vars, force evaluation at syntactic location */
if (bms_is_empty(phinfo->ph_eval_at))
{
phinfo->ph_eval_at = bms_copy(phv->phrels);
Assert(!bms_is_empty(phinfo->ph_eval_at));
}
/* ph_eval_at may change later, see update_placeholder_eval_levels */
phinfo->ph_needed = NULL; /* initially it's unused */
/* for the moment, estimate width using just the datatype info */
phinfo->ph_width = get_typavgwidth(exprType((Node *) phv->phexpr),
exprTypmod((Node *) phv->phexpr));
root->placeholder_list = lappend(root->placeholder_list, phinfo);//添加到优化器信息中
/*
* The PHV's contained expression may contain other, lower-level PHVs. We
* now know we need to get those into the PlaceHolderInfo list, too, so we
* may as well do that immediately.
*/
find_placeholders_in_expr(root, (Node *) phinfo->ph_var->phexpr);//如存在子表达式,递归进去
return phinfo;
}
find_lateral_references
/*
************************ find_lateral_references *****************************
*/
/*
* find_lateral_references
* For each LATERAL subquery, extract all its references to Vars and
* PlaceHolderVars of the current query level, and make sure those values
* will be available for evaluation of the subquery.
*
* 对于LATERAL子查询,获取当前查询层次的Vars&PHVars,并且确保这些值在解析子查询时是可用的
*
* While later planning steps ensure that the Var/PHV source rels are on the
* outside of nestloops relative to the LATERAL subquery, we also need to
* ensure that the Vars/PHVs propagate up to the nestloop join level; this
* means setting suitable where_needed values for them.
*
* Note that this only deals with lateral references in unflattened LATERAL
* subqueries. When we flatten a LATERAL subquery, its lateral references
* become plain Vars in the parent query, but they may have to be wrapped in
* PlaceHolderVars if they need to be forced NULL by outer joins that don't
* also null the LATERAL subquery. That's all handled elsewhere.
*
* This has to run before deconstruct_jointree, since it might result in
* creation of PlaceHolderInfos.
*/
void
find_lateral_references(PlannerInfo *root)
{
Index rti;
/* We need do nothing if the query contains no LATERAL RTEs */
if (!root->hasLateralRTEs)
return;
/*
* Examine all baserels (the rel array has been set up by now).
*/
for (rti = 1; rti < root->simple_rel_array_size; rti++)//遍历RelOptInfo
{
RelOptInfo *brel = root->simple_rel_array[rti];
/* there may be empty slots corresponding to non-baserel RTEs */
if (brel == NULL)
continue;
Assert(brel->relid == rti); /* sanity check on array */
/*
* This bit is less obvious than it might look. We ignore appendrel
* otherrels and consider only their parent baserels. In a case where
* a LATERAL-containing UNION ALL subquery was pulled up, it is the
* otherrel that is actually going to be in the plan. However, we
* want to mark all its lateral references as needed by the parent,
* because it is the parent's relid that will be used for join
* planning purposes. And the parent's RTE will contain all the
* lateral references we need to know, since the pulled-up member is
* nothing but a copy of parts of the original RTE's subquery. We
* could visit the parent's children instead and transform their
* references back to the parent's relid, but it would be much more
* complicated for no real gain. (Important here is that the child
* members have not yet received any processing beyond being pulled
* up.) Similarly, in appendrels created by inheritance expansion,
* it's sufficient to look at the parent relation.
*/
/* ignore RTEs that are "other rels" */
if (brel->reloptkind != RELOPT_BASEREL)
continue;
extract_lateral_references(root, brel, rti);//获取LATERAL依赖
}
}
static void
extract_lateral_references(PlannerInfo *root, RelOptInfo *brel, Index rtindex)
{
RangeTblEntry *rte = root->simple_rte_array[rtindex];//相应的RTE
List *vars;
List *newvars;
Relids where_needed;
ListCell *lc;
/* No cross-references are possible if it's not LATERAL */
if (!rte->lateral)//非LATERAL,退出
return;
/* Fetch the appropriate variables */
//获取相应的Vars
if (rte->rtekind == RTE_RELATION)//基表
vars = pull_vars_of_level((Node *) rte->tablesample, 0);
else if (rte->rtekind == RTE_SUBQUERY)//子查询
vars = pull_vars_of_level((Node *) rte->subquery, 1);
else if (rte->rtekind == RTE_FUNCTION)//函数
vars = pull_vars_of_level((Node *) rte->functions, 0);
else if (rte->rtekind == RTE_TABLEFUNC)//TABLEFUNC
vars = pull_vars_of_level((Node *) rte->tablefunc, 0);
else if (rte->rtekind == RTE_VALUES)//VALUES
vars = pull_vars_of_level((Node *) rte->values_lists, 0);
else
{
Assert(false);
return; /* keep compiler quiet */
}
if (vars == NIL)
return; /* nothing to do */
/* Copy each Var (or PlaceHolderVar) and adjust it to match our level */
newvars = NIL;
foreach(lc, vars)//遍历Vars
{
Node *node = (Node *) lfirst(lc);
node = copyObject(node);
if (IsA(node, Var))//Var
{
Var *var = (Var *) node;
/* Adjustment is easy since it's just one node */
var->varlevelsup = 0;
}
else if (IsA(node, PlaceHolderVar))//PHVar
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
int levelsup = phv->phlevelsup;
/* Have to work harder to adjust the contained expression too */
if (levelsup != 0)
IncrementVarSublevelsUp(node, -levelsup, 0);//调整其中的表达式
/*
* If we pulled the PHV out of a subquery RTE, its expression
* needs to be preprocessed. subquery_planner() already did this
* for level-zero PHVs in function and values RTEs, though.
*/
if (levelsup > 0)
phv->phexpr = preprocess_phv_expression(root, phv->phexpr);//预处理PHVar表达式
}
else
Assert(false);
newvars = lappend(newvars, node);//添加到新的结果Vars中
}
list_free(vars);
/*
* We mark the Vars as being "needed" at the LATERAL RTE. This is a bit
* of a cheat: a more formal approach would be to mark each one as needed
* at the join of the LATERAL RTE with its source RTE. But it will work,
* and it's much less tedious than computing a separate where_needed for
* each Var.
*/
where_needed = bms_make_singleton(rtindex);//获取Rel编号
/*
* Push Vars into their source relations' targetlists, and PHVs into
* root->placeholder_list.
*/
add_vars_to_targetlist(root, newvars, where_needed, true);//添加到相应的Rel中
/* Remember the lateral references for create_lateral_join_info */
brel->lateral_vars = newvars;//RelOptInfo赋值
}
/*
* pull_vars_of_level
* Create a list of all Vars (and PlaceHolderVars) referencing the
* specified query level in the given parsetree.
*
* Caution: the Vars are not copied, only linked into the list.
*/
List *
pull_vars_of_level(Node *node, int levelsup)
{
pull_vars_context context;
context.vars = NIL;
context.sublevels_up = levelsup;
/*
* Must be prepared to start with a Query or a bare expression tree; if
* it's a Query, we don't want to increment sublevels_up.
*/
query_or_expression_tree_walker(node,
pull_vars_walker,
(void *) &context,
0);//调用XX_walker函数遍历
return context.vars;
}
static bool
pull_vars_walker(Node *node, pull_vars_context *context)//遍历函数
{
if (node == NULL)
return false;
if (IsA(node, Var))
{
Var *var = (Var *) node;
if (var->varlevelsup == context->sublevels_up)
context->vars = lappend(context->vars, var);
return false;
}
if (IsA(node, PlaceHolderVar))
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
if (phv->phlevelsup == context->sublevels_up)
context->vars = lappend(context->vars, phv);
/* we don't want to look into the contained expression */
return false;
}
if (IsA(node, Query))
{
/* Recurse into RTE subquery or not-yet-planned sublink subquery */
bool result;
context->sublevels_up++;
result = query_tree_walker((Query *) node, pull_vars_walker,
(void *) context, 0);
context->sublevels_up--;
return result;
}
return expression_tree_walker(node, pull_vars_walker,
(void *) context);
}
公共部分
/*
************************ 公共 *****************************
*/
/*
* pull_var_clause
* Recursively pulls all Var nodes from an expression clause.
*
* 递归的方式推送(pull)所有的Var节点
*
* Aggrefs are handled according to these bits in 'flags':
* PVC_INCLUDE_AGGREGATES include Aggrefs in output list
* PVC_RECURSE_AGGREGATES recurse into Aggref arguments
* neither flag throw error if Aggref found
* Vars within an Aggref's expression are included in the result only
* when PVC_RECURSE_AGGREGATES is specified.
*
* WindowFuncs are handled according to these bits in 'flags':
* PVC_INCLUDE_WINDOWFUNCS include WindowFuncs in output list
* PVC_RECURSE_WINDOWFUNCS recurse into WindowFunc arguments
* neither flag throw error if WindowFunc found
* Vars within a WindowFunc's expression are included in the result only
* when PVC_RECURSE_WINDOWFUNCS is specified.
*
* PlaceHolderVars are handled according to these bits in 'flags':
* PVC_INCLUDE_PLACEHOLDERS include PlaceHolderVars in output list
* PVC_RECURSE_PLACEHOLDERS recurse into PlaceHolderVar arguments
* neither flag throw error if PlaceHolderVar found
* Vars within a PHV's expression are included in the result only
* when PVC_RECURSE_PLACEHOLDERS is specified.
*
* GroupingFuncs are treated mostly like Aggrefs, and so do not need
* their own flag bits.
*
* CurrentOfExpr nodes are ignored in all cases.
*
* Upper-level vars (with varlevelsup > 0) should not be seen here,
* likewise for upper-level Aggrefs and PlaceHolderVars.
*
* Returns list of nodes found. Note the nodes themselves are not
* copied, only referenced.
*
* Does not examine subqueries, therefore must only be used after reduction
* of sublinks to subplans!
*/
List *
pull_var_clause(Node *node, int flags)
{
pull_var_clause_context context;//上下文
//互斥选项检测
/* Assert that caller has not specified inconsistent flags */
Assert((flags & (PVC_INCLUDE_AGGREGATES | PVC_RECURSE_AGGREGATES))
!= (PVC_INCLUDE_AGGREGATES | PVC_RECURSE_AGGREGATES));
Assert((flags & (PVC_INCLUDE_WINDOWFUNCS | PVC_RECURSE_WINDOWFUNCS))
!= (PVC_INCLUDE_WINDOWFUNCS | PVC_RECURSE_WINDOWFUNCS));
Assert((flags & (PVC_INCLUDE_PLACEHOLDERS | PVC_RECURSE_PLACEHOLDERS))
!= (PVC_INCLUDE_PLACEHOLDERS | PVC_RECURSE_PLACEHOLDERS));
context.varlist = NIL;
context.flags = flags;
pull_var_clause_walker(node, &context);//调用XX_walker函数遍历,结果保存在context.varlist中
return context.varlist;
}
static bool
pull_var_clause_walker(Node *node, pull_var_clause_context *context)
{
if (node == NULL)
return false;
if (IsA(node, Var))//Var类型
{
if (((Var *) node)->varlevelsup != 0)//非本级Var
elog(ERROR, "Upper-level Var found where not expected");
context->varlist = lappend(context->varlist, node);//添加到结果链表中
return false;
}
else if (IsA(node, Aggref))//聚合
{
if (((Aggref *) node)->agglevelsup != 0)
elog(ERROR, "Upper-level Aggref found where not expected");
if (context->flags & PVC_INCLUDE_AGGREGATES)//包含聚合
{
context->varlist = lappend(context->varlist, node);//添加到结果
/* we do NOT descend into the contained expression */
return false;
}
else if (context->flags & PVC_RECURSE_AGGREGATES)//递归搜索
{
/* fall through to recurse into the aggregate's arguments */
}
else
elog(ERROR, "Aggref found where not expected");
}
else if (IsA(node, GroupingFunc))//分组
{
if (((GroupingFunc *) node)->agglevelsup != 0)
elog(ERROR, "Upper-level GROUPING found where not expected");
if (context->flags & PVC_INCLUDE_AGGREGATES)//包含标记
{
context->varlist = lappend(context->varlist, node);
/* we do NOT descend into the contained expression */
return false;
}
else if (context->flags & PVC_RECURSE_AGGREGATES)//递归标记
{
/*
* We do NOT descend into the contained expression, even if the
* caller asked for it, because we never actually evaluate it -
* the result is driven entirely off the associated GROUP BY
* clause, so we never need to extract the actual Vars here.
*/
return false;//直接返回,需与GROUP BY语句一起
}
else
elog(ERROR, "GROUPING found where not expected");
}
else if (IsA(node, WindowFunc))//窗口函数
{
/* WindowFuncs have no levelsup field to check ... */
if (context->flags & PVC_INCLUDE_WINDOWFUNCS)//包含标记
{
context->varlist = lappend(context->varlist, node);
/* we do NOT descend into the contained expressions */
return false;
}
else if (context->flags & PVC_RECURSE_WINDOWFUNCS)//递归标记
{
/* fall through to recurse into the windowfunc's arguments */
}
else
elog(ERROR, "WindowFunc found where not expected");
}
else if (IsA(node, PlaceHolderVar))//PH
{
if (((PlaceHolderVar *) node)->phlevelsup != 0)
elog(ERROR, "Upper-level PlaceHolderVar found where not expected");
if (context->flags & PVC_INCLUDE_PLACEHOLDERS)
{
context->varlist = lappend(context->varlist, node);
/* we do NOT descend into the contained expression */
return false;
}
else if (context->flags & PVC_RECURSE_PLACEHOLDERS)
{
/* fall through to recurse into the placeholder's expression */
}
else
elog(ERROR, "PlaceHolderVar found where not expected");
}
return expression_tree_walker(node, pull_var_clause_walker,
(void *) context);//表达式解析
}
三、跟踪分析
重点考察root->simple_rel_array[n]->attr_needed、root->simple_rel_array[n]->reltarget、root->placeholder_list、root->simple_rel_array[n]->lateral_vars.
启动gdb跟踪
(gdb) b build_base_rel_tlists
Breakpoint 1 at 0x76551b: file initsplan.c, line 153.
(gdb) c
Continuing.
Breakpoint 1, build_base_rel_tlists (root=0x171ae40, final_tlist=0x1734750) at initsplan.c:153
153 List *tlist_vars = pull_var_clause((Node *) final_tlist,
(gdb)
final_tlist是最终的输出列(投影列),一共有5个,分别是t_dwxx.dwmc/dwbh/dwdz,t_grxx.grbh,t_jfxx.je
(gdb) p *final_tlist
$1 = {type = T_List, length = 5, head = 0x1734730, tail = 0x1734a60}
(gdb) p *(Node *)final_tlist->head->data.ptr_value
$2 = {type = T_TargetEntry}
(gdb) p *(TargetEntry *)final_tlist->head->data.ptr_value
$3 = {xpr = {type = T_TargetEntry}, expr = 0x17346e0, resno = 1, resname = 0x171a5a0 "dwmc", ressortgroupref = 0,
resorigtbl = 16394, resorigcol = 1, resjunk = false}
跟踪函数build_base_rel_tlists
(gdb) n
158 if (tlist_vars != NIL)
(gdb)
160 add_vars_to_targetlist(root, tlist_vars, bms_make_singleton(0), true);
#5个Vars
(gdb) p *tlist_vars
$4 = {type = T_List, length = 5, head = 0x17369e0, tail = 0x1737cb8}
(gdb) p *(Var *)tlist_vars->head->data.ptr_value
$5 = {xpr = {type = T_Var}, varno = 1, varattno = 1, vartype = 1043, vartypmod = 104, varcollid = 100, varlevelsup = 0,
varnoold = 1, varoattno = 1, location = 7}
执行函数build_base_rel_tlists,检查final_rel->attr_needed和final_rel->reltarget
(gdb) finish
Run till exit from #0 build_base_rel_tlists (root=0x171ae40, final_tlist=0x1734750) at initsplan.c:160
query_planner (root=0x171ae40, tlist=0x1734750, qp_callback=0x76e97d , qp_extra=0x7ffe20fc33d0)
at planmain.c:152
152 find_placeholders_in_jointree(root);
检查root内存结构
(gdb) p *root
$15 = {type = T_PlannerInfo, parse = 0x1711680, glob = 0x1732118, query_level = 1, parent_root = 0x0, plan_params = 0x0,
outer_params = 0x0, simple_rel_array = 0x1736578, simple_rel_array_size = 6, simple_rte_array = 0x17365c8,
all_baserels = 0x0, nullable_baserels = 0x0, join_rel_list = 0x0, join_rel_hash = 0x0, join_rel_level = 0x0,
join_cur_level = 0, init_plans = 0x0, cte_plan_ids = 0x0, multiexpr_params = 0x0, eq_classes = 0x0, canon_pathkeys = 0x0,
left_join_clauses = 0x0, right_join_clauses = 0x0, full_join_clauses = 0x0, join_info_list = 0x0, append_rel_list = 0x0,
rowMarks = 0x0, placeholder_list = 0x0, fkey_list = 0x0, query_pathkeys = 0x0, group_pathkeys = 0x0,
window_pathkeys = 0x0, distinct_pathkeys = 0x0, sort_pathkeys = 0x0, part_schemes = 0x0, initial_rels = 0x0,
upper_rels = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, upper_targets = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
processed_tlist = 0x1734750, grouping_map = 0x0, minmax_aggs = 0x0, planner_cxt = 0x165a040, total_table_pages = 0,
tuple_fraction = 0, limit_tuples = -1, qual_security_level = 0, inhTargetKind = INHKIND_NONE, hasJoinRTEs = true,
hasLateralRTEs = true, hasDeletedRTEs = false, hasHavingQual = false, hasPseudoConstantQuals = false,
hasRecursion = false, wt_param_id = -1, non_recursive_path = 0x0, curOuterRels = 0x0, curOuterParams = 0x0,
join_search_private = 0x0, partColsUpdated = false}
RelOptInfo数组,注意数组的第0(下标)个元素为NULL(无用),有用的元素下标从1开始:
第1个元素是基础关系(相应的RTE=t_dwxx),第2个元素为NULL(相应的RTE=子查询),第3个元素为基础关系(相应的RTE=t_grxx),第4个元素为基础关系(相应的RTE=t_jfxx),第5个元素为NULL(相应的RTE=连接)
(gdb) p *root->simple_rel_array[0]
Cannot access memory at address 0x0
(gdb) p *root->simple_rel_array[1]
$31 = {type = T_RelOptInfo, reloptkind = RELOPT_BASEREL, relids = 0x1736828, rows = 0, consider_startup = false,
consider_param_startup = false, consider_parallel = false, reltarget = 0x1736840, pathlist = 0x0, ppilist = 0x0,
partial_pathlist = 0x0, cheapest_startup_path = 0x0, cheapest_total_path = 0x0, cheapest_unique_path = 0x0,
cheapest_parameterized_paths = 0x0, direct_lateral_relids = 0x0, lateral_relids = 0x0, relid = 1, reltablespace = 0,
rtekind = RTE_RELATION, min_attr = -7, max_attr = 3, attr_needed = 0x1736890, attr_widths = 0x1736920,
lateral_vars = 0x0, lateral_referencers = 0x0, indexlist = 0x1736cc8, statlist = 0x0, pages = 1, tuples = 3,
allvisfrac = 0, subroot = 0x0, subplan_params = 0x0, rel_parallel_workers = -1, serverid = 0, userid = 0,
useridiscurrent = false, fdwroutine = 0x0, fdw_private = 0x0, unique_for_rels = 0x0, non_unique_for_rels = 0x0,
baserestrictinfo = 0x0, baserestrictcost = {startup = 0, per_tuple = 0}, baserestrict_min_security = 4294967295,
joininfo = 0x0, has_eclass_joins = false, top_parent_relids = 0x0, part_scheme = 0x0, nparts = 0, boundinfo = 0x0,
partition_qual = 0x0, part_rels = 0x0, partexprs = 0x0, nullable_partexprs = 0x0, partitioned_child_rels = 0x0}
(gdb) p *root->simple_rel_array[2]
Cannot access memory at address 0x0
(gdb) p *root->simple_rel_array[3]
$32 = {type = T_RelOptInfo, reloptkind = RELOPT_BASEREL, relids = 0x17377f8, rows = 0, consider_startup = false,
consider_param_startup = false, consider_parallel = false, reltarget = 0x1737810, pathlist = 0x0, ppilist = 0x0,
partial_pathlist = 0x0, cheapest_startup_path = 0x0, cheapest_total_path = 0x0, cheapest_unique_path = 0x0,
cheapest_parameterized_paths = 0x0, direct_lateral_relids = 0x0, lateral_relids = 0x0, relid = 3, reltablespace = 0,
rtekind = RTE_RELATION, min_attr = -7, max_attr = 5, attr_needed = 0x1737860, attr_widths = 0x17378f0,
lateral_vars = 0x0, lateral_referencers = 0x0, indexlist = 0x0, statlist = 0x0, pages = 10, tuples = 400, allvisfrac = 0,
subroot = 0x0, subplan_params = 0x0, rel_parallel_workers = -1, serverid = 0, userid = 0, useridiscurrent = false,
fdwroutine = 0x0, fdw_private = 0x0, unique_for_rels = 0x0, non_unique_for_rels = 0x0, baserestrictinfo = 0x0,
baserestrictcost = {startup = 0, per_tuple = 0}, baserestrict_min_security = 4294967295, joininfo = 0x0,
has_eclass_joins = false, top_parent_relids = 0x0, part_scheme = 0x0, nparts = 0, boundinfo = 0x0, partition_qual = 0x0,
part_rels = 0x0, partexprs = 0x0, nullable_partexprs = 0x0, partitioned_child_rels = 0x0}
(gdb) p *root->simple_rel_array[4]
$33 = {type = T_RelOptInfo, reloptkind = RELOPT_BASEREL, relids = 0x1737b50, rows = 0, consider_startup = false,
consider_param_startup = false, consider_parallel = false, reltarget = 0x1737b68, pathlist = 0x0, ppilist = 0x0,
partial_pathlist = 0x0, cheapest_startup_path = 0x0, cheapest_total_path = 0x0, cheapest_unique_path = 0x0,
cheapest_parameterized_paths = 0x0, direct_lateral_relids = 0x0, lateral_relids = 0x0, relid = 4, reltablespace = 0,
rtekind = RTE_RELATION, min_attr = -7, max_attr = 3, attr_needed = 0x1737bb8, attr_widths = 0x1737c48,
lateral_vars = 0x0, lateral_referencers = 0x0, indexlist = 0x0, statlist = 0x0, pages = 10, tuples = 720, allvisfrac = 0,
subroot = 0x0, subplan_params = 0x0, rel_parallel_workers = -1, serverid = 0, userid = 0, useridiscurrent = false,
fdwroutine = 0x0, fdw_private = 0x0, unique_for_rels = 0x0, non_unique_for_rels = 0x0, baserestrictinfo = 0x0,
baserestrictcost = {startup = 0, per_tuple = 0}, baserestrict_min_security = 4294967295, joininfo = 0x0,
has_eclass_joins = false, top_parent_relids = 0x0, part_scheme = 0x0, nparts = 0, boundinfo = 0x0, partition_qual = 0x0,
part_rels = 0x0, partexprs = 0x0, nullable_partexprs = 0x0, partitioned_child_rels = 0x0}
(gdb) p *root->simple_rel_array[5]
Cannot access memory at address 0x0
查看root->simple_rel_array[n]->attr_needed、root->simple_rel_array[n]->reltarget的内存结构,以第1个元素为例:
#attr_needed(类型为Relids)为NULL
(gdb) p *root->simple_rel_array[1]->attr_needed
$47 = (Relids) 0x0
#reltarget->exprs为3个Var的链表
(gdb) p *root->simple_rel_array[1]->reltarget->exprs
$38 = {type = T_List, length = 3, head = 0x1737d40, tail = 0x1737e80}
(gdb) p *(Var *)root->simple_rel_array[1]->reltarget->exprs->head->data.ptr_value
$40 = {xpr = {type = T_Var}, varno = 1, varattno = 1, vartype = 1043, vartypmod = 104, varcollid = 100, varlevelsup = 0,
varnoold = 1, varoattno = 1, location = 7}
继续执行,调用函数find_placeholders_in_jointree
152 find_placeholders_in_jointree(root);
(gdb) step
find_placeholders_in_jointree (root=0x171ae40) at placeholder.c:148
148 if (root->glob->lastPHId != 0)
(gdb) n
155 }
(gdb) n
154 find_lateral_references(root);
#链表为NULL
#psql中的:v1似乎不是占位符,理解有偏差//TODO...
(gdb) p root->placeholder_list
$48 = (List *) 0x0
下面调用子函数find_lateral_references,由于RelOptInfo中的3个不为NULL的元素对应的lateral均为FALSE(只有子查询的lateral为true,但子查询对应的RelOptInfo为NULL),因此root->simple_rel_array[1/3/4/5]->lateral_vars均为NULL
四、参考资料
initsplan.c
当前标题：PostgreSQL源码解读（41）-查询语句#26（query_planner函数#4）
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