PostgreSQL 原始碼解讀（38）- 查詢語句#23（query_planner函式#1）

上一小節已介紹了grouping_planner函式,該函式透過呼叫query_planner函式為掃描/連線部分生成最優的未排序和預排序(presorted)路徑,本節主要介紹query_planner函式對簡單SQL如SELECT 2+2;的處理邏輯。

一、重要的資料結構

RelOptInfo
查詢語句經過查詢重寫/表示式簡化/外連線消除等處理後,查詢樹Query已完成規範化,查詢樹中的RangeTblEntry(RTE)資料結構已完成其歷史使命,由此可進入邏輯最佳化處理,在此階段使用RelOptInfo資料結構.

 typedef enum RelOptKind
 {
     RELOPT_BASEREL,//基本關係(如基表/子查詢等)
     RELOPT_JOINREL,//連線產生的關係,要注意的是透過連線等方式產生的結果亦可以視為關係
     RELOPT_OTHER_MEMBER_REL,
     RELOPT_OTHER_JOINREL,
     RELOPT_UPPER_REL,//上層的關係
     RELOPT_OTHER_UPPER_REL,
     RELOPT_DEADREL
 } RelOptKind;
 
 /*
  * Is the given relation a simple relation i.e a base or "other" member
  * relation?
  */
 #define IS_SIMPLE_REL(rel) \
     ((rel)->reloptkind == RELOPT_BASEREL || \
      (rel)->reloptkind == RELOPT_OTHER_MEMBER_REL)
 
 /* Is the given relation a join relation? */
 #define IS_JOIN_REL(rel)    \
     ((rel)->reloptkind == RELOPT_JOINREL || \
      (rel)->reloptkind == RELOPT_OTHER_JOINREL)
 
 /* Is the given relation an upper relation? */
 #define IS_UPPER_REL(rel)   \
     ((rel)->reloptkind == RELOPT_UPPER_REL || \
      (rel)->reloptkind == RELOPT_OTHER_UPPER_REL)
 
 /* Is the given relation an "other" relation? */
 #define IS_OTHER_REL(rel) \
     ((rel)->reloptkind == RELOPT_OTHER_MEMBER_REL || \
      (rel)->reloptkind == RELOPT_OTHER_JOINREL || \
      (rel)->reloptkind == RELOPT_OTHER_UPPER_REL)
 
 typedef struct RelOptInfo
 {
     NodeTag     type;//節點標識
 
     RelOptKind  reloptkind;//RelOpt型別
 
     /* all relations included in this RelOptInfo */
     Relids      relids;         /*Relids(rtindex)集合 set of base relids (rangetable indexes) */
 
     /* size estimates generated by planner */
     double      rows;           /*結果元組的估算數量 estimated number of result tuples */
 
     /* per-relation planner control flags */
     bool        consider_startup;   /*是否考慮啟動成本?是,需要保留啟動成本低的路徑 keep cheap-startup-cost paths? */
     bool        consider_param_startup; /*是否考慮引數化?的路徑 ditto, for parameterized paths? */
     bool        consider_parallel;  /*是否考慮並行處理路徑 consider parallel paths? */
 
     /* default result targetlist for Paths scanning this relation */
     struct PathTarget *reltarget;   /*掃描該Relation時預設的結果 list of Vars/Exprs, cost, width */
 
     /* materialization information */
     List       *pathlist;       /*訪問路徑連結串列 Path structures */
     List       *ppilist;        /*路徑連結串列中使用引數化路徑進行 ParamPathInfos used in pathlist */
     List       *partial_pathlist;   /* partial Paths */
     struct Path *cheapest_startup_path;//代價最低的啟動路徑
     struct Path *cheapest_total_path;//代價最低的整體路徑
     struct Path *cheapest_unique_path;//代價最低的獲取唯一值的路徑
     List       *cheapest_parameterized_paths;//代價最低的引數化?路徑連結串列
 
     /* parameterization information needed for both base rels and join rels */
     /* (see also lateral_vars and lateral_referencers) */
     Relids      direct_lateral_relids;  /*使用lateral語法,需依賴的Relids rels directly laterally referenced */
     Relids      lateral_relids; /* minimum parameterization of rel */
 
     /* information about a base rel (not set for join rels!) */
   //reloptkind=RELOPT_BASEREL時使用的資料結構
     Index       relid;      /* Relation ID */
     Oid         reltablespace;  /* 表空間 containing tablespace */
     RTEKind     rtekind;        /* 基表?子查詢?還是函式等等?RELATION, SUBQUERY, FUNCTION, etc */
     AttrNumber  min_attr;       /* 最小的屬性編號 smallest attrno of rel (often <0) */
     AttrNumber  max_attr;       /* 最大的屬性編號 largest attrno of rel */
     Relids     *attr_needed;    /* 陣列 array indexed [min_attr .. max_attr] */
     int32      *attr_widths;    /* 屬性寬度 array indexed [min_attr .. max_attr] */
     List       *lateral_vars;   /* 關係依賴的Vars/PHVs LATERAL Vars and PHVs referenced by rel */
     Relids      lateral_referencers;    /*依賴該關係的Relids rels that reference me laterally */
     List       *indexlist;      /* 該關係的IndexOptInfo連結串列 list of IndexOptInfo */
     List       *statlist;       /* 統計資訊連結串列 list of StatisticExtInfo */
     BlockNumber pages;          /* 塊數 size estimates derived from pg_class */
     double      tuples;     /* 元組數 */
     double      allvisfrac;   /* ? */
     PlannerInfo *subroot;       /* 如為子查詢,儲存子查詢的root if subquery */
     List       *subplan_params; /* 如為子查詢,儲存子查詢的引數 if subquery */
     int         rel_parallel_workers;   /* 並行執行,需要多少個workers? wanted number of parallel workers */
 
     /* Information about foreign tables and foreign joins */
   //FWD相關資訊
     Oid         serverid;       /* identifies server for the table or join */
     Oid         userid;         /* identifies user to check access as */
     bool        useridiscurrent;    /* join is only valid for current user */
     /* use "struct FdwRoutine" to avoid including fdwapi.h here */
     struct FdwRoutine *fdwroutine;
     void       *fdw_private;
 
     /* cache space for remembering if we have proven this relation unique */
   //已知的,可保證唯一的Relids連結串列
     List       *unique_for_rels;    /* known unique for these other relid
                                      * set(s) */
     List       *non_unique_for_rels;    /* 已知的,不唯一的Relids連結串列 known not unique for these set(s) */
 
     /* used by various scans and joins: */
     List       *baserestrictinfo;   /* 如為基本關係,儲存約束條件 RestrictInfo structures (if base rel) */
     QualCost    baserestrictcost;   /* 解析約束表示式的成本? cost of evaluating the above */
     Index       baserestrict_min_security;  /* 最低安全等級 min security_level found in
                                              * baserestrictinfo */
     List       *joininfo;       /* 連線語句的約束條件資訊 RestrictInfo structures for join clauses
                                  * involving this rel */
     bool        has_eclass_joins;   /* 是否存在等價類連線? T means joininfo is incomplete */
 
     /* used by partitionwise joins: */
     bool        consider_partitionwise_join;    /* 分割槽? consider partitionwise
                                                  * join paths? (if
                                                  * partitioned rel) */
     Relids      top_parent_relids;  /* Relids of topmost parents (if "other"
                                      * rel) */
 
     /* used for partitioned relations */
   //分割槽表使用
     PartitionScheme part_scheme;    /* 分割槽的schema Partitioning scheme. */
     int         nparts;         /* 分割槽數 number of partitions */
     struct PartitionBoundInfoData *boundinfo;   /* 分割槽邊界資訊 Partition bounds */
     List       *partition_qual; /* 分割槽約束 partition constraint */
     struct RelOptInfo **part_rels;  /* 分割槽的RelOptInfo陣列 Array of RelOptInfos of partitions,
                                      * stored in the same order of bounds */
     List      **partexprs;      /* 非空分割槽鍵表示式 Non-nullable partition key expressions. */
     List      **nullable_partexprs; /* 可為空的分割槽鍵表示式 Nullable partition key expressions. */
     List       *partitioned_child_rels; /* RT Indexes連結串列 List of RT indexes. */
 } RelOptInfo;

PathCostComparison

 typedef enum
 {
     COSTS_EQUAL,                /* 近似相等 path costs are fuzzily equal */
     COSTS_BETTER1,              /* 第一個路徑成本較低 first path is cheaper than second */
     COSTS_BETTER2,              /* 第二個相對較低 second path is cheaper than first */
     COSTS_DIFFERENT             /* 不管是哪個路徑,成本上都不佔優勢 neither path dominates the other on cost */
 } PathCostComparison;
 

ResultPath

 /*
  * ResultPath represents use of a Result plan node to compute a variable-free
  * targetlist with no underlying tables (a "SELECT expressions" query).
  * The query could have a WHERE clause, too, represented by "quals".
  *
  * Note that quals is a list of bare clauses, not RestrictInfos.
  */
 typedef struct ResultPath //表示無基礎表的結果計劃節點
 {
     Path        path;//掃描路徑
     List       *quals;//where語句表示式,bare clauses, not RestrictInfos
 } ResultPath;

二、原始碼解讀

 /*
  * query_planner
  *    Generate a path (that is, a simplified plan) for a basic query,
  *    which may involve joins but not any fancier features.
  *
  * 為一個基本的查詢(可能涉及連線)生成訪問路徑(也可以視為一個簡化的計劃).
  *
  * Since query_planner does not handle the toplevel processing (grouping,
  * sorting, etc) it cannot select the best path by itself.  Instead, it
  * returns the RelOptInfo for the top level of joining, and the caller
  * (grouping_planner) can choose among the surviving paths for the rel.
  *
  * query_planner不會處理頂層的處理過程(如最後的分組/排序等操作),因此,不能選擇最優的訪問路徑
  * 該函式會返回RelOptInfo給最高層的連線,grouping_planner可以在剩下的路徑中進行選擇
  *
  * root describes the query to plan
  * tlist is the target list the query should produce
  *      (this is NOT necessarily root->parse->targetList!)
  * qp_callback is a function to compute query_pathkeys once it's safe to do so
  * qp_extra is optional extra data to pass to qp_callback
  *
  * root是計劃資訊/tlist是投影列
  * qp_callback是計算query_pathkeys的函式/qp_extra是傳遞給qp_callback的函式
  *
  * Note: the PlannerInfo node also includes a query_pathkeys field, which
  * tells query_planner the sort order that is desired in the final output
  * plan.  This value is *not* available at call time, but is computed by
  * qp_callback once we have completed merging the query's equivalence classes.
  * (We cannot construct canonical pathkeys until that's done.)
  */
 RelOptInfo *
 query_planner(PlannerInfo *root, List *tlist,
               query_pathkeys_callback qp_callback, void *qp_extra)
 {
     Query      *parse = root->parse;//查詢樹
     List       *joinlist;
     RelOptInfo *final_rel;//結果
     Index       rti;//RTE的index
     double      total_pages;//總pages數
 
     /*
      * If the query has an empty join tree, then it's something easy like
      * "SELECT 2+2;" or "INSERT ... VALUES()".  Fall through quickly.
      */
     if (parse->jointree->fromlist == NIL)//簡單SQL,無FROM/WHERE語句
     {
         /* We need a dummy joinrel to describe the empty set of baserels */
         final_rel = build_empty_join_rel(root);//建立返回結果
 
         /*
          * If query allows parallelism in general, check whether the quals are
          * parallel-restricted.  (We need not check final_rel->reltarget
          * because it's empty at this point.  Anything parallel-restricted in
          * the query tlist will be dealt with later.)
          */
         if (root->glob->parallelModeOK)//並行模式?
             final_rel->consider_parallel =
                 is_parallel_safe(root, parse->jointree->quals);
 
         /* The only path for it is a trivial Result path */
         add_path(final_rel, (Path *)
                  create_result_path(root, final_rel,
                                     final_rel->reltarget,
                                     (List *) parse->jointree->quals));//新增訪問路徑
 
         /* Select cheapest path (pretty easy in this case...) */
         set_cheapest(final_rel);//選擇最優的訪問路徑
 
         /*
          * We still are required to call qp_callback, in case it's something
          * like "SELECT 2+2 ORDER BY 1".
          */
         root->canon_pathkeys = NIL;
         (*qp_callback) (root, qp_extra);//回撥函式
 
         return final_rel;//返回
     }
 
     //其他程式碼
     ...
 }

add_path/set_cheapest
後續再行介紹

create_result_path

 /*
  * create_result_path
  *    Creates a path representing a Result-and-nothing-else plan.
  *
  * This is only used for degenerate cases, such as a query with an empty
  * jointree.
  */
 ResultPath *
 create_result_path(PlannerInfo *root, RelOptInfo *rel,
                    PathTarget *target, List *resconstantqual)
 {
     ResultPath *pathnode = makeNode(ResultPath);//結果
 
     pathnode->path.pathtype = T_Result;//掃描路徑型別
     pathnode->path.parent = rel;//路徑的partentbuild_empty_join_rel
     pathnode->path.pathtarget = target;//目標列
     pathnode->path.param_info = NULL;   /* there are no other rels... */
     pathnode->path.parallel_aware = false;
     pathnode->path.parallel_safe = rel->consider_parallel;
     pathnode->path.parallel_workers = 0;//並行workers數目,設定為0
     pathnode->path.pathkeys = NIL;//
     pathnode->quals = resconstantqual;//表示式
 
     /* Hardly worth defining a cost_result() function ... just do it */
     pathnode->path.rows = 1;//行數為1
     pathnode->path.startup_cost = target->cost.startup;
     pathnode->path.total_cost = target->cost.startup +
         cpu_tuple_cost + target->cost.per_tuple;
 
     /*
      * Add cost of qual, if any --- but we ignore its selectivity, since our
      * rowcount estimate should be 1 no matter what the qual is.
      */
     if (resconstantqual)
     {
         QualCost    qual_cost;
 
         cost_qual_eval(&qual_cost, resconstantqual, root);
         /* resconstantqual is evaluated once at startup */
         pathnode->path.startup_cost += qual_cost.startup + qual_cost.per_tuple;
         pathnode->path.total_cost += qual_cost.startup + qual_cost.per_tuple;
     }
 
     return pathnode;
 }

build_empty_join_rel

/*
  * build_empty_join_rel
  *      Build a dummy join relation describing an empty set of base rels.
  *
  * This is used for queries with empty FROM clauses, such as "SELECT 2+2" or
  * "INSERT INTO foo VALUES(...)".  We don't try very hard to make the empty
  * joinrel completely valid, since no real planning will be done with it ---
  * we just need it to carry a simple Result path out of query_planner().
  */
 RelOptInfo *
 build_empty_join_rel(PlannerInfo *root)
 {
     RelOptInfo *joinrel;
 
     /* The dummy join relation should be the only one ... */
     Assert(root->join_rel_list == NIL);
 
     joinrel = makeNode(RelOptInfo);
     joinrel->reloptkind = RELOPT_JOINREL;
     joinrel->relids = NULL;     /* empty set */
     joinrel->rows = 1;          /* we produce one row for such cases */
     joinrel->rtekind = RTE_JOIN;
     joinrel->reltarget = create_empty_pathtarget();
 
     root->join_rel_list = lappend(root->join_rel_list, joinrel);
 
     return joinrel;
 }

三、跟蹤分析

(gdb) b query_planner
Breakpoint 1 at 0x76942c: file planmain.c, line 57.
(gdb) c
Continuing.

Breakpoint 1, query_planner (root=0x275c878, tlist=0x277fd78, qp_callback=0x76e97d <standard_qp_callback>, 
    qp_extra=0x7ffdd435d490) at planmain.c:57
57    Query    *parse = root->parse;
(gdb) n
67    if (parse->jointree->fromlist == NIL)
(gdb) 
70      final_rel = build_empty_join_rel(root);
(gdb) 
78      if (root->glob->parallelModeOK)
#建立的空RELOPT_JOINREL
(gdb) p *final_rel
$4 = {type = T_RelOptInfo, reloptkind = RELOPT_JOINREL, relids = 0x0, rows = 1, consider_startup = false, 
  consider_param_startup = false, consider_parallel = false, reltarget = 0x277fda8, pathlist = 0x0, ppilist = 0x0, 
  partial_pathlist = 0x0, cheapest_startup_path = 0x0, cheapest_total_path = 0x0, cheapest_unique_path = 0x0, 
  cheapest_parameterized_paths = 0x0, direct_lateral_relids = 0x0, lateral_relids = 0x0, relid = 0, reltablespace = 0, 
  rtekind = RTE_JOIN, min_attr = 0, max_attr = 0, attr_needed = 0x0, attr_widths = 0x0, lateral_vars = 0x0, 
  lateral_referencers = 0x0, indexlist = 0x0, statlist = 0x0, pages = 0, tuples = 0, allvisfrac = 0, subroot = 0x0, 
  subplan_params = 0x0, rel_parallel_workers = 0, serverid = 0, userid = 0, useridiscurrent = false, fdwroutine = 0x0, 
  fdw_private = 0x0, unique_for_rels = 0x0, non_unique_for_rels = 0x0, baserestrictinfo = 0x0, baserestrictcost = {
    startup = 0, per_tuple = 0}, baserestrict_min_security = 0, joininfo = 0x0, has_eclass_joins = false, 
  top_parent_relids = 0x0, part_scheme = 0x0, nparts = 0, boundinfo = 0x0, partition_qual = 0x0, part_rels = 0x0, 
  partexprs = 0x0, nullable_partexprs = 0x0, partitioned_child_rels = 0x0}
...
(gdb) step
add_path (parent_rel=0x275cc88, new_path=0x275c498) at pathnode.c:424
424   bool    accept_new = true;  /* unless we find a superior old path */
#建立的path(ResultPath)
(gdb) p *new_path
$6 = {type = T_ResultPath, pathtype = T_Result, parent = 0x275cc88, pathtarget = 0x277fda8, param_info = 0x0, 
  parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 1, startup_cost = 0, total_cost = 0.01, 
  pathkeys = 0x0}
(gdb) finish
Run till exit from #0  add_path (parent_rel=0x275cc88, new_path=0x275c498) at pathnode.c:425
query_planner (root=0x275c878, tlist=0x277fd78, qp_callback=0x76e97d <standard_qp_callback>, qp_extra=0x7ffdd435d490)
    at planmain.c:89
89      set_cheapest(final_rel);
...
98      return final_rel;
(gdb) 
267 }
#返回值
(gdb) p *final_rel
$8 = {type = T_RelOptInfo, reloptkind = RELOPT_JOINREL, relids = 0x0, rows = 1, consider_startup = false, 
  consider_param_startup = false, consider_parallel = true, reltarget = 0x277fda8, pathlist = 0x277fe68, ppilist = 0x0, 
  partial_pathlist = 0x0, cheapest_startup_path = 0x275c498, cheapest_total_path = 0x275c498, cheapest_unique_path = 0x0, 
  cheapest_parameterized_paths = 0x277feb8, direct_lateral_relids = 0x0, lateral_relids = 0x0, relid = 0, 
  reltablespace = 0, rtekind = RTE_JOIN, min_attr = 0, max_attr = 0, attr_needed = 0x0, attr_widths = 0x0, 
  lateral_vars = 0x0, lateral_referencers = 0x0, indexlist = 0x0, statlist = 0x0, pages = 0, tuples = 0, allvisfrac = 0, 
  subroot = 0x0, subplan_params = 0x0, rel_parallel_workers = 0, serverid = 0, userid = 0, useridiscurrent = false, 
  fdwroutine = 0x0, fdw_private = 0x0, unique_for_rels = 0x0, non_unique_for_rels = 0x0, baserestrictinfo = 0x0, 
  baserestrictcost = {startup = 0, per_tuple = 0}, baserestrict_min_security = 0, joininfo = 0x0, has_eclass_joins = false, 
  top_parent_relids = 0x0, part_scheme = 0x0, nparts = 0, boundinfo = 0x0, partition_qual = 0x0, part_rels = 0x0, 
  partexprs = 0x0, nullable_partexprs = 0x0, partitioned_child_rels = 0x0}
(gdb) p *final_rel->cheapest_total_path
$9 = {type = T_ResultPath, pathtype = T_Result, parent = 0x275cc88, pathtarget = 0x277fda8, param_info = 0x0, 
  parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 1, startup_cost = 0, total_cost = 0.01, 
  pathkeys = 0x0}
(gdb) 
#DONE!

四、參考資料

planmain.c