先前的章节已介绍了函数query_planner中子函数remove_useless_joins、reduce_unique_semijoins和add_placeholders_to_base_rels的主要实现逻辑,本节继续介绍create_lateral_join_info、match_foreign_keys_to_quals和extract_restriction_or_clauses的实现逻辑。
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query_planner代码片段:
//...
/*
* Construct the lateral reference sets now that we have finalized
* PlaceHolderVar eval levels.
*/
create_lateral_join_info(root);//创建Lateral连接信息
/*
* Match foreign keys to equivalence classes and join quals. This must be
* done after finalizing equivalence classes, and it's useful to wait till
* after join removal so that we can skip processing foreign keys
* involving removed relations.
*/
match_foreign_keys_to_quals(root);//匹配外键信息
/*
* Look for join OR clauses that we can extract single-relation
* restriction OR clauses from.
*/
extract_restriction_or_clauses(root);//在OR语句中抽取约束条件
/*
* We should now have size estimates for every actual table involved in
* the query, and we also know which if any have been deleted from the
* query by join removal; so we can compute total_table_pages.
*
* Note that appendrels are not double-counted here, even though we don't
* bother to distinguish RelOptInfos for appendrel parents, because the
* parents will still have size zero.
*
* XXX if a table is self-joined, we will count it once per appearance,
* which perhaps is the wrong thing ... but that's not completely clear,
* and detecting self-joins here is difficult, so ignore it for now.
*/
total_pages = 0;
for (rti = 1; rti < root->simple_rel_array_size; rti++)//计算总pages
{
RelOptInfo *brel = root->simple_rel_array[rti];
if (brel == NULL)
continue;
Assert(brel->relid == rti); /* sanity check on array */
if (IS_SIMPLE_REL(brel))
total_pages += (double) brel->pages;
}
root->total_table_pages = total_pages;//赋值
//...
一、数据结构
RelOptInfo
RelOptInfo中,与LATERAL相关的数据结构
typedef struct RelOptInfo
{
NodeTag type;//节点标识
RelOptKind reloptkind;//RelOpt类型
//...
/* 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 */
//...
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 */
//...
} RelOptInfo;
二、源码解读
create_lateral_join_info
PG在提供LATERAL语法之前,假定所有的子查询都可以独立存在,不能互相引用属性或者引用上层的属性,为了可以引用其他或上层的属性,需要在子查询前面显式指定LATERAL关键字.
比如以下的SQL语句,不显式指定LATERAL关键字无法正常运行:
testdb=# select a.*,b.grbh,b.je
testdb-# from t_dwxx a,(select t1.dwbh,t1.grbh,t2.je from t_grxx t1 inner join t_jfxx t2 on t1.dwbh = a.dwbh and t1.grbh = t2.grbh) b
testdb-# where a.dwbh = '1001'
testdb-# order by b.dwbh;
ERROR: invalid reference to FROM-clause entry for table "a"
LINE 2: ... from t_grxx t1 inner join t_jfxx t2 on t1.dwbh = a.dwbh and...
^
HINT: There is an entry for table "a", but it cannot be referenced from this part of the query.
在子查询前显式指定LATERAL后,可以正常运行:
testdb=# select a.*,b.grbh,b.je
testdb-# from t_dwxx a,lateral (select t1.dwbh,t1.grbh,t2.je from t_grxx t1 inner join t_jfxx t2 on t1.dwbh = a.dwbh and t1.grbh = t2.grbh) b
testdb-# where a.dwbh = '1001'
testdb-# order by b.dwbh;
dwmc | dwbh | dwdz | grbh | je
-----------+------+--------------------+------+-------
X有限公司 | 1001 | 广东省广州市荔湾区 | 901 | 401.3
X有限公司 | 1001 | 广东省广州市荔湾区 | 901 | 401.3
X有限公司 | 1001 | 广东省广州市荔湾区 | 901 | 401.3
如函数注释所描述的,create_lateral_join_info函数的作用是填充RelOptInfo中的相关四个变量,"Fill in the per-base-relation direct_lateral_relids, lateral_relids和and lateral_referencers sets"
源代码如下:
/*
* create_lateral_join_info
* Fill in the per-base-relation direct_lateral_relids, lateral_relids
* and lateral_referencers sets.
*
* This has to run after deconstruct_jointree, because we need to know the
* final ph_eval_at values for PlaceHolderVars.
*/
void
create_lateral_join_info(PlannerInfo *root)
{
bool found_laterals = false;
Index rti;
ListCell *lc;
/* We need do nothing if the query contains no LATERAL RTEs */
if (!root->hasLateralRTEs)//是否存在LateralRTE
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 *brel = root->simple_rel_array[rti];
Relids lateral_relids;
/* there may be empty slots corresponding to non-baserel RTEs */
if (brel == NULL)
continue;
Assert(brel->relid == rti); /* sanity check on array */
/* ignore RTEs that are "other rels" */
if (brel->reloptkind != RELOPT_BASEREL)
continue;
lateral_relids = NULL;
/* consider each laterally-referenced Var or PHV */
foreach(lc, brel->lateral_vars)
{
Node *node = (Node *) lfirst(lc);
if (IsA(node, Var))
{
Var *var = (Var *) node;
found_laterals = true;
lateral_relids = bms_add_member(lateral_relids,
var->varno);
}
else if (IsA(node, PlaceHolderVar))
{
PlaceHolderVar *phv = (PlaceHolderVar *) node;
PlaceHolderInfo *phinfo = find_placeholder_info(root, phv,
false);
found_laterals = true;
lateral_relids = bms_add_members(lateral_relids,
phinfo->ph_eval_at);
}
else
Assert(false);
}
/* We now have all the simple lateral refs from this rel */
brel->direct_lateral_relids = lateral_relids;
brel->lateral_relids = bms_copy(lateral_relids);
}
/*
* Now check for lateral references within PlaceHolderVars, and mark their
* eval_at rels as having lateral references to the source rels.
*
* For a PHV that is due to be evaluated at a baserel, mark its source(s)
* as direct lateral dependencies of the baserel (adding onto the ones
* recorded above). If it's due to be evaluated at a join, mark its
* source(s) as indirect lateral dependencies of each baserel in the join,
* ie put them into lateral_relids but not direct_lateral_relids. This is
* appropriate because we can't put any such baserel on the outside of a
* join to one of the PHV's lateral dependencies, but on the other hand we
* also can't yet join it directly to the dependency.
*/
foreach(lc, root->placeholder_list)
{
PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
Relids eval_at = phinfo->ph_eval_at;
int varno;
if (phinfo->ph_lateral == NULL)
continue; /* PHV is uninteresting if no lateral refs */
found_laterals = true;
if (bms_get_singleton_member(eval_at, &varno))
{
/* Evaluation site is a baserel */
RelOptInfo *brel = find_base_rel(root, varno);
brel->direct_lateral_relids =
bms_add_members(brel->direct_lateral_relids,
phinfo->ph_lateral);
brel->lateral_relids =
bms_add_members(brel->lateral_relids,
phinfo->ph_lateral);
}
else
{
/* Evaluation site is a join */
varno = -1;
while ((varno = bms_next_member(eval_at, varno)) >= 0)
{
RelOptInfo *brel = find_base_rel(root, varno);
brel->lateral_relids = bms_add_members(brel->lateral_relids,
phinfo->ph_lateral);
}
}
}
/*
* If we found no actual lateral references, we're done; but reset the
* hasLateralRTEs flag to avoid useless work later.
*/
if (!found_laterals)
{
root->hasLateralRTEs = false;
return;
}
/*
* Calculate the transitive closure of the lateral_relids sets, so that
* they describe both direct and indirect lateral references. If relation
* X references Y laterally, and Y references Z laterally, then we will
* have to scan X on the inside of a nestloop with Z, so for all intents
* and purposes X is laterally dependent on Z too.
*
* This code is essentially Warshall's algorithm for transitive closure.
* The outer loop considers each baserel, and propagates its lateral
* dependencies to those baserels that have a lateral dependency on it.
*/
for (rti = 1; rti < root->simple_rel_array_size; rti++)
{
RelOptInfo *brel = root->simple_rel_array[rti];
Relids outer_lateral_relids;
Index rti2;
if (brel == NULL || brel->reloptkind != RELOPT_BASEREL)
continue;
/* need not consider baserel further if it has no lateral refs */
outer_lateral_relids = brel->lateral_relids;
if (outer_lateral_relids == NULL)
continue;
/* else scan all baserels */
for (rti2 = 1; rti2 < root->simple_rel_array_size; rti2++)
{
RelOptInfo *brel2 = root->simple_rel_array[rti2];
if (brel2 == NULL || brel2->reloptkind != RELOPT_BASEREL)
continue;
/* if brel2 has lateral ref to brel, propagate brel's refs */
if (bms_is_member(rti, brel2->lateral_relids))
brel2->lateral_relids = bms_add_members(brel2->lateral_relids,
outer_lateral_relids);
}
}
/*
* Now that we've identified all lateral references, mark each baserel
* with the set of relids of rels that reference it laterally (possibly
* indirectly) --- that is, the inverse mapping of lateral_relids.
*/
for (rti = 1; rti < root->simple_rel_array_size; rti++)
{
RelOptInfo *brel = root->simple_rel_array[rti];
Relids lateral_relids;
int rti2;
if (brel == NULL || brel->reloptkind != RELOPT_BASEREL)
continue;
/* Nothing to do at rels with no lateral refs */
lateral_relids = brel->lateral_relids;
if (lateral_relids == NULL)
continue;
/*
* We should not have broken the invariant that lateral_relids is
* exactly NULL if empty.
*/
Assert(!bms_is_empty(lateral_relids));
/* Also, no rel should have a lateral dependency on itself */
Assert(!bms_is_member(rti, lateral_relids));
/* Mark this rel's referencees */
rti2 = -1;
while ((rti2 = bms_next_member(lateral_relids, rti2)) >= 0)
{
RelOptInfo *brel2 = root->simple_rel_array[rti2];
Assert(brel2 != NULL && brel2->reloptkind == RELOPT_BASEREL);
brel2->lateral_referencers =
bms_add_member(brel2->lateral_referencers, rti);
}
}
/*
* Lastly, propagate lateral_relids and lateral_referencers from appendrel
* parent rels to their child rels. We intentionally give each child rel
* the same minimum parameterization, even though it's quite possible that
* some don't reference all the lateral rels. This is because any append
* path for the parent will have to have the same parameterization for
* every child anyway, and there's no value in forcing extra
* reparameterize_path() calls. Similarly, a lateral reference to the
* parent prevents use of otherwise-movable join rels for each child.
*/
for (rti = 1; rti < root->simple_rel_array_size; rti++)
{
RelOptInfo *brel = root->simple_rel_array[rti];
RangeTblEntry *brte = root->simple_rte_array[rti];
/*
* Skip empty slots. Also skip non-simple relations i.e. dead
* relations.
*/
if (brel == NULL || !IS_SIMPLE_REL(brel))
continue;
/*
* In the case of table inheritance, the parent RTE is directly linked
* to every child table via an AppendRelInfo. In the case of table
* partitioning, the inheritance hierarchy is expanded one level at a
* time rather than flattened. Therefore, an other member rel that is
* a partitioned table may have children of its own, and must
* therefore be marked with the appropriate lateral info so that those
* children eventually get marked also.
*/
Assert(brte);
if (brel->reloptkind == RELOPT_OTHER_MEMBER_REL &&
(brte->rtekind != RTE_RELATION ||
brte->relkind != RELKIND_PARTITIONED_TABLE))
continue;
if (brte->inh)
{
foreach(lc, root->append_rel_list)
{
AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(lc);
RelOptInfo *childrel;
if (appinfo->parent_relid != rti)
continue;
childrel = root->simple_rel_array[appinfo->child_relid];
Assert(childrel->reloptkind == RELOPT_OTHER_MEMBER_REL);
Assert(childrel->direct_lateral_relids == NULL);
childrel->direct_lateral_relids = brel->direct_lateral_relids;
Assert(childrel->lateral_relids == NULL);
childrel->lateral_relids = brel->lateral_relids;
Assert(childrel->lateral_referencers == NULL);
childrel->lateral_referencers = brel->lateral_referencers;
}
}
}
}
跟踪分析:
(gdb) b planmain.c:173
Breakpoint 1 at 0x76961a: file planmain.c, line 173.
(gdb) c
Continuing.
Breakpoint 1, query_planner (root=0x1702b80, tlist=0x174a870, qp_callback=0x76e97d ,
qp_extra=0x7ffd35e059c0) at planmain.c:177
...
(gdb)
212 create_lateral_join_info(root);
查看root变量:
(gdb) p *root
$11 = {..., hasLateralRTEs = false, ...}
经过处理后,LATERAL已经消失(hasLateralRTEs = false),不需要进行处理.
match_foreign_keys_to_quals
这是外键相关的处理,等价类与外键约束进行匹配并加入到条件语句(quals)中
/*
* match_foreign_keys_to_quals
* Match foreign-key constraints to equivalence classes and join quals
*
* The idea here is to see which query join conditions match equality
* constraints of a foreign-key relationship. For such join conditions,
* we can use the FK semantics to make selectivity estimates that are more
* reliable than estimating from statistics, especially for multiple-column
* FKs, where the normal assumption of independent conditions tends to fail.
*
* In this function we annotate the ForeignKeyOptInfos in root->fkey_list
* with info about which eclasses and join qual clauses they match, and
* discard any ForeignKeyOptInfos that are irrelevant for the query.
*/
void
match_foreign_keys_to_quals(PlannerInfo *root)
{
List *newlist = NIL;
ListCell *lc;
foreach(lc, root->fkey_list)
{
ForeignKeyOptInfo *fkinfo = (ForeignKeyOptInfo *) lfirst(lc);
RelOptInfo *con_rel;
RelOptInfo *ref_rel;
int colno;
/*
* Either relid might identify a rel that is in the query's rtable but
* isn't referenced by the jointree so won't have a RelOptInfo. Hence
* don't use find_base_rel() here. We can ignore such FKs.
*/
if (fkinfo->con_relid >= root->simple_rel_array_size ||
fkinfo->ref_relid >= root->simple_rel_array_size)
continue; /* just paranoia */
con_rel = root->simple_rel_array[fkinfo->con_relid];
if (con_rel == NULL)
continue;
ref_rel = root->simple_rel_array[fkinfo->ref_relid];
if (ref_rel == NULL)
continue;
/*
* Ignore FK unless both rels are baserels. This gets rid of FKs that
* link to inheritance child rels (otherrels) and those that link to
* rels removed by join removal (dead rels).
*/
if (con_rel->reloptkind != RELOPT_BASEREL ||
ref_rel->reloptkind != RELOPT_BASEREL)
continue;
/*
* Scan the columns and try to match them to eclasses and quals.
*
* Note: for simple inner joins, any match should be in an eclass.
* "Loose" quals that syntactically match an FK equality must have
* been rejected for EC status because they are outer-join quals or
* similar. We can still consider them to match the FK if they are
* not outerjoin_delayed.
*/
for (colno = 0; colno < fkinfo->nkeys; colno++)
{
AttrNumber con_attno,
ref_attno;
Oid fpeqop;
ListCell *lc2;
fkinfo->eclass[colno] = match_eclasses_to_foreign_key_col(root,
fkinfo,
colno);
/* Don't bother looking for loose quals if we got an EC match */
if (fkinfo->eclass[colno] != NULL)
{
fkinfo->nmatched_ec++;
continue;
}
/*
* Scan joininfo list for relevant clauses. Either rel's joininfo
* list would do equally well; we use con_rel's.
*/
con_attno = fkinfo->conkey[colno];
ref_attno = fkinfo->confkey[colno];
fpeqop = InvalidOid; /* we'll look this up only if needed */
foreach(lc2, con_rel->joininfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc2);
OpExpr *clause = (OpExpr *) rinfo->clause;
Var *leftvar;
Var *rightvar;
/* Ignore outerjoin-delayed clauses */
if (rinfo->outerjoin_delayed)
continue;
/* Only binary OpExprs are useful for consideration */
if (!IsA(clause, OpExpr) ||
list_length(clause->args) != 2)
continue;
leftvar = (Var *) get_leftop((Expr *) clause);
rightvar = (Var *) get_rightop((Expr *) clause);
/* Operands must be Vars, possibly with RelabelType */
while (leftvar && IsA(leftvar, RelabelType))
leftvar = (Var *) ((RelabelType *) leftvar)->arg;
if (!(leftvar && IsA(leftvar, Var)))
continue;
while (rightvar && IsA(rightvar, RelabelType))
rightvar = (Var *) ((RelabelType *) rightvar)->arg;
if (!(rightvar && IsA(rightvar, Var)))
continue;
/* Now try to match the vars to the current foreign key cols */
if (fkinfo->ref_relid == leftvar->varno &&
ref_attno == leftvar->varattno &&
fkinfo->con_relid == rightvar->varno &&
con_attno == rightvar->varattno)
{
/* Vars match, but is it the right operator? */
if (clause->opno == fkinfo->conpfeqop[colno])
{
fkinfo->rinfos[colno] = lappend(fkinfo->rinfos[colno],
rinfo);
fkinfo->nmatched_ri++;
}
}
else if (fkinfo->ref_relid == rightvar->varno &&
ref_attno == rightvar->varattno &&
fkinfo->con_relid == leftvar->varno &&
con_attno == leftvar->varattno)
{
/*
* Reverse match, must check commutator operator. Look it
* up if we didn't already. (In the worst case we might
* do multiple lookups here, but that would require an FK
* equality operator without commutator, which is
* unlikely.)
*/
if (!OidIsValid(fpeqop))
fpeqop = get_commutator(fkinfo->conpfeqop[colno]);
if (clause->opno == fpeqop)
{
fkinfo->rinfos[colno] = lappend(fkinfo->rinfos[colno],
rinfo);
fkinfo->nmatched_ri++;
}
}
}
/* If we found any matching loose quals, count col as matched */
if (fkinfo->rinfos[colno])
fkinfo->nmatched_rcols++;
}
/*
* Currently, we drop multicolumn FKs that aren't fully matched to the
* query. Later we might figure out how to derive some sort of
* estimate from them, in which case this test should be weakened to
* "if ((fkinfo->nmatched_ec + fkinfo->nmatched_rcols) > 0)".
*/
if ((fkinfo->nmatched_ec + fkinfo->nmatched_rcols) == fkinfo->nkeys)
newlist = lappend(newlist, fkinfo);
}
/* Replace fkey_list, thereby discarding any useless entries */
root->fkey_list = newlist;
}
extract_restriction_or_clauses
检查join连接条件的OR-of-AND语句,如存在有用的OR约束条件,则提取出来.
如代码注释所描述,((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45)),可以提取条件(a.x = 42 OR a.x = 44) AND (b.y = 43 OR b.z = 45),提取这些条件的目的是为了在连接前能把这些条件下推到关系中,减少参与连接运算的元组数量.
比如:
testdb=# explain verbose select t1.*
from t_dwxx t1 inner join t_grxx t2
on (t1.dwbh = '1001' and t2.grbh = '901') OR (t1.dwbh = '1002' and t2.grbh = '902');
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
--------------------------------------
Nested Loop (cost=0.00..17.23 rows=5 width=474)
Output: t1.dwmc, t1.dwbh, t1.dwdz
Join Filter: ((((t1.dwbh)::text = '1001'::text) AND ((t2.grbh)::text = '901'::text)) OR (((t1.dwbh)::text = '1002'::text)
AND ((t2.grbh)::text = '902'::text)))
-> Seq Scan on public.t_grxx t2 (cost=0.00..16.00 rows=4 width=38)
Output: t2.dwbh, t2.grbh, t2.xm, t2.xb, t2.nl
Filter: (((t2.grbh)::text = '901'::text) OR ((t2.grbh)::text = '902'::text))
-> Materialize (cost=0.00..1.05 rows=2 width=474)
Output: t1.dwmc, t1.dwbh, t1.dwdz
-> Seq Scan on public.t_dwxx t1 (cost=0.00..1.04 rows=2 width=474)
Output: t1.dwmc, t1.dwbh, t1.dwdz
Filter: (((t1.dwbh)::text = '1001'::text) OR ((t1.dwbh)::text = '1002'::text))
(11 rows)
可以看到,t1.dwbh = '1001' OR t1.dwbh = '1002'和t2.grbh = '901' OR t2.grbh = '902'在连接前下推到数据表扫描作为过滤条件.
/*
* extract_restriction_or_clauses
* Examine join OR-of-AND clauses to see if any useful restriction OR
* clauses can be extracted. If so, add them to the query.
*
* Although a join clause must reference multiple relations overall,
* an OR of ANDs clause might contain sub-clauses that reference just one
* relation and can be used to build a restriction clause for that rel.
* For example consider
* WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45));
* We can transform this into
* WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45))
* AND (a.x = 42 OR a.x = 44)
* AND (b.y = 43 OR b.z = 45);
* which allows the latter clauses to be applied during the scans of a and b,
* perhaps as index qualifications, and in any case reducing the number of
* rows arriving at the join. In essence this is a partial transformation to
* CNF (AND of ORs format). It is not complete, however, because we do not
* unravel the original OR --- doing so would usually bloat the qualification
* expression to little gain.
*
* The added quals are partially redundant with the original OR, and therefore
* would cause the size of the joinrel to be underestimated when it is finally
* formed. (This would be true of a full transformation to CNF as well; the
* fault is not really in the transformation, but in clauselist_selectivity's
* inability to recognize redundant conditions.) We can compensate for this
* redundancy by changing the cached selectivity of the original OR clause,
* canceling out the (valid) reduction in the estimated sizes of the base
* relations so that the estimated joinrel size remains the same. This is
* a MAJOR HACK: it depends on the fact that clause selectivities are cached
* and on the fact that the same RestrictInfo node will appear in every
* joininfo list that might be used when the joinrel is formed.
* And it doesn't work in cases where the size estimation is nonlinear
* (i.e., outer and IN joins). But it beats not doing anything.
*
* We examine each base relation to see if join clauses associated with it
* contain extractable restriction conditions. If so, add those conditions
* to the rel's baserestrictinfo and update the cached selectivities of the
* join clauses. Note that the same join clause will be examined afresh
* from the point of view of each baserel that participates in it, so its
* cached selectivity may get updated multiple times.
*/
void
extract_restriction_or_clauses(PlannerInfo *root)
{
Index rti;
/* Examine each baserel for potential join OR clauses */
for (rti = 1; rti < root->simple_rel_array_size; rti++)
{
RelOptInfo *rel = root->simple_rel_array[rti];
ListCell *lc;
/* there may be empty slots corresponding to non-baserel RTEs */
if (rel == NULL)
continue;
Assert(rel->relid == rti); /* sanity check on array */
/* ignore RTEs that are "other rels" */
if (rel->reloptkind != RELOPT_BASEREL)
continue;
/*
* Find potentially interesting OR joinclauses. We can use any
* joinclause that is considered safe to move to this rel by the
* parameterized-path machinery, even though what we are going to do
* with it is not exactly a parameterized path.
*
* However, it seems best to ignore clauses that have been marked
* redundant (by setting norm_selec > 1). That likely can't happen
* for OR clauses, but let's be safe.
*/
foreach(lc, rel->joininfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
if (restriction_is_or_clause(rinfo) &&
join_clause_is_movable_to(rinfo, rel) &&
rinfo->norm_selec <= 1)
{
/* Try to extract a qual for this rel only */
Expr *orclause = extract_or_clause(rinfo, rel);
/*
* If successful, decide whether we want to use the clause,
* and insert it into the rel's restrictinfo list if so.
*/
if (orclause)
consider_new_or_clause(root, rel, orclause, rinfo);
}
}
}
}
三、参考资料
planmain.c
