PostgreSQL 原始碼解讀(28)- 查詢語句#13(查詢優化-上拉子連結#3)

husthxd發表於2018-08-27

本節簡單介紹了PG查詢邏輯優化中的子查詢連結(subLink),以EXISTS子連結為例介紹了子查詢連結上拉主函式處理邏輯以及使用gdb跟蹤分析。

一、原始碼解讀

上一節介紹了ANY子連結,本節介紹了EXISTS子連結.
為便於方便解析,根據日誌分析,得出查詢樹如下圖所示:


PostgreSQL 原始碼解讀(28)- 查詢語句#13(查詢優化-上拉子連結#3)
查詢樹

convert_EXISTS_sublink_to_join函式原始碼:

 /*
  * 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".
  * 邏輯與ANY一致,為了支援NOT EXISTS,多加了一個引數under_not
  */
  
 JoinExpr *
 convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
                                bool under_not, Relids available_rels)
 {
     JoinExpr   *result;//返回結果
     Query      *parse = root->parse;//查詢樹
     Query      *subselect = (Query *) sublink->subselect;//子查詢
     Node       *whereClause;//where語句
     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,則退出
     //如果含有集合操作/聚合操作/Having子句等,子連結不能提升
     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)//子查詢沒有查詢主體,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;//子查詢條件語句單獨儲存
     subselect->jointree->quals = NULL;//子查詢的條件語句設定為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))//去掉條件語句後,如仍依賴父查詢的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))//條件語句必須含有父查詢的Vars,否則不構成連線,退出
         return NULL;
 
     /*
      * We don't risk optimizing if the WHERE clause is volatile, either.
      */
     if (contain_volatile_functions(whereClause))//條件語句存在易變函式(如隨機函式等)
         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);//獲取rtable的長度(新RTE插入的偏移)
     //調整子查詢中varno為0(指向rtable)的Vars,varno調整為父查詢rtable的index
     //(詳見依賴函式解析)
     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.
      */
     //子查詢中與父查詢相關的Vars,varlevelsup需要從Leve 1變為Level 0
     //在子查詢中,這些Vars的varlevelsup為1,表示依賴於父查詢(上一層)的Vars,提升後不存在此依賴,需改為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 */
     //把子查詢的rtable拼接到父查詢的rtable中
     parse->rtable = list_concat(parse->rtable, subselect->rtable);
 
     //構造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、基礎資訊

相關資料結構
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
 };
 

依賴的函式
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;//儲存傳入的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、跟蹤分析

測試指令碼:

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見上一節
#2.sublink,子連結,見本節查詢樹結構
#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;
...
#調整subselect->jointree->fromlist->head(型別為RTR)的rtindex,原為1,調整為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
$32 = {type = T_Var}
#第1個引數是t_dwxx.dwbh
(gdb) p *(Var *)$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 $arg2=(RelabelType *)((OpExpr *)whereClause)->args->tail->data.ptr_value
#第2個引數是t_grxx.dwbh
#varno/varnoold從原來的值1調整為2
(gdb) p *(Var *)$arg2->arg
$34 = {xpr = {type = T_Var}, varno = 2, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 2, varoattno = 1, location = 81}
(gdb) 
(gdb) n
1521        IncrementVarSublevelsUp(whereClause, -1, 1);
(gdb) 
1529        clause_varnos = pull_varnos(whereClause);
#調整varlevelsup為0
(gdb) p *(Var *)$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}
...
#構造返回值
(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;
#上拉成為半連線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、小結

1、上拉過程:與ANY子連結類似,主要邏輯是調整varno&varlevelsup等資訊;
2、重要函式:XX_walker,遍歷Node節點的函式,用於統計或更新Node資訊。

來自 “ ITPUB部落格 ” ,連結:http://blog.itpub.net/6906/viewspace-2374888/,如需轉載,請註明出處,否則將追究法律責任。

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