PostgreSQL 原始碼解讀（30）- 查詢語句#15（查詢優化-扁平化處理UNION ALL）

本文簡單介紹了PG查詢優化中對頂層UNION ALL語句進行的扁平化(flatten)處理過程。扁平化處理的目的是把UNION ALL的集合操作轉換為Append Relation以進行優化處理。

一、測試指令碼

測試指令碼：

testdb=# select a.dwbh 
from t_dwxx a
union all
select b.dwbh
from t_grxx b
union all
select c.grbh
from t_jfxx c;

查詢樹如下圖所示：

二、原始碼解讀

/*
 * flatten_simple_union_all
 *      Try to optimize top-level UNION ALL structure into an appendrel
 *
 * If a query's setOperations tree consists entirely of simple UNION ALL
 * operations, flatten it into an append relation, which we can process more
 * intelligently than the general setops case.  Otherwise, do nothing.
 *
 * In most cases, this can succeed only for a top-level query, because for a
 * subquery in FROM, the parent query's invocation of pull_up_subqueries would
 * already have flattened the UNION via pull_up_simple_union_all.  But there
 * are a few cases we can support here but not in that code path, for example
 * when the subquery also contains ORDER BY.
 */
void
flatten_simple_union_all(PlannerInfo *root)
{
    Query      *parse = root->parse;//查詢樹
    SetOperationStmt *topop;//集合操作語句
    Node       *leftmostjtnode;//最左邊節點
    int         leftmostRTI;//最左邊節點對應的RTI(rtable中的位置,即rtindex)
    RangeTblEntry *leftmostRTE;//最左邊節點對應的RTE
    int         childRTI;//child的RTI
    RangeTblEntry *childRTE;//子RTE
    RangeTblRef *rtr;//RTR

    /* Shouldn't be called unless query has setops */
    topop = castNode(SetOperationStmt, parse->setOperations);//獲取集合操作語句
    Assert(topop);

    /* Can't optimize away a recursive UNION */
    if (root->hasRecursion)
        return;//不支援遞迴UNION

    /*
     * Recursively check the tree of set operations.  If not all UNION ALL
     * with identical column types, punt.
     */
    if (!is_simple_union_all_recurse((Node *) topop, parse, topop->colTypes))
        return;//不是簡單的UNION ALL,返回

    /*
     * Locate the leftmost leaf query in the setops tree.  The upper query's
     * Vars all refer to this RTE (see transformSetOperationStmt).
     */
    leftmostjtnode = topop->larg;//左邊的child
    while (leftmostjtnode && IsA(leftmostjtnode, SetOperationStmt))
        leftmostjtnode = ((SetOperationStmt *) leftmostjtnode)->larg;//獲取最左邊的葉子節點(非SetOperationStmt)
    Assert(leftmostjtnode && IsA(leftmostjtnode, RangeTblRef));
    leftmostRTI = ((RangeTblRef *) leftmostjtnode)->rtindex;//最左邊節點的RTI
    leftmostRTE = rt_fetch(leftmostRTI, parse->rtable);//從rtable中獲取leftmostRTI指向的RTE
    Assert(leftmostRTE->rtekind == RTE_SUBQUERY);//該RTE一定是子查詢

    /*
     * Make a copy of the leftmost RTE and add it to the rtable.  This copy
     * will represent the leftmost leaf query in its capacity as a member of
     * the appendrel.  The original will represent the appendrel as a whole.
     * (We must do things this way because the upper query's Vars have to be
     * seen as referring to the whole appendrel.)
     */
    childRTE = copyObject(leftmostRTE);//Make a copy
    parse->rtable = lappend(parse->rtable, childRTE);//把RTE追加到上層rtable
    childRTI = list_length(parse->rtable);//相應的rtindex

    /* Modify the setops tree to reference the child copy */
    ((RangeTblRef *) leftmostjtnode)->rtindex = childRTI;//調整RTR的rtindex

    /* Modify the formerly-leftmost RTE to mark it as an appendrel parent */
    leftmostRTE->inh = true;

    /*
     * Form a RangeTblRef for the appendrel, and insert it into FROM.  The top
     * Query of a setops tree should have had an empty FromClause initially.
     */
    rtr = makeNode(RangeTblRef);//構造一個RTR
    rtr->rtindex = leftmostRTI;//rtindex=最左邊節點的rtindex
    Assert(parse->jointree->fromlist == NIL);//Query的jointree(FromExpr)->fromlist應為NULL
    parse->jointree->fromlist = list_make1(rtr);//把RTR放到jointree的fromlist中

    /*
     * Now pretend the query has no setops.  We must do this before trying to
     * do subquery pullup, because of Assert in pull_up_simple_subquery.
     */
    parse->setOperations = NULL;//集合操作設定為NULL

    /*
     * Build AppendRelInfo information, and apply pull_up_subqueries to the
     * leaf queries of the UNION ALL.  (We must do that now because they
     * weren't previously referenced by the jointree, and so were missed by
     * the main invocation of pull_up_subqueries.)
     */
    pull_up_union_leaf_queries((Node *) topop, root, leftmostRTI, parse, 0);
}
 
/*
 * pull_up_union_leaf_queries -- recursive guts of pull_up_simple_union_all
 *
 * Build an AppendRelInfo for each leaf query in the setop tree, and then
 * apply pull_up_subqueries to the leaf query.
 *
 * Note that setOpQuery is the Query containing the setOp node, whose tlist
 * contains references to all the setop output columns.  When called from
 * pull_up_simple_union_all, this is *not* the same as root->parse, which is
 * the parent Query we are pulling up into.
 *
 * parentRTindex is the appendrel parent's index in root->parse->rtable.
 *
 * The child RTEs have already been copied to the parent.  childRToffset
 * tells us where in the parent's range table they were copied.  When called
 * from flatten_simple_union_all, childRToffset is 0 since the child RTEs
 * were already in root->parse->rtable and no RT index adjustment is needed.
 */
static void
pull_up_union_leaf_queries(Node *setOp, PlannerInfo *root, int parentRTindex,
                           Query *setOpQuery, int childRToffset)
{
    if (IsA(setOp, RangeTblRef))//RTR
    {
        RangeTblRef *rtr = (RangeTblRef *) setOp;
        int         childRTindex;
        AppendRelInfo *appinfo;

        /*
         * Calculate the index in the parent's range table
         */
        childRTindex = childRToffset + rtr->rtindex;

        /*
         * Build a suitable AppendRelInfo, and attach to parent's list.
         */
        appinfo = makeNode(AppendRelInfo);//構造AppendRelInfo
        appinfo->parent_relid = parentRTindex;
        appinfo->child_relid = childRTindex;
        appinfo->parent_reltype = InvalidOid;
        appinfo->child_reltype = InvalidOid;
        make_setop_translation_list(setOpQuery, childRTindex,
                                    &appinfo->translated_vars);//把相關的Vars新增到AppendRelInfo中
        appinfo->parent_reloid = InvalidOid;
        root->append_rel_list = lappend(root->append_rel_list, appinfo);//新增到PlannerInfo中

        /*
         * Recursively apply pull_up_subqueries to the new child RTE.  (We
         * must build the AppendRelInfo first, because this will modify it.)
         * Note that we can pass NULL for containing-join info even if we're
         * actually under an outer join, because the child's expressions
         * aren't going to propagate up to the join.  Also, we ignore the
         * possibility that pull_up_subqueries_recurse() returns a different
         * jointree node than what we pass it; if it does, the important thing
         * is that it replaced the child relid in the AppendRelInfo node.
         */
        rtr = makeNode(RangeTblRef);
        rtr->rtindex = childRTindex;
        (void) pull_up_subqueries_recurse(root, (Node *) rtr,
                                          NULL, NULL, appinfo, false);//提升子查詢
    }
    else if (IsA(setOp, SetOperationStmt))//型別為SetOperationStmt
    {
        SetOperationStmt *op = (SetOperationStmt *) setOp;

        /* Recurse to reach leaf queries */
        pull_up_union_leaf_queries(op->larg, root, parentRTindex, setOpQuery,
                                   childRToffset);//左Child
        pull_up_union_leaf_queries(op->rarg, root, parentRTindex, setOpQuery,
                                   childRToffset);//右Child
    }
    else
    {
        elog(ERROR, "unrecognized node type: %d",
             (int) nodeTag(setOp));
    }
}

三、基礎資訊

資料結構/巨集定義
1、SetOperationStmt

 /* ----------------------
  *      Set Operation node for post-analysis query trees
  *
  * After parse analysis, a SELECT with set operations is represented by a
  * top-level Query node containing the leaf SELECTs as subqueries in its
  * range table.  Its setOperations field shows the tree of set operations,
  * with leaf SelectStmt nodes replaced by RangeTblRef nodes, and internal
  * nodes replaced by SetOperationStmt nodes.  Information about the output
  * column types is added, too.  (Note that the child nodes do not necessarily
  * produce these types directly, but we've checked that their output types
  * can be coerced to the output column type.)  Also, if it's not UNION ALL,
  * information about the types' sort/group semantics is provided in the form
  * of a SortGroupClause list (same representation as, eg, DISTINCT).
  * The resolved common column collations are provided too; but note that if
  * it's not UNION ALL, it's okay for a column to not have a common collation,
  * so a member of the colCollations list could be InvalidOid even though the
  * column has a collatable type.
  * ----------------------
  */
 typedef enum SetOperation
 {
     SETOP_NONE = 0,
     SETOP_UNION,
     SETOP_INTERSECT,
     SETOP_EXCEPT
 } SetOperation;

 typedef struct SetOperationStmt
 {
     NodeTag     type;//節點型別
     SetOperation op;            /* 操作型別,type of set op */
     bool        all;            /* 是否UNION ALL,ALL specified? */
     Node       *larg;           /* left child,左Child,型別為RTR或SetOperationStmt */
     Node       *rarg;           /* right child,右Child,型別為RTR或SetOperationStmt */
     /* Eventually add fields for CORRESPONDING spec here */
 
     /* Fields derived during parse analysis: */
     List       *colTypes;       /* OID list of output column type OIDs,資料列型別 */
     List       *colTypmods;     /* integer list of output column typmods,精度/刻度資訊 */
     List       *colCollations;  /* OID list of output column collation OIDs,collation資訊*/
     List       *groupClauses;   /* a list of SortGroupClause's,SortGroup語句*/
     /* groupClauses is NIL if UNION ALL, but must be set otherwise */
 } SetOperationStmt;
 

依賴的函式
1、make_setop_translation_list

 /*
  * make_setop_translation_list
  *    Build the list of translations from parent Vars to child Vars for
  *    a UNION ALL member.  (At this point it's just a simple list of
  *    referencing Vars, but if we succeed in pulling up the member
  *    subquery, the Vars will get replaced by pulled-up expressions.)
  */
 static void
 make_setop_translation_list(Query *query, Index newvarno,
                             List **translated_vars)
 {
     List       *vars = NIL;
     ListCell   *l;
 
     foreach(l, query->targetList)
     {
         TargetEntry *tle = (TargetEntry *) lfirst(l);
 
         if (tle->resjunk)
             continue;
 
         vars = lappend(vars, makeVarFromTargetEntry(newvarno, tle));
     }
 
     *translated_vars = vars;
 }

四、跟蹤分析

測試指令碼見上,啟動gdb跟蹤:

(gdb) b flatten_simple_union_all
Breakpoint 1 at 0x77f964: file prepjointree.c, line 2355.
(gdb) c
Continuing.

Breakpoint 1, flatten_simple_union_all (root=0x16a6338) at prepjointree.c:2355
2355        Query      *parse = root->parse;
#輸入引數
#1.root,見前面章節,root->parse見先前小節的圖(查詢樹)
...
#獲取最左邊的葉子節點(型別為RTR)
2384        while (leftmostjtnode && IsA(leftmostjtnode, SetOperationStmt))
(gdb) 
2387        leftmostRTI = ((RangeTblRef *) leftmostjtnode)->rtindex;
(gdb) p *leftmostjtnode
$9 = {type = T_RangeTblRef}
...
#新增到root->rtable中
(gdb) n
2399        parse->rtable = lappend(parse->rtable, childRTE);
(gdb) 
2400        childRTI = list_length(parse->rtable);
#原來的rtable為3個子查詢,在最後面新增1個
(gdb) p *parse->rtable
$12 = {type = T_List, length = 4, head = 0x15e9dc8, tail = 0x16b09d0}
(gdb) p leftmostRTI
$19 = 1
#copy了一份"子查詢",放在rtable的最後面
(gdb) p *((RangeTblEntry *)parse->rtable->head->next->next->next->data.ptr_value)->alias
$20 = {type = T_Alias, aliasname = 0x16b08d8 "*SELECT* 1", colnames = 0x0}
(gdb) p *((RangeTblEntry *)parse->rtable->head->data.ptr_value)->alias
$21 = {type = T_Alias, aliasname = 0x15e9cd0 "*SELECT* 1", colnames = 0x0}
(gdb) 
#構造RTR(rtindex=1),放在jointree的fromlist中
2406        leftmostRTE->inh = true;
(gdb) 
2412        rtr = makeNode(RangeTblRef);
(gdb) 
2413        rtr->rtindex = leftmostRTI;
(gdb) 
2415        parse->jointree->fromlist = list_make1(rtr);
(gdb) p *rtr
$23 = {type = T_RangeTblRef, rtindex = 1}
(gdb) p *(RangeTblRef *) leftmostjtnode
$24 = {type = T_RangeTblRef, rtindex = 4}
(gdb) p *parse->jointree->fromlist
$26 = {type = T_List, length = 1, head = 0x16b0a08, tail = 0x16b0a08}
#進入pull_up_union_leaf_queries
(gdb) n
2429        pull_up_union_leaf_queries((Node *) topop, root, leftmostRTI, parse, 0);
(gdb) step
pull_up_union_leaf_queries (setOp=0x15c59c8, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1344
1344        if (IsA(setOp, RangeTblRef))
#輸入引數
#1.setOp,SetOperationStmt
#2.root,同flatten_simple_union_all
#3.leftmostRTI=1,最左邊節點的rtindex
#4.parse,查詢樹
#5.childRToffset,RT的初始偏移量
1344        if (IsA(setOp, RangeTblRef))
#setOp為SetOperationStmt型別,遞迴呼叫SetOperationStmt->larg->larg
(gdb) n
1383        else if (IsA(setOp, SetOperationStmt))
...
1388            pull_up_union_leaf_queries(op->larg, root, parentRTindex, setOpQuery,
(gdb) step
pull_up_union_leaf_queries (setOp=0x15c5a18, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1344
1344        if (IsA(setOp, RangeTblRef))
1344        if (IsA(setOp, RangeTblRef))
(gdb) n
1383        else if (IsA(setOp, SetOperationStmt))
(gdb) 
1385            SetOperationStmt *op = (SetOperationStmt *) setOp;
(gdb) 
1388            pull_up_union_leaf_queries(op->larg, root, parentRTindex, setOpQuery,
(gdb) step
pull_up_union_leaf_queries (setOp=0x15e9e18, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1344
1344        if (IsA(setOp, RangeTblRef))
...
1346            RangeTblRef *rtr = (RangeTblRef *) setOp;
(gdb) 
1353            childRTindex = childRToffset + rtr->rtindex;
(gdb) 
1358            appinfo = makeNode(AppendRelInfo);
#最左邊的RTR在flatten_simple_union_all中已調整為4
(gdb) p *rtr
$27 = {type = T_RangeTblRef, rtindex = 4}
(gdb) p childRTindex
$28 = 4
#構造AppendRelInfo,插入到root->append_rel_list中
(gdb) n
1359            appinfo->parent_relid = parentRTindex;
...
#進入pull_up_subqueries_recurse
(gdb) step
pull_up_subqueries_recurse (root=0x16a6338, jtnode=0x16b0b98, lowest_outer_join=0x0, lowest_nulling_outer_join=0x0, 
    containing_appendrel=0x16b0a58, deletion_ok=false) at prepjointree.c:680
680     if (IsA(jtnode, RangeTblRef))
#查詢樹的rtable中增加RTE(RTE_RELATION)
(gdb) p *parse->rtable
$47 = {type = T_List, length = 5, head = 0x15e9dc8, tail = 0x16b0cf0}
(gdb) p *(RangeTblEntry *)parse->rtable->tail->data.ptr_value
$49 = {type = T_RangeTblEntry, rtekind = RTE_RELATION, relid = 16394, relkind = 114 'r', tablesample = 0x0, subquery = 0x0, 
  security_barrier = false, jointype = JOIN_INNER, joinaliasvars = 0x0, functions = 0x0, funcordinality = false, 
  tablefunc = 0x0, values_lists = 0x0, ctename = 0x0, ctelevelsup = 0, self_reference = false, coltypes = 0x0, 
  coltypmods = 0x0, colcollations = 0x0, enrname = 0x0, enrtuples = 0, alias = 0x16b0e20, eref = 0x16b0e68, 
  lateral = false, inh = true, inFromCl = true, requiredPerms = 2, checkAsUser = 0, selectedCols = 0x16b0fe8, 
  insertedCols = 0x0, updatedCols = 0x0, securityQuals = 0x0}
(gdb) n
pull_up_subqueries_recurse (root=0x16a6338, jtnode=0x16b0b98, lowest_outer_join=0x0, lowest_nulling_outer_join=0x0, 
    containing_appendrel=0x16b0a58, deletion_ok=false) at prepjointree.c:853
853 }
(gdb) 
pull_up_union_leaf_queries (setOp=0x15e9e18, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1398
1398    }
(gdb) n
#處理SetOperationStmt->larg(SetOperationStmt)->rarg
pull_up_union_leaf_queries (setOp=0x15c5a18, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1390
1390            pull_up_union_leaf_queries(op->rarg, root, parentRTindex, setOpQuery,
(gdb) 
1398    }
(gdb)
#處理SetOperationStmt->rarg 
pull_up_union_leaf_queries (setOp=0x15c59c8, root=0x16a6338, parentRTindex=1, setOpQuery=0x15c58b8, childRToffset=0)
    at prepjointree.c:1390
1390            pull_up_union_leaf_queries(op->rarg, root, parentRTindex, setOpQuery,
(gdb) 
1398    }
(gdb) 
flatten_simple_union_all (root=0x16a6338) at prepjointree.c:2430
2430    }
#
(gdb) p *root
$54 = {type = T_PlannerInfo, parse = 0x15c58b8, glob = 0x15eb8b0, query_level = 1, parent_root = 0x0, plan_params = 0x0, 
  outer_params = 0x0, simple_rel_array = 0x0, simple_rel_array_size = 0, simple_rte_array = 0x0, all_baserels = 0x0, 
  nullable_baserels = 0x0, join_rel_list = 0x0, join_rel_hash = 0x0, join_rel_level = 0x0, join_cur_level = 0, 
  init_plans = 0x0, cte_plan_ids = 0x0, multiexpr_params = 0x0, eq_classes = 0x0, canon_pathkeys = 0x0, 
  left_join_clauses = 0x0, right_join_clauses = 0x0, full_join_clauses = 0x0, join_info_list = 0x0, 
  append_rel_list = 0x16b0b68, rowMarks = 0x0, placeholder_list = 0x0, fkey_list = 0x0, query_pathkeys = 0x0, 
  group_pathkeys = 0x0, window_pathkeys = 0x0, distinct_pathkeys = 0x0, sort_pathkeys = 0x0, part_schemes = 0x0, 
  initial_rels = 0x0, upper_rels = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, upper_targets = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 
    0x0}, processed_tlist = 0x0, grouping_map = 0x0, minmax_aggs = 0x0, planner_cxt = 0x15c3e90, total_table_pages = 0, 
  tuple_fraction = 0, limit_tuples = 0, qual_security_level = 0, inhTargetKind = INHKIND_NONE, hasJoinRTEs = false, 
  hasLateralRTEs = false, hasDeletedRTEs = false, hasHavingQual = false, hasPseudoConstantQuals = false, 
  hasRecursion = false, wt_param_id = -1, non_recursive_path = 0x0, curOuterRels = 0x0, curOuterParams = 0x0, 
  join_search_private = 0x0, partColsUpdated = false}
(gdb) p *root->append_rel_list
$55 = {type = T_List, length = 3, head = 0x16b0b48, tail = 0x16b2790}
(gdb) p root->parse
$56 = (Query *) 0x15c58b8
(gdb) p *root->parse
$57 = {type = T_Query, commandType = CMD_SELECT, querySource = QSRC_ORIGINAL, queryId = 0, canSetTag = true, 
  utilityStmt = 0x0, resultRelation = 0, hasAggs = false, hasWindowFuncs = false, hasTargetSRFs = false, 
  hasSubLinks = false, hasDistinctOn = false, hasRecursive = false, hasModifyingCTE = false, hasForUpdate = false, 
  hasRowSecurity = false, cteList = 0x0, rtable = 0x15e9de8, jointree = 0x15eb880, targetList = 0x16b3348, 
  override = OVERRIDING_NOT_SET, onConflict = 0x0, returningList = 0x0, groupClause = 0x0, groupingSets = 0x0, 
  havingQual = 0x0, windowClause = 0x0, distinctClause = 0x0, sortClause = 0x0, limitOffset = 0x0, limitCount = 0x0, 
  rowMarks = 0x0, setOperations = 0x0, constraintDeps = 0x0, withCheckOptions = 0x0, stmt_location = 0, stmt_len = 104}
#rtable中已有7個元素,其中第4個是子查詢,第5-7個是實際的RTE_RELATION(relid分別是16394/16397/16400)
(gdb) p *root->parse->rtable
$58 = {type = T_List, length = 7, head = 0x15e9dc8, tail = 0x16b2908}
(gdb) 
#第4個元素
(gdb) p *((RangeTblEntry *)root->parse->rtable->head->next->next->next->data.ptr_value)
$80 = {type = T_RangeTblEntry, rtekind = RTE_SUBQUERY, relid = 0, relkind = 0 '\000', tablesample = 0x0, 
  subquery = 0x15c57a8, security_barrier = false, jointype = JOIN_INNER, joinaliasvars = 0x0, functions = 0x0, 
  funcordinality = false, tablefunc = 0x0, values_lists = 0x0, ctename = 0x0, ctelevelsup = 0, self_reference = false, 
  coltypes = 0x0, coltypmods = 0x0, colcollations = 0x0, enrname = 0x0, enrtuples = 0, alias = 0x16b08a8, eref = 0x16b08f8, 
  lateral = false, inh = false, inFromCl = false, requiredPerms = 0, checkAsUser = 0, selectedCols = 0x0, 
  insertedCols = 0x0, updatedCols = 0x0, securityQuals = 0x0}
(gdb) p *((RangeTblEntry *)root->parse->rtable->head->next->next->next->data.ptr_value)->eref
$81 = {type = T_Alias, aliasname = 0x16b0928 "*SELECT* 1", colnames = 0x16b0948}
#第5個元素
(gdb) p *((RangeTblEntry *)root->parse->rtable->head->next->next->next->next->data.ptr_value)
$82 = {type = T_RangeTblEntry, rtekind = RTE_RELATION, relid = 16394, relkind = 114 'r', tablesample = 0x0, subquery = 0x0, 
  security_barrier = false, jointype = JOIN_INNER, joinaliasvars = 0x0, functions = 0x0, funcordinality = false, 
  tablefunc = 0x0, values_lists = 0x0, ctename = 0x0, ctelevelsup = 0, self_reference = false, coltypes = 0x0, 
  coltypmods = 0x0, colcollations = 0x0, enrname = 0x0, enrtuples = 0, alias = 0x16b0e20, eref = 0x16b0e68, 
  lateral = false, inh = true, inFromCl = true, requiredPerms = 2, checkAsUser = 0, selectedCols = 0x16b0fe8, 
  insertedCols = 0x0, updatedCols = 0x0, securityQuals = 0x0}
(gdb) p *((RangeTblEntry *)root->parse->rtable->head->next->next->next->next->data.ptr_value)->eref
$83 = {type = T_Alias, aliasname = 0x16b0e98 "a", colnames = 0x16b0eb0}

#root->append_rel_list中有3個元素,分別對應rtable中的5-7
(gdb) p *root->append_rel_list
$69 = {type = T_List, length = 3, head = 0x16b0b48, tail = 0x16b2790}
(gdb) p *(Node *)root->append_rel_list->head->data.ptr_value
$70 = {type = T_AppendRelInfo}
(gdb) p *(AppendRelInfo *)root->append_rel_list->head->data.ptr_value
$71 = {type = T_AppendRelInfo, parent_relid = 1, child_relid = 5, parent_reltype = 0, child_reltype = 0, 
  translated_vars = 0x16b33e8, parent_reloid = 0}
#查詢樹的jointree
(gdb) p *root->parse->jointree
$87 = {type = T_FromExpr, fromlist = 0x16b0a28, quals = 0x0}
(gdb) p *root->parse->jointree->fromlist
$88 = {type = T_List, length = 1, head = 0x16b0a08, tail = 0x16b0a08}
(gdb) p *(RangeTblRef *)root->parse->jointree->fromlist->head->data.ptr_value
$90 = {type = T_RangeTblRef, rtindex = 1}
(gdb) n
subquery_planner (glob=0x15eb8b0, parse=0x15c58b8, parent_root=0x0, hasRecursion=false, tuple_fraction=0) at planner.c:680
680     root->hasJoinRTEs = false;
#DONE!

優化後的查詢樹如下圖所示:

五、小結

UNION ALL優化過程：把集合操作扁平化為AppendRelInfo，通過增加AppendRelInfo、修改rtable等方式實現。

參考資料：
prepjointree.c
list.c