SQL语句解析完成后被解析成Query结构，在进行优化时是以Query为单位进行的，Query的优化分为基于规则的逻辑优化（查询重写）和基于代价的物理优化（计划生成），主入口函数为subquery_planner。subquery_planner函数接收Query(查询树)，返回一个Plan(计划树)。
Plan* subquery_planner(PlannerGlobal* glob, Query* parse, PlannerInfo* parent_root,boolhasRecursion,doubletuple_fraction, PlannerInfo** subroot,intoptions, ItstDisKey* diskeys,List* subqueryRestrictInfo){ PlannerInfo* root =NULL;Plan* plan =NULL;//返回结果…preprocess_const_params(root, (Node*)parse->jointree);// 常数替换等式…if(parse->hasSubLinks) { pull_up_sublinks(root);//提升子链接DEBUG_QRW("After sublink pullup"); }/* Reduce orderby clause in subquery for join */reduce_orderby(parse,false);//减少orderbyDEBUG_QRW("After order by reduce");if(u_sess->attr.attr_sql.enable_constraint_optimization) { removeNotNullTest(root);//删除NotNullTestDEBUG_QRW("After soft constraint removal"); }…if((LAZY_AGG & u_sess->attr.attr_sql.rewrite_rule) && permit_from_rewrite_hint(root, LAZY_AGG)) { lazyagg_main(parse);// lazyagg重写DEBUG_QRW("After lazyagg");}…parse->jointree = (FromExpr*)pull_up_subqueries(root, (Node*)parse->jointree);//提升子查询…if(parse->setOperations) { flatten_simple_union_all(root);//UNIONALL优化DEBUG_QRW("After simple union all flatten");}…expand_inherited_tables(root);//展开继承表…parse->targetList = (List*)preprocess_expression(root, (Node*)parse->targetList, EXPRKIND_TARGET);//预处理表达式…parse->havingQual = (Node *) newHaving;//处理HAVING子句… reduce_outer_joins(root);//外连接消除… reduce_inequality_fulljoins(root);//全连接重写…plan = grouping_planner(root, tuple_fraction);//主要的计划过程…returnplan;}
subquery_planner函数由函数standard_planner调用，standard_planner函数由exec_simple_query->pg_plan_queries->pg_plan_query->planner函数调用。standard_planner将Query(查询树)生成规划好的语句，可用于执行器实际执行。
PlannedStmt* standard_planner(Query* parse,intcursorOptions, ParamListInfo boundParams){ PlannedStmt* result =NULL;//返回结果PlannerGlobal* glob =NULL;doubletuple_fraction; PlannerInfo* root =NULL;Plan* top_plan =NULL;… glob = makeNode(PlannerGlobal);/* primary planning entry point (may recurse for subqueries) */top_plan = subquery_planner(glob, parse,NULL,false, tuple_fraction, &root);//主规划过程入口…/* build the PlannedStmt result */result = makeNode(PlannedStmt);//构造PlannedStmtresult->commandType = parse->commandType;result->queryId = parse->queryId;result->uniqueSQLId = parse->uniqueSQLId;result->hasReturning = (parse->returningList != NIL);result->hasModifyingCTE = parse->hasModifyingCTE;result->canSetTag = parse->canSetTag;result->transientPlan = glob->transientPlan;result->dependsOnRole = glob->dependsOnRole;result->planTree = top_plan;//执行计划result->rtable = glob->finalrtable;result->resultRelations = glob->resultRelations;…returnresult;}
仍然以前文的join列子来说明
SELECT* FROM t1inner JOIN t2ONt1.c1= t2.c1;
在planner函数打断点，用gdb查看standard_planner返回的PlannedStmt
(gdb) bt#0 planner (parse=0x7fd93a410288, cursorOptions=0, boundParams=0x0) at planner.cpp:389#1 0x0000000001936fbdinpg_plan_query (querytree=0x7fd93a410288, cursorOptions=0, boundParams=0x0, underExplain=false) at postgres.cpp:1197#2 0x0000000001937381inpg_plan_queries (querytrees=0x7fd939b81090, cursorOptions=0, boundParams=0x0) at postgres.cpp:1315#3 0x000000000193a6b8inexec_simple_query (query_string=0x7fd966ad2060"SELECT * FROM t1 inner JOIN t2 ON t1.c1 = t2.c1;", messageType=QUERY_MESSAGE, msg=0x7fd931056210)at postgres.cpp:2560#4 0x0000000001947104inPostgresMain (argc=1, argv=0x7fd93a2cf1c0, dbname=0x7fd93a2ce1f8"postgres", username=0x7fd93a2ce1b0"test") at postgres.cpp:8403#5 0x0000000001890740inBackendRun (port=0x7fd931056720) at postmaster.cpp:8053#6 0x00000000018a00b1inGaussDbThreadMain<(knl_thread_role)1> (arg=0x7fd97c55c5f0) at postmaster.cpp:12181#7 0x000000000189c0deinInternalThreadFunc (args=0x7fd97c55c5f0) at postmaster.cpp:12755#8 0x00000000024bf7d8inThreadStarterFunc (arg=0x7fd97c55c5e0) at gs_thread.cpp:382#9 0x00007fd9a60cfdd5instart_thread () from /lib64/libpthread.so.0#10 0x00007fd9a5df8eadinclone() from /lib64/libc.so.6
(gdb)p*result$14={type=T_PlannedStmt,commandType=CMD_SELECT,queryId=0,hasReturning=false,hasModifyingCTE=false,canSetTag=true,transientPlan=false,dependsOnRole=false,planTree=0x7fd93a409d58,rtable=0x7fd939b81660,…}(gdb)p*result->planTree->lefttree$46={type=T_SeqScan,plan_node_id=2,parent_node_id=1,exec_type=EXEC_ON_DATANODES,startup_cost=0,total_cost=1.03,plan_rows=3,multiple=1,plan_width=8,…}
将Query规划后得到PlannedStmt
可以看到，Plannedstmt 与explain执行计划是一致的
更多内容请参考：
