PostgreSQL查询代价估算（五）

1.1.1.1     嵌套循环的代价：final_cost_nestloop

函数功能：

计算两个关系使用嵌套循环连接算法的花费。内外表的元组数决定着CPU的花费。

对于nestloop花费的计算，分为两个函数：

1.        一是initial_cost_ nestloop函数，初步估计nestloop算法的花费，形成的结果会作为final_cost_nestloop的入口参数（JoinCostWorkspace *workspace）传入final_cost_nestloop函数；

2.        二是final_cost_nestloop函数，对nestloop算法的花费进行计算，区分了半连接、反半连接和其他连接类型的差别。

 

代码分析：

void

final_cost_nestloop(PlannerInfo *root, NestPath *path,

                                   JoinCostWorkspace*workspace,

                                   SpecialJoinInfo*sjinfo,

                                   SemiAntiJoinFactors*semifactors)

{......

       //半连接或反半连接，其连接后的元组数与其他连接方式不同，表现在选择率的计算上

       if(path->jointype == JOIN_SEMI || path->jointype == JOIN_ANTI)

       {......

              //半连接或反半连接，在遇到第一个满足条件的，即停止扫描，所以，需要考虑选择率（在initial_cost_nestloop函数中考虑选择率）

              ntuples= outer_matched_rows * inner_path_rows * inner_scan_frac;

 

              /*

               * For unmatched outer-rel rows, there are twocases.  If the inner

               * path is an indexscan using all the joinqualsas indexquals, then an

               * unmatched row results in an indexscanreturning no rows, which is

               * probably quite cheap.  We estimate this case as the same cost to

               * return the first tuple of a nonemptyscan.  Otherwise, the executor

               * will have to scan the whole inner rel; notso cheap.

               */

              if(has_indexed_join_quals(path))//对于半连接和反半连接考虑索引造成的影响

              {

                     run_cost+= (outer_path_rows - outer_matched_rows) *

                            inner_rescan_run_cost/ inner_path_rows;

              }

              else

              {

                     run_cost+= (outer_path_rows - outer_matched_rows) *

                            inner_rescan_run_cost;

                     ntuples+= (outer_path_rows - outer_matched_rows) * inner_path_rows;

              }

       }

       else//內连接，外表和内表的元组都要进行连接

       {  //对于半连接和反半连接考虑了索引造成的影响；对于內连接也应该考虑，但PostgreSQL没有考虑索引对非半连接和非反半连接的其他连接造成的影响

              ntuples= outer_path_rows * inner_path_rows;

       }

 

       //计算CPU花费

       cost_qual_eval(&restrict_qual_cost,path->joinrestrictinfo, root);

       startup_cost+= restrict_qual_cost.startup;

       cpu_per_tuple= cpu_tuple_cost + restrict_qual_cost.per_tuple;

       run_cost+= cpu_per_tuple * ntuples; //内外表的元组数决定着CPU的花费

 

       path->path.startup_cost= startup_cost;

       path->path.total_cost= startup_cost + run_cost;

}

 

调用关系图解：

 

1.        final_cost_nestloop函数被create_nestloop_path函数调用，直至被add_paths_to_joinrel函数调用，完成两表连接的nestloop连接算法的花费估算。