PostgreSQL 源码解读（33）- 查询语句#18（查询优化-表达式预处理#3）

2024-04-02 19:04:59 735人浏览安东尼

摘要

本节简单介绍了PG查询优化表达式预处理中的规范化过程。规范化具体的做法一是忽略NULL以及OR中的False,And中的True(实现函数find_duplicate_ors),二

本节简单介绍了PG查询优化表达式预处理中的规范化过程。规范化具体的做法一是忽略NULL以及OR中的False,And中的True(实现函数find_duplicate_ors),二是拉平谓词(实现函数:pull_ors/pull_ands),三是清除重复的ORs(实现函数process_duplicate_ors)。这些函数位于文件src/backend/optimizer/prep/prepqual.c中。

一、布尔代数基础

规范化处理基于布尔/逻辑代数运算的相关基本定律:
幂等律　
A∪A = A
A∩A = A
交换律
A∪B = B∪A
A∩B = B∩A
结合律　
A∪(B∪С) = (A∪B) ∪ С
A∩(B∩С) = (A∩B）∩ С
吸收律　
A∪(A∩B) = A
A∩(A∪B) = A
分配律　
A∪（B∩С）=(A∪B）∩（A∪С）
A∩（B∪С）=(A∩B）∪（A∩С）
幺元律　
0∪A = A
1∩A = A
1∪A = 1
0∩A = 0
补余律　
A∪A' = 1
A∩A' = 0

二、基本概念

PG源码对规范化表达式的注释如下:

忽略NULL以及OR中的False/AND中的TRUE
Where条件语句中的NULL/FALSE/TRUE,如能忽略,忽略之.如:NULL OR FALSE OR dwbh = '1001',则忽略NULL/ FALSE

testdb=# explain verbose select * from t_dwxx where NULL OR FALSE OR dwbh = '1001' ;
                           QUERY PLAN                           
----------------------------------------------------------------
 Seq Scan on public.t_dwxx  (cost=0.00..12.00 rows=1 width=474)
   Output: dwmc, dwbh, dwdz
   Filter: ((t_dwxx.dwbh)::text = '1001'::text)
(3 rows)

拉平谓词
sql语句中的X1 OR/AND (X2 OR/AND X3),拉平简化为OR/AND(X1,X2,X3)
X1 OR/AND (X2 OR/AND X3),在查询树中为树状结构,第一层节点是BoolExpr,该node中的args链表有2个元素,args->1=X1,args->2=BoolExpr,args->2->1=X2,args->2->2=X3,组成树状结构.简化后args->1/2/3=X1/X2/X3,所有条件处于同一个层次上,并不是树状结构.

testdb=# explain select * from t_dwxx where dwbh = '1001' OR (dwbh = '1002' OR dwbh = '1003');
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Seq Scan on t_dwxx  (cost=0.00..12.80 rows=3 width=474)
   Filter: (((dwbh)::text = '1001'::text) OR ((dwbh)::text = '1002'::text) OR ((dwbh)::text = '1003'::text))
(2 rows)

清除重复ORs
清除重复ORs的数学基础是布尔(逻辑)代数:
(X1 AND X2) OR (X1 AND X3) 应用分配律可以改写为 X1 AND (X2 OR X3),这样改写的目的是把X1抽取出来,为后续下推谓词X1作准备.
如(dwbh = '1001' AND dwbh = '1002') OR (dwbh = '1001' AND dwbh = '1003')条件,会改写为dwbh = '1001' AND (dwbh = '1002' OR dwbh = '1003')

testdb=# explain verbose select * from t_dwxx where (dwbh = '1001' AND dwbh = '1002')  OR (dwbh = '1001' AND dwbh = '1003');
                                                             QUERY PLAN                                                      
        
-----------------------------------------------------------------------------------------------------------------------------
--------
 Seq Scan on public.t_dwxx  (cost=0.00..12.80 rows=1 width=474)
   Output: dwmc, dwbh, dwdz
   Filter: (((t_dwxx.dwbh)::text = '1001'::text) AND (((t_dwxx.dwbh)::text = '1002'::text) OR ((t_dwxx.dwbh)::text = '1003'::
text)))
(3 rows)

三、源码解读

主函数入口:
subquery_planner


 PlannerInfo *
 subquery_planner(PlannerGlobal *glob, Query *parse,
                  PlannerInfo *parent_root,
                  bool hasRecursion, double tuple_fraction)
 {
     PlannerInfo *root;//返回值
     List       *newWithCheckOptions;//
     List       *newHaving;//Having子句
     bool        hasOuterJoins;//是否存在Outer Join?
     RelOptInfo *final_rel;//
     ListCell   *l;//临时变量
 
     
     root = makeNode(PlannerInfo);//构造返回值
     root->parse = parse;
     root->glob = glob;
     root->query_level = parent_root ? parent_root->query_level + 1 : 1;
     root->parent_root = parent_root;
     root->plan_params = NIL;
     root->outer_params = NULL;
     root->planner_cxt = CurrentMemoryContext;
     root->init_plans = NIL;
     root->cte_plan_ids = NIL;
     root->multiexpr_params = NIL;
     root->eq_classes = NIL;
     root->append_rel_list = NIL;
     root->rowMarks = NIL;
     memset(root->upper_rels, 0, sizeof(root->upper_rels));
     memset(root->upper_targets, 0, sizeof(root->upper_targets));
     root->processed_tlist = NIL;
     root->grouping_map = NULL;
     root->minmax_aggs = NIL;
     root->qual_security_level = 0;
     root->inhTargetKind = INHKIND_NONE;
     root->hasRecursion = hasRecursion;
     if (hasRecursion)
         root->wt_param_id = SS_assign_special_param(root);
     else
         root->wt_param_id = -1;
     root->non_recursive_path = NULL;
     root->partColsUpdated = false;
 
     
     if (parse->cteList)
         SS_process_ctes(root);//处理With 语句
 
     
     if (parse->hasSubLinks)
         pull_up_sublinks(root); //上拉子链接
 
     
     inline_set_returning_functions(root);//
 
     
     pull_up_subqueries(root);//上拉子查询
 
     
     if (parse->setOperations)
         flatten_simple_uNIOn_all(root);//扁平化处理UNION ALL
 
     
     //判断RTE中是否存在RTE_JOIN?
     root->hasJoinRTEs = false;
     root->hasLateralRTEs = false;
     hasOuterJoins = false;
     foreach(l, parse->rtable)
     {
         RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);
 
         if (rte->rtekind == RTE_JOIN)
         {
             root->hasJoinRTEs = true;
             if (IS_OUTER_JOIN(rte->jointype))
                 hasOuterJoins = true;
         }
         if (rte->lateral)
             root->hasLateralRTEs = true;
     }
 
     
      //预处理RowMark信息
     preprocess_rowmarks(root);
 
     
      //展开继承表
     expand_inherited_tables(root);
 
     
     //是否存在Having表达式
     root->hasHavingQual = (parse->havingQual != NULL);
 
     
     root->hasPseudoConstantQuals = false;
 
     
     //预处理表达式:targetList(投影列)
     parse->targetList = (List *)
         preprocess_expression(root, (Node *) parse->targetList,
                               EXPRKIND_TARGET);
 
     
     if (parse->hasTargetSRFs)
         parse->hasTargetSRFs = expression_returns_set((Node *) parse->targetList);
 
     newWithCheckOptions = NIL;
     foreach(l, parse->withCheckOptions)//witch Check Options
     {
         WithCheckOption *wco = lfirst_node(WithCheckOption, l);
 
         wco->qual = preprocess_expression(root, wco->qual,
                                           EXPRKIND_QUAL);
         if (wco->qual != NULL)
             newWithCheckOptions = lappend(newWithCheckOptions, wco);
     }
     parse->withCheckOptions = newWithCheckOptions;
     //返回列信息returningList
     parse->returningList = (List *)
         preprocess_expression(root, (Node *) parse->returningList,
                               EXPRKIND_TARGET);
     //预处理条件表达式
     preprocess_qual_conditions(root, (Node *) parse->jointree);
     //预处理Having表达式
     parse->havingQual = preprocess_expression(root, parse->havingQual,
                                               EXPRKIND_QUAL);
     //窗口函数
     foreach(l, parse->windowClause)
     {
         WindowClause *wc = lfirst_node(WindowClause, l);
 
         
         wc->startOffset = preprocess_expression(root, wc->startOffset,
                                                 EXPRKIND_LIMIT);
         wc->endOffset = preprocess_expression(root, wc->endOffset,
                                               EXPRKIND_LIMIT);
     }
     //Limit子句
     parse->limitOffset = preprocess_expression(root, parse->limitOffset,
                                                EXPRKIND_LIMIT);
     parse->limitCount = preprocess_expression(root, parse->limitCount,
                                               EXPRKIND_LIMIT);
     //On Conflict子句
     if (parse->onConflict)
     {
         parse->onConflict->arbiterElems = (List *)
             preprocess_expression(root,
                                   (Node *) parse->onConflict->arbiterElems,
                                   EXPRKIND_ARBITER_ELEM);
         parse->onConflict->arbiterWhere =
             preprocess_expression(root,
                                   parse->onConflict->arbiterWhere,
                                   EXPRKIND_QUAL);
         parse->onConflict->onConflictSet = (List *)
             preprocess_expression(root,
                                   (Node *) parse->onConflict->onConflictSet,
                                   EXPRKIND_TARGET);
         parse->onConflict->onConflictWhere =
             preprocess_expression(root,
                                   parse->onConflict->onConflictWhere,
                                   EXPRKIND_QUAL);
         
     }
     //集合操作(AppendRelInfo)
     root->append_rel_list = (List *)
         preprocess_expression(root, (Node *) root->append_rel_list,
                               EXPRKIND_APPINFO);
     //RTE
     
     foreach(l, parse->rtable)
     {
         RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);
         int         kind;
         ListCell   *lcsq;
 
         if (rte->rtekind == RTE_RELATION)
         {
             if (rte->tablesample)
                 rte->tablesample = (TableSampleClause *)
                     preprocess_expression(root,
                                           (Node *) rte->tablesample,
                                           EXPRKIND_TABLESAMPLE);//数据表采样语句
         }
         else if (rte->rtekind == RTE_SUBQUERY)//子查询
         {
             
             if (rte->lateral && root->hasJoinRTEs)
                 rte->subquery = (Query *)
                     flatten_join_alias_vars(root, (Node *) rte->subquery);
         }
         else if (rte->rtekind == RTE_FUNCTION)//函数
         {
             
             kind = rte->lateral ? EXPRKIND_RTFUNC_LATERAL : EXPRKIND_RTFUNC;
             rte->functions = (List *)
                 preprocess_expression(root, (Node *) rte->functions, kind);
         }
         else if (rte->rtekind == RTE_TABLEFUNC)//TABLE FUNC
         {
             
             kind = rte->lateral ? EXPRKIND_TABLEFUNC_LATERAL : EXPRKIND_TABLEFUNC;
             rte->tablefunc = (TableFunc *)
                 preprocess_expression(root, (Node *) rte->tablefunc, kind);
         }
         else if (rte->rtekind == RTE_VALUES)//VALUES子句
         {
             
             kind = rte->lateral ? EXPRKIND_VALUES_LATERAL : EXPRKIND_VALUES;
             rte->values_lists = (List *)
                 preprocess_expression(root, (Node *) rte->values_lists, kind);
         }
 
         
         foreach(lcsq, rte->securityQuals)
         {
             lfirst(lcsq) = preprocess_expression(root,
                                                  (Node *) lfirst(lcsq),
                                                  EXPRKIND_QUAL);
         }
     }
 
     ...//其他
     
     return root;
 }

preprocess_expression

 
 static Node *
 preprocess_expression(PlannerInfo *root, Node *expr, int kind)
 {
     
     if (expr == NULL)
         return NULL;
 
     
     if (root->hasJoinRTEs &&
         !(kind == EXPRKIND_RTFUNC ||
           kind == EXPRKIND_VALUES ||
           kind == EXPRKIND_TABLESAMPLE ||
           kind == EXPRKIND_TABLEFUNC))
         expr = flatten_join_alias_vars(root, expr);//扁平化处理joinaliasvars,上节已介绍
 
     
     expr = eval_const_expressions(root, expr);//简化常量表达式
 
     
     if (kind == EXPRKIND_QUAL)
     {
         expr = (Node *) canonicalize_qual((Expr *) expr, false);//表达式规范化
 
 #ifdef OPTIMIZER_DEBUG
         printf("After canonicalize_qual()\n");
         pprint(expr);
 #endif
     }
 
     
     if (root->parse->hasSubLinks)//扩展子链接为子计划
         expr = SS_process_sublinks(root, expr, (kind == EXPRKIND_QUAL));
 
     
 
     
     if (root->query_level > 1)
         expr = SS_replace_correlation_vars(root, expr);//使用Param节点替换上层的Vars
 
     
     if (kind == EXPRKIND_QUAL)
         expr = (Node *) make_ands_implicit((Expr *) expr);
 
     return expr;
 }

canonicalize_qual

 
 Expr *
 canonicalize_qual(Expr *qual, bool is_check)//规范化表达式
 {
     Expr       *newqual;
 
     
     if (qual == NULL)
         return NULL;
 
     
     Assert(!IsA(qual, List));
 
     
     newqual = find_duplicate_ors(qual, is_check);//执行实际处理逻辑
 
     return newqual;
 }

find_duplicate_ors

 
 static Expr *
 find_duplicate_ors(Expr *qual, bool is_check)
 {
     if (or_clause((Node *) qual))//OR语句
     {
         List       *orlist = NIL;
         ListCell   *temp;
 
         
         foreach(temp, ((BoolExpr *) qual)->args)//遍历args链表
         {
             Expr       *arg = (Expr *) lfirst(temp);//获取链表中的元素
 
             arg = find_duplicate_ors(arg, is_check);//递归调用
 
             
             if (arg && IsA(arg, Const))//arg为常量
             {
                 Const      *carg = (Const *) arg;
 
                 if (is_check)//Check语句
                 {
                     
                     if (!carg->constisnull && !DatumGetBool(carg->constvalue))
                         continue;//不为NULL而且为FALSE,继续循环
                     //arg为NULL或者TURE,即NULL OR TRUE,简化为TRUE
                     
                     return (Expr *) makeBoolConst(true, false);
                 }
                 else//Where条件语句
                 {
                     
                     if (carg->constisnull || !DatumGetBool(carg->constvalue))
                         continue;//arg为NULL或者FALSE,继续循环
                     //arg不为NULL而且为TRUE,即TRUE常量,直接返回arg
                     
                     return arg;
                 }
             }
             //加入结果链表
             orlist = lappend(orlist, arg);
         }
 
         
         orlist = pull_ors(orlist);//扁平化ORs
 
         
         return process_duplicate_ors(orlist);//处理重复的ORs
     }
     else if (and_clause((Node *) qual))//AND语句
     {
         List       *andlist = NIL;
         ListCell   *temp;
 
         
         foreach(temp, ((BoolExpr *) qual)->args)//遍历链表
         {
             Expr       *arg = (Expr *) lfirst(temp);
 
             arg = find_duplicate_ors(arg, is_check);
 
             
             if (arg && IsA(arg, Const))
             {
                 Const      *carg = (Const *) arg;
 
                 if (is_check)
                 {
                     
                     if (carg->constisnull || DatumGetBool(carg->constvalue))
                         continue;//CHECK语句
                     
                     //不为空且值为FALSE,返回FALSE
                     return arg;
                 }
                 else
                 {
                     
                     if (!carg->constisnull && DatumGetBool(carg->constvalue))
                         continue;
                     
                     //NULL OR FALSE,返回FALSE
                     return (Expr *) makeBoolConst(false, false);
                 }
             }
 
             andlist = lappend(andlist, arg);//加入到结果列表
         }
 
         
         andlist = pull_ands(andlist);//扁平化处理AND
 
         
         if (andlist == NIL)//为空指针
             return (Expr *) makeBoolConst(true, false);//返回TRUE
 
         
         if (list_length(andlist) == 1)//单个表达式
             return (Expr *) linitial(andlist);//返回此表达式链表
 
         
         return make_andclause(andlist);//否则返回结果链表
     }
     else
         return qual;//非AND/OR语句,直接返回结果
 }

process_duplicate_ors

 
 static Expr *
 process_duplicate_ors(List *orlist)
 {
     List       *reference = NIL;
     int         num_subclauses = 0;
     List       *winners;
     List       *neworlist;
     ListCell   *temp;
 
     
     if (orlist == NIL)
         return (Expr *) makeBoolConst(false, false);
 
     
     if (list_length(orlist) == 1)
         return (Expr *) linitial(orlist);
 
     
     //遍历OR链表,找到AND语句中约束条件最少的那个表达式
     //与求解最小公约数同理,公共的谓词只可能在此最小的表达式中产生 
     foreach(temp, orlist)
     {
         Expr       *clause = (Expr *) lfirst(temp);
 
         if (and_clause((Node *) clause))//AND语句
         {
             List       *subclauses = ((BoolExpr *) clause)->args;
             int         nclauses = list_length(subclauses);
 
             if (reference == NIL || nclauses < num_subclauses)
             {
                 reference = subclauses;
                 num_subclauses = nclauses;
             }
         }
         else//单个约束条件或者带有表达式的约束条件,如(X1 AND X2) OR X3等
         {
             reference = list_make1(clause);
             break;
         }
     }
 
     
     reference = list_union(NIL, reference);//去掉重复的谓词
 
     
     winners = NIL;
     foreach(temp, reference)//遍历链表
     {
         Expr       *refclause = (Expr *) lfirst(temp);
         bool        win = true;
         ListCell   *temp2;
 
         foreach(temp2, orlist)
         {
             Expr       *clause = (Expr *) lfirst(temp2);
 
             if (and_clause((Node *) clause))//该谓词是否在链表中存在?
             {
                 if (!list_member(((BoolExpr *) clause)->args, refclause))
                 {
                     win = false;
                     break;
                 }
             }
             else//该谓词是否与单个条件表达式等价?
             {
                 if (!equal(refclause, clause))
                 {
                     win = false;
                     break;
                 }
             }
         }
 
         if (win)//找到了公共的谓词
             winners = lappend(winners, refclause);//加入到结果中
     }
 
     
     if (winners == NIL)
         return make_orclause(orlist);//如果找到,原样返回
 
     
     neworlist = NIL;
     foreach(temp, orlist)//遍历OR链表
     {
         Expr       *clause = (Expr *) lfirst(temp);
 
         if (and_clause((Node *) clause))//AND语句
         {
             List       *subclauses = ((BoolExpr *) clause)->args;//获取条件语句参数
 
             subclauses = list_difference(subclauses, winners);//剔除相同部分
             if (subclauses != NIL)//成功剔除,产生新的AND语句
             {
                 if (list_length(subclauses) == 1)
                     neworlist = lappend(neworlist, linitial(subclauses));
                 else
                     neworlist = lappend(neworlist, make_andclause(subclauses));
             }
             else
             {
                 neworlist = NIL;    
                 break;
             }
         }
         else//不是AND语句
         {
             if (!list_member(winners, clause))//单个条件语句,直接添加条件
                 neworlist = lappend(neworlist, clause);
             else
             //OR语句?公共部分已提出,无需加入其他条件,直接返回
             //根据吸收律,X AND (X OR B) 等价于X
             {
                 neworlist = NIL;    
                 break;
             }
         }
     }
 
     
     if (neworlist != NIL)//新产生的链表
     {
         if (list_length(neworlist) == 1)
             winners = lappend(winners, linitial(neworlist));
         else
             winners = lappend(winners, make_orclause(pull_ors(neworlist)));//拉平OR
     }
 
     
     //返回结果 
     if (list_length(winners) == 1)
         return (Expr *) linitial(winners);
     else
         return make_andclause(pull_ands(winners));//拉平AND
 }

pull_ors

 
 static List *
 pull_ors(List *orlist)
 {
     List       *out_list = NIL;
     ListCell   *arg;
 
     foreach(arg, orlist)
     {
         Node       *subexpr = (Node *) lfirst(arg);
 
         
         if (or_clause(subexpr))
             out_list = list_concat(out_list,
                                    pull_ors(((BoolExpr *) subexpr)->args));//递归拉平
         else
             out_list = lappend(out_list, subexpr);
     }
     return out_list;
 }

pull_ands

 
 static List *
 pull_ands(List *andlist)
 {
     List       *out_list = NIL;
     ListCell   *arg;
 
     foreach(arg, andlist)
     {
         Node       *subexpr = (Node *) lfirst(arg);
 
         
         if (and_clause(subexpr))
             out_list = list_concat(out_list,
                                    pull_ands(((BoolExpr *) subexpr)->args));//递归拉平
         else
             out_list = lappend(out_list, subexpr);
     }
     return out_list;
 }

四、跟踪分析

测试脚本:

select * from t_dwxx 
where (FALSE OR dwbh = '1001') 
       AND ((dwbh = '1001' AND dwbh = '1002')  
             OR (dwbh = '1001' AND dwbh = '1003'));

该语句经规范化后与以下SQL语句无异:

select * from t_dwxx 
where  dwbh = '1001' AND (dwbh='1002' OR dwbh='1003');

-- 执行计划
testdb=# explain verbose select * from t_dwxx 
where (FALSE OR dwbh = '1001') 
       AND ((dwbh = '1001' AND dwbh = '1002')  
             OR (dwbh = '1001' AND dwbh = '1003'));
                                                             QUERY PLAN                                                      
        
-----------------------------------------------------------------------------------------------------------------------------
--------
 Seq Scan on public.t_dwxx  (cost=0.00..12.80 rows=1 width=474)
   Output: dwmc, dwbh, dwdz
   Filter: (((t_dwxx.dwbh)::text = '1001'::text) AND (((t_dwxx.dwbh)::text = '1002'::text) OR ((t_dwxx.dwbh)::text = '1003'::
text)))
(3 rows)

gdb跟踪:

(gdb) b find_duplicate_ors
Breakpoint 1 at 0x7811b4: file prepqual.c, line 418.
(gdb) c
Continuing.

Breakpoint 1, find_duplicate_ors (qual=0x2727528, is_check=false) at prepqual.c:418
418   if (or_clause((Node *) qual))
#输入参数,BoolExpr
#args链表有两个元素,一个是(FALSE OR dwbh = '1001'),
#另外一个是((dwbh = '1001' AND dwbh = '1002')  
             OR (dwbh = '1001' AND dwbh = '1003'))
(gdb) p *(BoolExpr *)qual
$2 = {xpr = {type = T_BoolExpr}, boolop = AND_EXPR, args = 0x2726c88, location = -1}
(gdb) p *((BoolExpr *)qual)->args
$3 = {type = T_List, length = 2, head = 0x2726c68, tail = 0x2727508}
...
#进入AND分支
462   else if (and_clause((Node *) qual))
...
#获得链表第一个元素
470       Expr     *arg = (Expr *) lfirst(temp);
#再次进入find_duplicate_ors
#FALSE OR dwbh='1001'在此前已被简化为dwbh='1001',所以在这里直接返回
Breakpoint 1, find_duplicate_ors (qual=0x2726bc8, is_check=false) at prepqual.c:418
418   if (or_clause((Node *) qual))
(gdb) n
462   else if (and_clause((Node *) qual))
(gdb) 
#
515     return qual;
(gdb) n
516 }
(gdb) 
475       if (arg && IsA(arg, Const))
...
497       andlist = lappend(andlist, arg);
(gdb) n
468     foreach(temp, ((BoolExpr *) qual)->args)
(gdb) n
470       Expr     *arg = (Expr *) lfirst(temp);
(gdb) 
472       arg = find_duplicate_ors(arg, is_check);
#链表的下一个元素,即
#((dwbh = '1001' AND dwbh = '1002')  
             OR (dwbh = '1001' AND dwbh = '1003'))
#由两个BoolExpr组成
(gdb) p *(BoolExpr *)arg
$23 = {xpr = {type = T_BoolExpr}, boolop = OR_EXPR, args = 0x27270d8, location = -1}
(gdb) p *((BoolExpr *)arg)->args
$24 = {type = T_List, length = 2, head = 0x27270b8, tail = 0x27274b8}
(gdb) p *(Node *)((BoolExpr *)arg)->args->head->data.ptr_value
$25 = {type = T_BoolExpr}
(gdb) p *(Node *)((BoolExpr *)arg)->args->head->next->data.ptr_value
$26 = {type = T_BoolExpr}
...
#args左右两边的的arg处理完毕
424     foreach(temp, ((BoolExpr *) qual)->args)
(gdb) 
457     orlist = pull_ors(orlist);
...
#进入process_duplicate_ors
460     return process_duplicate_ors(orlist);
(gdb) step
process_duplicate_ors (orlist=0x2727858) at prepqual.c:529
529   List     *reference = NIL;
...
#获得最少谓词的链表
(gdb) p *reference
$36 = {type = T_List, length = 2, head = 0x27278a8, tail = 0x27278f8}
...
#获得了公共的谓词winner!即dwbh = '1001'
(gdb) p *winners
$40 = {type = T_List, length = 1, head = 0x2727918, tail = 0x2727918}
...
(gdb) 
canonicalize_qual (qual=0x2727528, is_check=false) at prepqual.c:309
309   return newqual;
(gdb) p *newqual
$43 = {type = T_BoolExpr}
(gdb) p *(BoolExpr *)newqual
$45 = {xpr = {type = T_BoolExpr}, boolop = AND_EXPR, args = 0x2727c18, location = -1}
#DONE!