本节简单介绍了PG查询逻辑优化中的子查询链接(subLink),以EXISTS子链接为例介绍了子查询链接上拉主函数处理逻辑以及使用gdb跟踪分析。
一、源码解读
上一节介绍了ANY子链接,本节介绍了EXISTS子链接.
为便于方便解析,根据日志分析,得出查询树如下图所示:
查询树
convert_EXISTS_sublink_to_join函数源码:
/*
* convert_EXISTS_sublink_to_join: try to convert an EXISTS SubLink to a join
*
* The API of this function is identical to convert_ANY_sublink_to_join's,
* except that we also support the case where the caller has found NOT EXISTS,
* so we need an additional input parameter "under_not".
* 逻辑与ANY一致,为了支持NOT EXISTS,多加了一个参数under_not
*/
JoinExpr *
convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
bool under_not, Relids available_rels)
{
JoinExpr *result;//返回结果
Query *parse = root->parse;//查询树
Query *subselect = (Query *) sublink->subselect;//子查询
Node *whereClause;//where语句
int rtoffset;//
int varno;
Relids clause_varnos;
Relids upper_varnos;
Assert(sublink->subLinkType == EXISTS_SUBLINK);
/*
* Can't flatten if it contains WITH. (We could arrange to pull up the
* WITH into the parent query's cteList, but that risks changing the
* semantics, since a WITH ought to be executed once per associated query
* call.) Note that convert_ANY_sublink_to_join doesn't have to reject
* this case, since it just produces a subquery RTE that doesn't have to
* get flattened into the parent query.
*/
if (subselect->cteList)//存在With子句,返回
return NULL;
/*
* Copy the subquery so we can modify it safely (see comments in
* make_subplan).
*/
subselect = copyObject(subselect);
/*
* See if the subquery can be simplified based on the knowledge that it's
* being used in EXISTS(). If we aren't able to get rid of its
* targetlist, we have to fail, because the pullup operation leaves us
* with noplace to evaluate the targetlist.
*/
//能否handle targetList?不行,则退出
//如果含有集合操作/聚合操作/Having子句等,子链接不能提升
if (!simplify_EXISTS_query(root, subselect))
return NULL;
/*
* The subquery must have a nonempty jointree, else we won't have a join.
*/
if (subselect->jointree->fromlist == NIL)//子查询没有查询主体,退出
return NULL;
/*
* Separate out the WHERE clause. (We could theoretically also remove
* top-level plain JOIN/ON clauses, but it's probably not worth the
* trouble.)
*/
whereClause = subselect->jointree->quals;//子查询条件语句单独保存
subselect->jointree->quals = NULL;//子查询的条件语句设置为NULL
/*
* The rest of the sub-select must not refer to any Vars of the parent
* query. (Vars of higher levels should be okay, though.)
*/
if (contain_vars_of_level((Node *) subselect, 1))//去掉条件语句后,如仍依赖父查询的Vars,退出
return NULL;
/*
* On the other hand, the WHERE clause must contain some Vars of the
* parent query, else it's not gonna be a join.
*/
if (!contain_vars_of_level(whereClause, 1))//条件语句必须含有父查询的Vars,否则不构成连接,退出
return NULL;
/*
* We don't risk optimizing if the WHERE clause is volatile, either.
*/
if (contain_volatile_functions(whereClause))//条件语句存在易变函数(如随机函数等)
return NULL;
/*
* Prepare to pull up the sub-select into top range table.
*
* We rely here on the assumption that the outer query has no references
* to the inner (necessarily true). Therefore this is a lot easier than
* what pull_up_subqueries has to go through.
*
* In fact, it's even easier than what convert_ANY_sublink_to_join has to
* do. The machinations of simplify_EXISTS_query ensured that there is
* nothing interesting in the subquery except an rtable and jointree, and
* even the jointree FromExpr no longer has quals. So we can just append
* the rtable to our own and use the FromExpr in our jointree. But first,
* adjust all level-zero varnos in the subquery to account for the rtable
* merger.
*/
rtoffset = list_length(parse->rtable);//获取rtable的长度(新RTE插入的偏移)
//调整子查询中varno为0(指向rtable)的Vars,varno调整为父查询rtable的index
//(详见依赖函数解析)
OffsetVarNodes((Node *) subselect, rtoffset, 0);
OffsetVarNodes(whereClause, rtoffset, 0);
/*
* Upper-level vars in subquery will now be one level closer to their
* parent than before; in particular, anything that had been level 1
* becomes level zero.
*/
//子查询中与父查询相关的Vars,varlevelsup需要从Leve 1变为Level 0
//在子查询中,这些Vars的varlevelsup为1,表示依赖于父查询(上一层)的Vars,提升后不存在此依赖,需改为0
IncrementVarSublevelsUp((Node *) subselect, -1, 1);
IncrementVarSublevelsUp(whereClause, -1, 1);
/*
* Now that the WHERE clause is adjusted to match the parent query
* environment, we can easily identify all the level-zero rels it uses.
* The ones <= rtoffset belong to the upper query; the ones > rtoffset do
* not.
*/
clause_varnos = pull_varnos(whereClause);
upper_varnos = NULL;
while ((varno = bms_first_member(clause_varnos)) >= 0)
{
if (varno <= rtoffset)
upper_varnos = bms_add_member(upper_varnos, varno);
}
bms_free(clause_varnos);
Assert(!bms_is_empty(upper_varnos));
/*
* Now that we've got the set of upper-level varnos, we can make the last
* check: only available_rels can be referenced.
*/
if (!bms_is_subset(upper_varnos, available_rels))
return NULL;
/* Now we can attach the modified subquery rtable to the parent */
//把子查询的rtable拼接到父查询的rtable中
parse->rtable = list_concat(parse->rtable, subselect->rtable);
//构造JoinExpr
/*
* And finally, build the JoinExpr node.
*/
result = makeNode(JoinExpr);
result->jointype = under_not ? JOIN_ANTI : JOIN_SEMI;
result->isNatural = false;
result->larg = NULL; /* caller must fill this in */
/* flatten out the FromExpr node if it's useless */
if (list_length(subselect->jointree->fromlist) == 1)
result->rarg = (Node *) linitial(subselect->jointree->fromlist);
else
result->rarg = (Node *) subselect->jointree;
result->usingClause = NIL;
result->quals = whereClause;
result->alias = NULL;
result->rtindex = 0; /* we don't need an RTE for it */
return result;
}
二、基础信息
相关数据结构
1、Var
/*
* Var - expression node representing a variable (ie, a table column)
*
* Note: during parsing/planning, varnoold/varoattno are always just copies
* of varno/varattno. At the tail end of planning, Var nodes appearing in
* upper-level plan nodes are reassigned to point to the outputs of their
* subplans; for example, in a join node varno becomes INNER_VAR or OUTER_VAR
* and varattno becomes the index of the proper element of that subplan's
* target list. Similarly, INDEX_VAR is used to identify Vars that reference
* an index column rather than a heap column. (In ForeignScan and CustomScan
* plan nodes, INDEX_VAR is abused to signify references to columns of a
* custom scan tuple type.) In all these cases, varnoold/varoattno hold the
* original values. The code doesn't really need varnoold/varoattno, but they
* are very useful for debugging and interpreting completed plans, so we keep
* them around.
*/
#define INNER_VAR 65000 /* reference to inner subplan */
#define OUTER_VAR 65001 /* reference to outer subplan */
#define INDEX_VAR 65002 /* reference to index column */
#define IS_SPECIAL_VARNO(varno) ((varno) >= INNER_VAR)
/* Symbols for the indexes of the special RTE entries in rules */
#define PRS2_OLD_VARNO 1
#define PRS2_NEW_VARNO 2
typedef struct Var
{
Expr xpr;
Index varno; /* index of this var's relation in the range
* table, or INNER_VAR/OUTER_VAR/INDEX_VAR */
AttrNumber varattno; /* attribute number of this var, or zero for
* all attrs ("whole-row Var") */
Oid vartype; /* pg_type OID for the type of this var */
int32 vartypmod; /* pg_attribute typmod value */
Oid varcollid; /* OID of collation, or InvalidOid if none */
Index varlevelsup; /* for subquery variables referencing outer
* relations; 0 in a normal var, >0 means N
* levels up */
Index varnoold; /* original value of varno, for debugging */
AttrNumber varoattno; /* original value of varattno */
int location; /* token location, or -1 if unknown */
} Var;
XX_one_pos
/*
* Lookup tables to avoid need for bit-by-bit groveling
*
* rightmost_one_pos[x] gives the bit number (0-7) of the rightmost one bit
* in a nonzero byte value x. The entry for x=0 is never used.
*
* leftmost_one_pos[x] gives the bit number (0-7) of the leftmost one bit in a
* nonzero byte value x. The entry for x=0 is never used.
*
* number_of_ones[x] gives the number of one-bits (0-8) in a byte value x.
*
* We could make these tables larger and reduce the number of iterations
* in the functions that use them, but bytewise shifts and masks are
* especially fast on many machines, so working a byte at a time seems best.
*/
static const uint8 rightmost_one_pos[256] = {
0, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
static const uint8 leftmost_one_pos[256] = {
0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
};
static const uint8 number_of_ones[256] = {
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};
依赖的函数
simplify_EXISTS_query
/*
* simplify_EXISTS_query: remove any useless stuff in an EXISTS's subquery
*
* The only thing that matters about an EXISTS query is whether it returns
* zero or more than zero rows. Therefore, we can remove certain SQL features
* that won't affect that. The only part that is really likely to matter in
* typical usage is simplifying the targetlist: it's a common habit to write
* "SELECT * FROM" even though there is no need to evaluate any columns.
*
* Note: by suppressing the targetlist we could cause an observable behavioral
* change, namely that any errors that might occur in evaluating the tlist
* won't occur, nor will other side-effects of volatile functions. This seems
* unlikely to bother anyone in practice.
*
* Returns true if was able to discard the targetlist, else false.
*/
static bool
simplify_EXISTS_query(PlannerInfo *root, Query *query)
{
/*
* We don't try to simplify at all if the query uses set operations,
* aggregates, grouping sets, SRFs, modifying CTEs, HAVING, OFFSET, or FOR
* UPDATE/SHARE; none of these seem likely in normal usage and their
* possible effects are complex. (Note: we could ignore an "OFFSET 0"
* clause, but that traditionally is used as an optimization fence, so we
* don't.)
*/
if (query->commandType != CMD_SELECT ||
query->setOperations ||
query->hasAggs ||
query->groupingSets ||
query->hasWindowFuncs ||
query->hasTargetSRFs ||
query->hasModifyingCTE ||
query->havingQual ||
query->limitOffset ||
query->rowMarks)
return false;
/*
* LIMIT with a constant positive (or NULL) value doesn't affect the
* semantics of EXISTS, so let's ignore such clauses. This is worth doing
* because people accustomed to certain other DBMSes may be in the habit
* of writing EXISTS(SELECT ... LIMIT 1) as an optimization. If there's a
* LIMIT with anything else as argument, though, we can't simplify.
*/
if (query->limitCount)
{
/*
* The LIMIT clause has not yet been through eval_const_expressions,
* so we have to apply that here. It might seem like this is a waste
* of cycles, since the only case plausibly worth worrying about is
* "LIMIT 1" ... but what we'll actually see is "LIMIT int8(1::int4)",
* so we have to fold constants or we're not going to recognize it.
*/
Node *node = eval_const_expressions(root, query->limitCount);
Const *limit;
/* Might as well update the query if we simplified the clause. */
query->limitCount = node;
if (!IsA(node, Const))
return false;
limit = (Const *) node;
Assert(limit->consttype == INT8OID);
if (!limit->constisnull && DatumGetInt64(limit->constvalue) <= 0)
return false;
/* Whether or not the targetlist is safe, we can drop the LIMIT. */
query->limitCount = NULL;
}
/*
* Otherwise, we can throw away the targetlist, as well as any GROUP,
* WINDOW, DISTINCT, and ORDER BY clauses; none of those clauses will
* change a nonzero-rows result to zero rows or vice versa. (Furthermore,
* since our parsetree representation of these clauses depends on the
* targetlist, we'd better throw them away if we drop the targetlist.)
*/
query->targetList = NIL;
query->groupClause = NIL;
query->windowClause = NIL;
query->distinctClause = NIL;
query->sortClause = NIL;
query->hasDistinctOn = false;
return true;
}
OffsetVarNodes
void
OffsetVarNodes(Node *node, int offset, int sublevels_up)
{
OffsetVarNodes_context context;//上下文
context.offset = offset;//保存传入的offset
context.sublevels_up = sublevels_up;
/*
* Must be prepared to start with a Query or a bare expression tree; if
* it's a Query, go straight to query_tree_walker to make sure that
* sublevels_up doesn't get incremented prematurely.
*/
if (node && IsA(node, Query))
{
Query *qry = (Query *) node;
/*
* If we are starting at a Query, and sublevels_up is zero, then we
* must also fix rangetable indexes in the Query itself --- namely
* resultRelation, exclRelIndex and rowMarks entries. sublevels_up
* cannot be zero when recursing into a subquery, so there's no need
* to have the same logic inside OffsetVarNodes_walker.
*/
if (sublevels_up == 0)
{
ListCell *l;
if (qry->resultRelation)
qry->resultRelation += offset;
if (qry->onConflict && qry->onConflict->exclRelIndex)
qry->onConflict->exclRelIndex += offset;
foreach(l, qry->rowMarks)
{
RowMarkClause *rc = (RowMarkClause *) lfirst(l);
rc->rti += offset;
}
}
query_tree_walker(qry, OffsetVarNodes_walker,
(void *) &context, 0);
}
else
OffsetVarNodes_walker(node, &context);
}
IncrementVarSublevelsUp
void
IncrementVarSublevelsUp(Node *node, int delta_sublevels_up,
int min_sublevels_up)
{
IncrementVarSublevelsUp_context context;
context.delta_sublevels_up = delta_sublevels_up;
context.min_sublevels_up = min_sublevels_up;
/*
* 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,
IncrementVarSublevelsUp_walker,
(void *) &context,
QTW_EXAMINE_RTES);
}
IncrementVarSublevelsUp
/*
* IncrementVarSublevelsUp - adjust Var nodes when pushing them down in tree
*
* Find all Var nodes in the given tree having varlevelsup >= min_sublevels_up,
* and add delta_sublevels_up to their varlevelsup value. This is needed when
* an expression that's correct for some nesting level is inserted into a
* subquery. Ordinarily the initial call has min_sublevels_up == 0 so that
* all Vars are affected. The point of min_sublevels_up is that we can
* increment it when we recurse into a sublink, so that local variables in
* that sublink are not affected, only outer references to vars that belong
* to the expression's original query level or parents thereof.
*
* Likewise for other nodes containing levelsup fields, such as Aggref.
*
* NOTE: although this has the form of a walker, we cheat and modify the
* Var nodes in-place. The given expression tree should have been copied
* earlier to ensure that no unwanted side-effects occur!
*/
typedef struct
{
int delta_sublevels_up;
int min_sublevels_up;
} IncrementVarSublevelsUp_context;
static bool
IncrementVarSublevelsUp_walker(Node *node,
IncrementVarSublevelsUp_context *context)
{
if (node == NULL)
return false;
if (IsA(node, Var))
{
Var *var = (Var *) node;
if (var->varlevelsup >= context->min_sublevels_up)
var->varlevelsup += context->delta_sublevels_up;
return false; /* done here */
}
if (IsA(node, CurrentOfExpr))
{
/* this should not happen */
if (context->min_sublevels_up == 0)
elog(ERROR, "cannot push down CurrentOfExpr");
return false;
}
if (IsA(node, Aggref))
{
Aggref *agg = (Aggref *) node;
if (agg->agglevelsup >= context->min_sublevels_up)
agg->agglevelsup += context->delta_sublevels_up;
/* fall through to recurse into argument */
}
if (IsA(node, GroupingFunc))
{
GroupingFunc *grp = (GroupingFunc *) node;
if (grp->agglevelsup >= context->min_sublevels_up)
grp->agglevelsup += context->delta_sublevels_up;
/* fall through to recurse into argument */
}
if (IsA(node, PlaceHolderVar))
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
if (phv->phlevelsup >= context->min_sublevels_up)
phv->phlevelsup += context->delta_sublevels_up;
/* fall through to recurse into argument */
}
if (IsA(node, RangeTblEntry))
{
RangeTblEntry *rte = (RangeTblEntry *) node;
if (rte->rtekind == RTE_CTE)
{
if (rte->ctelevelsup >= context->min_sublevels_up)
rte->ctelevelsup += context->delta_sublevels_up;
}
return false; /* allow range_table_walker to continue */
}
if (IsA(node, Query))
{
/* Recurse into subselects */
bool result;
context->min_sublevels_up++;
result = query_tree_walker((Query *) node,
IncrementVarSublevelsUp_walker,
(void *) context,
QTW_EXAMINE_RTES);
context->min_sublevels_up--;
return result;
}
return expression_tree_walker(node, IncrementVarSublevelsUp_walker,
(void *) context);
}
pull_varnos
/*
* pull_varnos
* Create a set of all the distinct varnos present in a parsetree.
* Only varnos that reference level-zero rtable entries are considered.
*
* NOTE: this is used on not-yet-planned expressions. It may therefore find
* bare SubLinks, and if so it needs to recurse into them to look for uplevel
* references to the desired rtable level! But when we find a completed
* SubPlan, we only need to look at the parameters passed to the subplan.
*/
Relids
pull_varnos(Node *node)
{
pull_varnos_context context;
context.varnos = NULL;
context.sublevels_up = 0;
/*
* 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_varnos_walker,
(void *) &context,
0);
return context.varnos;
}
contain_vars_of_level
/*
* contain_vars_of_level
* Recursively scan a clause to discover whether it contains any Var nodes
* of the specified query level.
*
* Returns true if any such Var found.
*
* Will recurse into sublinks. Also, may be invoked directly on a Query.
*/
bool
contain_vars_of_level(Node *node, int levelsup)
{
int sublevels_up = levelsup;
return query_or_expression_tree_walker(node,
contain_vars_of_level_walker,
(void *) &sublevels_up,
0);
}
query_tree_walker
/*
* query_tree_walker --- initiate a walk of a Query's expressions
*
* This routine exists just to reduce the number of places that need to know
* where all the expression subtrees of a Query are. Note it can be used
* for starting a walk at top level of a Query regardless of whether the
* walker intends to descend into subqueries. It is also useful for
* descending into subqueries within a walker.
*
* Some callers want to suppress visitation of certain items in the sub-Query,
* typically because they need to process them specially, or don't actually
* want to recurse into subqueries. This is supported by the flags argument,
* which is the bitwise OR of flag values to suppress visitation of
* indicated items. (More flag bits may be added as needed.)
*/
bool
query_tree_walker(Query *query,
bool (*walker) (),
void *context,
int flags)
{
Assert(query != NULL && IsA(query, Query));
if (walker((Node *) query->targetList, context))
return true;
if (walker((Node *) query->withCheckOptions, context))
return true;
if (walker((Node *) query->onConflict, context))
return true;
if (walker((Node *) query->returningList, context))
return true;
if (walker((Node *) query->jointree, context))
return true;
if (walker(query->setOperations, context))
return true;
if (walker(query->havingQual, context))
return true;
if (walker(query->limitOffset, context))
return true;
if (walker(query->limitCount, context))
return true;
if (!(flags & QTW_IGNORE_CTE_SUBQUERIES))
{
if (walker((Node *) query->cteList, context))
return true;
}
if (!(flags & QTW_IGNORE_RANGE_TABLE))
{
if (range_table_walker(query->rtable, walker, context, flags))
return true;
}
return false;
}
OffsetVarNodes_walker
/*
* OffsetVarNodes - adjust Vars when appending one query's RT to another
*
* Find all Var nodes in the given tree with varlevelsup == sublevels_up,
* and increment their varno fields (rangetable indexes) by 'offset'.
* The varnoold fields are adjusted similarly. Also, adjust other nodes
* that contain rangetable indexes, such as RangeTblRef and JoinExpr.
*
* NOTE: although this has the form of a walker, we cheat and modify the
* nodes in-place. The given expression tree should have been copied
* earlier to ensure that no unwanted side-effects occur!
*/
typedef struct
{
int offset;
int sublevels_up;
} OffsetVarNodes_context;
static bool
OffsetVarNodes_walker(Node *node, OffsetVarNodes_context *context)
{
if (node == NULL)
return false;
if (IsA(node, Var))
{
Var *var = (Var *) node;
if (var->varlevelsup == context->sublevels_up)
{
var->varno += context->offset;
var->varnoold += context->offset;
}
return false;
}
if (IsA(node, CurrentOfExpr))
{
CurrentOfExpr *cexpr = (CurrentOfExpr *) node;
if (context->sublevels_up == 0)
cexpr->cvarno += context->offset;
return false;
}
if (IsA(node, RangeTblRef))
{
RangeTblRef *rtr = (RangeTblRef *) node;
if (context->sublevels_up == 0)
rtr->rtindex += context->offset;
/* the subquery itself is visited separately */
return false;
}
if (IsA(node, JoinExpr))
{
JoinExpr *j = (JoinExpr *) node;
if (j->rtindex && context->sublevels_up == 0)
j->rtindex += context->offset;
/* fall through to examine children */
}
if (IsA(node, PlaceHolderVar))
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
if (phv->phlevelsup == context->sublevels_up)
{
phv->phrels = offset_relid_set(phv->phrels,
context->offset);
}
/* fall through to examine children */
}
if (IsA(node, AppendRelInfo))
{
AppendRelInfo *appinfo = (AppendRelInfo *) node;
if (context->sublevels_up == 0)
{
appinfo->parent_relid += context->offset;
appinfo->child_relid += context->offset;
}
/* fall through to examine children */
}
/* Shouldn't need to handle other planner auxiliary nodes here */
Assert(!IsA(node, PlanRowMark));
Assert(!IsA(node, SpecialJoinInfo));
Assert(!IsA(node, PlaceHolderInfo));
Assert(!IsA(node, MinMaxAggInfo));
if (IsA(node, Query))
{
/* Recurse into subselects */
bool result;
context->sublevels_up++;
result = query_tree_walker((Query *) node, OffsetVarNodes_walker,
(void *) context, 0);
context->sublevels_up--;
return result;
}
return expression_tree_walker(node, OffsetVarNodes_walker,
(void *) context);
}
query_or_expression_tree_walker
/*
* query_or_expression_tree_walker --- hybrid form
*
* This routine will invoke query_tree_walker if called on a Query node,
* else will invoke the walker directly. This is a useful way of starting
* the recursion when the walker's normal change of state is not appropriate
* for the outermost Query node.
*/
bool
query_or_expression_tree_walker(Node *node,
bool (*walker) (),
void *context,
int flags)
{
if (node && IsA(node, Query))
return query_tree_walker((Query *) node,
walker,
context,
flags);
else
return walker(node, context);
}
pull_varnos_walker
static bool
pull_varnos_walker(Node *node, pull_varnos_context *context)
{
if (node == NULL)
return false;
if (IsA(node, Var))
{
Var *var = (Var *) node;
if (var->varlevelsup == context->sublevels_up)
context->varnos = bms_add_member(context->varnos, var->varno);
return false;
}
if (IsA(node, CurrentOfExpr))
{
CurrentOfExpr *cexpr = (CurrentOfExpr *) node;
if (context->sublevels_up == 0)
context->varnos = bms_add_member(context->varnos, cexpr->cvarno);
return false;
}
if (IsA(node, PlaceHolderVar))
{
/*
* A PlaceHolderVar acts as a variable of its syntactic scope, or
* lower than that if it references only a subset of the rels in its
* syntactic scope. It might also contain lateral references, but we
* should ignore such references when computing the set of varnos in
* an expression tree. Also, if the PHV contains no variables within
* its syntactic scope, it will be forced to be evaluated exactly at
* the syntactic scope, so take that as the relid set.
*/
PlaceHolderVar *phv = (PlaceHolderVar *) node;
pull_varnos_context subcontext;
subcontext.varnos = NULL;
subcontext.sublevels_up = context->sublevels_up;
(void) pull_varnos_walker((Node *) phv->phexpr, &subcontext);
if (phv->phlevelsup == context->sublevels_up)
{
subcontext.varnos = bms_int_members(subcontext.varnos,
phv->phrels);
if (bms_is_empty(subcontext.varnos))
context->varnos = bms_add_members(context->varnos,
phv->phrels);
}
context->varnos = bms_join(context->varnos, subcontext.varnos);
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_varnos_walker,
(void *) context, 0);
context->sublevels_up--;
return result;
}
return expression_tree_walker(node, pull_varnos_walker,
(void *) context);
}
contain_vars_of_level_walker
static bool
contain_vars_of_level_walker(Node *node, int *sublevels_up)
{
if (node == NULL)
return false;
if (IsA(node, Var))
{
if (((Var *) node)->varlevelsup == *sublevels_up)
return true; /* abort tree traversal and return true */
return false;
}
if (IsA(node, CurrentOfExpr))
{
if (*sublevels_up == 0)
return true;
return false;
}
if (IsA(node, PlaceHolderVar))
{
if (((PlaceHolderVar *) node)->phlevelsup == *sublevels_up)
return true; /* abort the tree traversal and return true */
/* else fall through to check the contained expr */
}
if (IsA(node, Query))
{
/* Recurse into subselects */
bool result;
(*sublevels_up)++;
result = query_tree_walker((Query *) node,
contain_vars_of_level_walker,
(void *) sublevels_up,
0);
(*sublevels_up)--;
return result;
}
return expression_tree_walker(node,
contain_vars_of_level_walker,
(void *) sublevels_up);
}
expression_tree_walker
/*
* Standard expression-tree walking support
*
* We used to have near-duplicate code in many different routines that
* understood how to recurse through an expression node tree. That was
* a pain to maintain, and we frequently had bugs due to some particular
* routine neglecting to support a particular node type. In most cases,
* these routines only actually care about certain node types, and don't
* care about other types except insofar as they have to recurse through
* non-primitive node types. Therefore, we now provide generic tree-walking
* logic to consolidate the redundant "boilerplate" code. There are
* two versions: expression_tree_walker() and expression_tree_mutator().
*/
/*
* expression_tree_walker() is designed to support routines that traverse
* a tree in a read-only fashion (although it will also work for routines
* that modify nodes in-place but never add/delete/replace nodes).
* A walker routine should look like this:
*
* bool my_walker (Node *node, my_struct *context)
* {
* if (node == NULL)
* return false;
* // check for nodes that special work is required for, eg:
* if (IsA(node, Var))
* {
* ... do special actions for Var nodes
* }
* else if (IsA(node, ...))
* {
* ... do special actions for other node types
* }
* // for any node type not specially processed, do:
* return expression_tree_walker(node, my_walker, (void *) context);
* }
*
* The "context" argument points to a struct that holds whatever context
* information the walker routine needs --- it can be used to return data
* gathered by the walker, too. This argument is not touched by
* expression_tree_walker, but it is passed down to recursive sub-invocations
* of my_walker. The tree walk is started from a setup routine that
* fills in the appropriate context struct, calls my_walker with the top-level
* node of the tree, and then examines the results.
*
* The walker routine should return "false" to continue the tree walk, or
* "true" to abort the walk and immediately return "true" to the top-level
* caller. This can be used to short-circuit the traversal if the walker
* has found what it came for. "false" is returned to the top-level caller
* iff no invocation of the walker returned "true".
*
* The node types handled by expression_tree_walker include all those
* normally found in target lists and qualifier clauses during the planning
* stage. In particular, it handles List nodes since a cnf-ified qual clause
* will have List structure at the top level, and it handles TargetEntry nodes
* so that a scan of a target list can be handled without additional code.
* Also, RangeTblRef, FromExpr, JoinExpr, and SetOperationStmt nodes are
* handled, so that query jointrees and setOperation trees can be processed
* without additional code.
*
* expression_tree_walker will handle SubLink nodes by recursing normally
* into the "testexpr" subtree (which is an expression belonging to the outer
* plan). It will also call the walker on the sub-Query node; however, when
* expression_tree_walker itself is called on a Query node, it does nothing
* and returns "false". The net effect is that unless the walker does
* something special at a Query node, sub-selects will not be visited during
* an expression tree walk. This is exactly the behavior wanted in many cases
* --- and for those walkers that do want to recurse into sub-selects, special
* behavior is typically needed anyway at the entry to a sub-select (such as
* incrementing a depth counter). A walker that wants to examine sub-selects
* should include code along the lines of:
*
* if (IsA(node, Query))
* {
* adjust context for subquery;
* result = query_tree_walker((Query *) node, my_walker, context,
* 0); // adjust flags as needed
* restore context if needed;
* return result;
* }
*
* query_tree_walker is a convenience routine (see below) that calls the
* walker on all the expression subtrees of the given Query node.
*
* expression_tree_walker will handle SubPlan nodes by recursing normally
* into the "testexpr" and the "args" list (which are expressions belonging to
* the outer plan). It will not touch the completed subplan, however. Since
* there is no link to the original Query, it is not possible to recurse into
* subselects of an already-planned expression tree. This is OK for current
* uses, but may need to be revisited in future.
*/
bool
expression_tree_walker(Node *node,
bool (*walker) (),
void *context)
{
ListCell *temp;
/*
* The walker has already visited the current node, and so we need only
* recurse into any sub-nodes it has.
*
* We assume that the walker is not interested in List nodes per se, so
* when we expect a List we just recurse directly to self without
* bothering to call the walker.
*/
if (node == NULL)
return false;
/* Guard against stack overflow due to overly complex expressions */
check_stack_depth();
switch (nodeTag(node))
{
case T_Var:
case T_Const:
case T_Param:
case T_CaseTestExpr:
case T_SQLValueFunction:
case T_CoerceToDomainValue:
case T_SetToDefault:
case T_CurrentOfExpr:
case T_NextValueExpr:
case T_RangeTblRef:
case T_SortGroupClause:
/* primitive node types with no expression subnodes */
break;
case T_WithCheckOption:
return walker(((WithCheckOption *) node)->qual, context);
case T_Aggref:
{
Aggref *expr = (Aggref *) node;
/* recurse directly on List */
if (expression_tree_walker((Node *) expr->aggdirectargs,
walker, context))
return true;
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
if (expression_tree_walker((Node *) expr->aggorder,
walker, context))
return true;
if (expression_tree_walker((Node *) expr->aggdistinct,
walker, context))
return true;
if (walker((Node *) expr->aggfilter, context))
return true;
}
break;
case T_GroupingFunc:
{
GroupingFunc *grouping = (GroupingFunc *) node;
if (expression_tree_walker((Node *) grouping->args,
walker, context))
return true;
}
break;
case T_WindowFunc:
{
WindowFunc *expr = (WindowFunc *) node;
/* recurse directly on List */
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
if (walker((Node *) expr->aggfilter, context))
return true;
}
break;
case T_ArrayRef:
{
ArrayRef *aref = (ArrayRef *) node;
/* recurse directly for upper/lower array index lists */
if (expression_tree_walker((Node *) aref->refupperindexpr,
walker, context))
return true;
if (expression_tree_walker((Node *) aref->reflowerindexpr,
walker, context))
return true;
/* walker must see the refexpr and refassgnexpr, however */
if (walker(aref->refexpr, context))
return true;
if (walker(aref->refassgnexpr, context))
return true;
}
break;
case T_FuncExpr:
{
FuncExpr *expr = (FuncExpr *) node;
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
}
break;
case T_NamedArgExpr:
return walker(((NamedArgExpr *) node)->arg, context);
case T_OpExpr:
case T_DistinctExpr: /* struct-equivalent to OpExpr */
case T_NullIfExpr: /* struct-equivalent to OpExpr */
{
OpExpr *expr = (OpExpr *) node;
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
}
break;
case T_ScalarArrayOpExpr:
{
ScalarArrayOpExpr *expr = (ScalarArrayOpExpr *) node;
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
}
break;
case T_BoolExpr:
{
BoolExpr *expr = (BoolExpr *) node;
if (expression_tree_walker((Node *) expr->args,
walker, context))
return true;
}
break;
case T_SubLink:
{
SubLink *sublink = (SubLink *) node;
if (walker(sublink->testexpr, context))
return true;
/*
* Also invoke the walker on the sublink's Query node, so it
* can recurse into the sub-query if it wants to.
*/
return walker(sublink->subselect, context);
}
break;
case T_SubPlan:
{
SubPlan *subplan = (SubPlan *) node;
/* recurse into the testexpr, but not into the Plan */
if (walker(subplan->testexpr, context))
return true;
/* also examine args list */
if (expression_tree_walker((Node *) subplan->args,
walker, context))
return true;
}
break;
case T_AlternativeSubPlan:
return walker(((AlternativeSubPlan *) node)->subplans, context);
case T_FieldSelect:
return walker(((FieldSelect *) node)->arg, context);
case T_FieldStore:
{
FieldStore *fstore = (FieldStore *) node;
if (walker(fstore->arg, context))
return true;
if (walker(fstore->newvals, context))
return true;
}
break;
case T_RelabelType:
return walker(((RelabelType *) node)->arg, context);
case T_CoerceViaIO:
return walker(((CoerceViaIO *) node)->arg, context);
case T_ArrayCoerceExpr:
{
ArrayCoerceExpr *acoerce = (ArrayCoerceExpr *) node;
if (walker(acoerce->arg, context))
return true;
if (walker(acoerce->elemexpr, context))
return true;
}
break;
case T_ConvertRowtypeExpr:
return walker(((ConvertRowtypeExpr *) node)->arg, context);
case T_CollateExpr:
return walker(((CollateExpr *) node)->arg, context);
case T_CaseExpr:
{
CaseExpr *caseexpr = (CaseExpr *) node;
if (walker(caseexpr->arg, context))
return true;
/* we assume walker doesn't care about CaseWhens, either */
foreach(temp, caseexpr->args)
{
CaseWhen *when = lfirst_node(CaseWhen, temp);
if (walker(when->expr, context))
return true;
if (walker(when->result, context))
return true;
}
if (walker(caseexpr->defresult, context))
return true;
}
break;
case T_ArrayExpr:
return walker(((ArrayExpr *) node)->elements, context);
case T_RowExpr:
/* Assume colnames isn't interesting */
return walker(((RowExpr *) node)->args, context);
case T_RowCompareExpr:
{
RowCompareExpr *rcexpr = (RowCompareExpr *) node;
if (walker(rcexpr->largs, context))
return true;
if (walker(rcexpr->rargs, context))
return true;
}
break;
case T_CoalesceExpr:
return walker(((CoalesceExpr *) node)->args, context);
case T_MinMaxExpr:
return walker(((MinMaxExpr *) node)->args, context);
case T_XmlExpr:
{
XmlExpr *xexpr = (XmlExpr *) node;
if (walker(xexpr->named_args, context))
return true;
/* we assume walker doesn't care about arg_names */
if (walker(xexpr->args, context))
return true;
}
break;
case T_NullTest:
return walker(((NullTest *) node)->arg, context);
case T_BooleanTest:
return walker(((BooleanTest *) node)->arg, context);
case T_CoerceToDomain:
return walker(((CoerceToDomain *) node)->arg, context);
case T_TargetEntry:
return walker(((TargetEntry *) node)->expr, context);
case T_Query:
/* Do nothing with a sub-Query, per discussion above */
break;
case T_WindowClause:
{
WindowClause *wc = (WindowClause *) node;
if (walker(wc->partitionClause, context))
return true;
if (walker(wc->orderClause, context))
return true;
if (walker(wc->startOffset, context))
return true;
if (walker(wc->endOffset, context))
return true;
}
break;
case T_CommonTableExpr:
{
CommonTableExpr *cte = (CommonTableExpr *) node;
/*
* Invoke the walker on the CTE's Query node, so it can
* recurse into the sub-query if it wants to.
*/
return walker(cte->ctequery, context);
}
break;
case T_List:
foreach(temp, (List *) node)
{
if (walker((Node *) lfirst(temp), context))
return true;
}
break;
case T_FromExpr:
{
FromExpr *from = (FromExpr *) node;
if (walker(from->fromlist, context))
return true;
if (walker(from->quals, context))
return true;
}
break;
case T_OnConflictExpr:
{
OnConflictExpr *onconflict = (OnConflictExpr *) node;
if (walker((Node *) onconflict->arbiterElems, context))
return true;
if (walker(onconflict->arbiterWhere, context))
return true;
if (walker(onconflict->onConflictSet, context))
return true;
if (walker(onconflict->onConflictWhere, context))
return true;
if (walker(onconflict->exclRelTlist, context))
return true;
}
break;
case T_PartitionPruneStepOp:
{
PartitionPruneStepOp *opstep = (PartitionPruneStepOp *) node;
if (walker((Node *) opstep->exprs, context))
return true;
}
break;
case T_PartitionPruneStepCombine:
/* no expression subnodes */
break;
case T_JoinExpr:
{
JoinExpr *join = (JoinExpr *) node;
if (walker(join->larg, context))
return true;
if (walker(join->rarg, context))
return true;
if (walker(join->quals, context))
return true;
/*
* alias clause, using list are deemed uninteresting.
*/
}
break;
case T_SetOperationStmt:
{
SetOperationStmt *setop = (SetOperationStmt *) node;
if (walker(setop->larg, context))
return true;
if (walker(setop->rarg, context))
return true;
/* groupClauses are deemed uninteresting */
}
break;
case T_PlaceHolderVar:
return walker(((PlaceHolderVar *) node)->phexpr, context);
case T_InferenceElem:
return walker(((InferenceElem *) node)->expr, context);
case T_AppendRelInfo:
{
AppendRelInfo *appinfo = (AppendRelInfo *) node;
if (expression_tree_walker((Node *) appinfo->translated_vars,
walker, context))
return true;
}
break;
case T_PlaceHolderInfo:
return walker(((PlaceHolderInfo *) node)->ph_var, context);
case T_RangeTblFunction:
return walker(((RangeTblFunction *) node)->funcexpr, context);
case T_TableSampleClause:
{
TableSampleClause *tsc = (TableSampleClause *) node;
if (expression_tree_walker((Node *) tsc->args,
walker, context))
return true;
if (walker((Node *) tsc->repeatable, context))
return true;
}
break;
case T_TableFunc:
{
TableFunc *tf = (TableFunc *) node;
if (walker(tf->ns_uris, context))
return true;
if (walker(tf->docexpr, context))
return true;
if (walker(tf->rowexpr, context))
return true;
if (walker(tf->colexprs, context))
return true;
if (walker(tf->coldefexprs, context))
return true;
}
break;
default:
elog(ERROR, "unrecognized node type: %d",
(int) nodeTag(node));
break;
}
return false;
}
三、跟踪分析
测试脚本:
select *
from t_dwxx a
where exists (select b.dwbh from t_grxx b where a.dwbh = b.dwbh);
gdb分析:
(gdb) b convert_EXISTS_sublink_to_join
Breakpoint 1 at 0x77a4fe: file subselect.c, line 1426.
(gdb) c
Continuing.
Breakpoint 1, convert_EXISTS_sublink_to_join (root=0x22de828, sublink=0x22292a0, under_not=false, available_rels=0x22defa8)
at subselect.c:1426
1426 Query *parse = root->parse;
(gdb)
#1.root见上一节
#2.sublink,子链接,见本节查询树结构
#3.under_not,false,表示EXISTS
#4.available_rels,可用的rels
(gdb) p *available_rels
$1 = {nwords = 1, words = 0x22defac}
(gdb) p available_rels->words[0]
$3 = 2
(gdb)
...
1511 rtoffset = list_length(parse->rtable);
(gdb)
1512 OffsetVarNodes((Node *) subselect, rtoffset, 0);
(gdb) p rtoffset
$6 = 1
(gdb) step
OffsetVarNodes (node=0x22dee48, offset=1, sublevels_up=0) at rewriteManip.c:428
428 context.offset = offset;
...
#调整subselect->jointree->fromlist->head(类型为RTR)的rtindex,原为1,调整为2
(gdb) p *node
$20 = {type = T_RangeTblRef}
(gdb) p *(RangeTblRef *)node
$21 = {type = T_RangeTblRef, rtindex = 1}
(gdb) n
368 rtr->rtindex += context->offset;
(gdb)
370 return false;
(gdb) p *(RangeTblRef *)node
$22 = {type = T_RangeTblRef, rtindex = 2}
(gdb)
1513 OffsetVarNodes(whereClause, rtoffset, 0);
(gdb)
1520 IncrementVarSublevelsUp((Node *) subselect, -1, 1);
(gdb) p whereClause
$23 = (Node *) 0x22def58
(gdb) p *whereClause
$24 = {type = T_OpExpr}
(gdb) set $arg1=(RelabelType *)((OpExpr *)whereClause)->args->head->data.ptr_value
(gdb) p *$arg1->arg
$32 = {type = T_Var}
#第1个参数是t_dwxx.dwbh
(gdb) p *(Var *)$arg1->arg
$33 = {xpr = {type = T_Var}, varno = 1, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 1,
varnoold = 1, varoattno = 2, location = 72}
(gdb) set $arg2=(RelabelType *)((OpExpr *)whereClause)->args->tail->data.ptr_value
#第2个参数是t_grxx.dwbh
#varno/varnoold从原来的值1调整为2
(gdb) p *(Var *)$arg2->arg
$34 = {xpr = {type = T_Var}, varno = 2, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0,
varnoold = 2, varoattno = 1, location = 81}
(gdb)
(gdb) n
1521 IncrementVarSublevelsUp(whereClause, -1, 1);
(gdb)
1529 clause_varnos = pull_varnos(whereClause);
#调整varlevelsup为0
(gdb) p *(Var *)$arg1->arg
$36 = {xpr = {type = T_Var}, varno = 1, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0,
varnoold = 1, varoattno = 2, location = 72}
(gdb) p *(Var *)$arg2->arg
$37 = {xpr = {type = T_Var}, varno = 2, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0,
varnoold = 2, varoattno = 1, location = 81}
...
#构造返回值
(gdb)
1552 result = makeNode(JoinExpr);
1566 return result;
(gdb)
1567 }
(gdb)
pull_up_sublinks_qual_recurse (root=0x22de828, node=0x22292a0, jtlink1=0x7ffdcabbc918, available_rels1=0x22defa8,
jtlink2=0x0, available_rels2=0x0) at prepjointree.c:404
404 j->larg = *jtlink1;
#上拉成为半连接SEMI JOIN
(gdb) p *(JoinExpr *)jtnode
$65 = {type = T_JoinExpr, jointype = JOIN_SEMI, isNatural = false, larg = 0x22e7118, rarg = 0x22e7438, usingClause = 0x0,
quals = 0x22def58, alias = 0x0, rtindex = 0}
#DONE!
四、小结
1、上拉过程：与ANY子链接类似，主要逻辑是调整varno&varlevelsup等信息；
2、重要函数：XX_walker，遍历Node节点的函数，用于统计或更新Node信息。