Postgresql源码（134）优化器针对volatile函数的排序优化分析

mingjie

发布于 2024-05-29 12:13:56

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发布于 2024-05-29 12:13:56

相关《Postgresql源码（133）优化器动态规划生成连接路径的实例分析》

上一篇对路径的生成进行了分析，通过make_one_rel最终拿到了一个带着路径的RelOptInfo。本篇针对带volatile函数的排序场景继续分析subquery_planner的后续流程。

subquery_planner
	grouping_planner
		query_planner
			make_one_rel   <<< 上一篇
		// 后续流程         <<< 本篇

总结速查

一句话总结：带有volatile的投影列会被SORT算子忽略，达到先排序在投影计算volatile的效果。

grouping_planner→make_one_rel层层生成path，每个path都会带pathtarget（不一定是SQL中最后需要的target列表），一般都是层层继承上来的。
make_one_rel生成的最终path中，会忽略volatile函数列，交给外层grouping_planner函数处理，所以生成的path中的pathtarget都是看不到volatile函数列的。
这里一个关键逻辑就path中的pathtarget和make_sort_input_target计算出来列表的是不是一样的
- 如果是一样的就不加投影节点，等后面加sort时（create_ordered_paths）先加sort在加投影，计算顺序就是先排序，在拿排序阶段投影（计算random函数）
- 如果不一样就直接加投影节点，后面sort会加到投影上面，计算顺序就是先投影（计算random函数），再排序。
path中的pathtarget会忽略volatile函数。
make_sort_input_target中的volatile函数正常也会被忽略掉（实例3），除非volatile函数就是排序列（实例4）。

最终效果是，投影列有**volatile**函数的SQL（函数非排序列），**sort**节点会忽略这类函数的执行，sort结束后，在投影节点使用sort的结果集来计算这类函数。

实例3：

1 实例：简单join

drop table student;
create table student(sno int primary key, sname varchar(10), ssex int);
insert into student values(1, 'stu1', 0);
insert into student values(2, 'stu2', 1);
insert into student values(3, 'stu3', 1);
insert into student values(4, 'stu4', 0);

drop table course;
create table course(cno int primary key, cname varchar(10), tno int);
insert into course values(20, 'meth', 10);
insert into course values(21, 'english', 11);

drop table teacher;
create table teacher(tno int primary key, tname varchar(10), tsex int);
insert into teacher values(10, 'te1', 1);
insert into teacher values(11, 'te2', 0);

drop table score;
create table score (sno int, cno int, degree int);
create index idx_score_sno on score(sno);
insert into score values (1, 20, 100);
insert into score values (1, 21, 89);
insert into score values (2, 20, 99);
insert into score values (2, 21, 90);
insert into score values (3, 20, 87);
insert into score values (3, 21, 20);
insert into score values (4, 20, 60);
insert into score values (4, 21, 70);


explain 
SELECT STUDENT.sname, COURSE.cname, SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno;

                                  QUERY PLAN
------------------------------------------------------------------------------
 Hash Left Join  (cost=69.50..110.65 rows=2040 width=80)
   Hash Cond: (score.cno = course.cno)
   ->  Hash Right Join  (cost=34.75..70.53 rows=2040 width=46)
         Hash Cond: (score.sno = student.sno)
         ->  Seq Scan on score  (cost=0.00..30.40 rows=2040 width=12)
         ->  Hash  (cost=21.00..21.00 rows=1100 width=42)
               ->  Seq Scan on student  (cost=0.00..21.00 rows=1100 width=42)
   ->  Hash  (cost=21.00..21.00 rows=1100 width=42)
         ->  Seq Scan on course  (cost=0.00..21.00 rows=1100 width=42)

1.1 subquery_planner→grouping_planner

grouping_planner
	current_rel = query_planner(root, standard_qp_callback, &qp_extra);

current_rel：

	final_target = create_pathtarget(root, root->processed_tlist);

得到final_target
- final_target->exprs->elements0 : {varno = 1, varattno = 2, vartype = 1043} STUDENT.sname
- final_target->exprs->elements1 : {varno = 4, varattno = 2, vartype = 1043} COURSE.cname
- final_target->exprs->elements2 : {varno = 2, varattno = 3, vartype = 23} SCORE.degree

	if (parse->sortClause)
		make_sort_input_target
	if (activeWindows)
	 	...
	if (have_grouping)
		...
	if (parse->hasTargetSRFs)
		...

apply_scanjoin_target_to_paths创建投影节点

	/* Apply scan/join target. */
	scanjoin_target_same_exprs = list_length(scanjoin_targets) == 1
		&& equal(scanjoin_target->exprs, current_rel->reltarget->exprs);
	apply_scanjoin_target_to_paths(root, current_rel, scanjoin_targets,
								   scanjoin_targets_contain_srfs,
								   scanjoin_target_parallel_safe,
								   scanjoin_target_same_exprs);

继续

	if (have_grouping)
		...
	if (activeWindows)
		...
	if (parse->distinctClause)
		...
	if (parse->sortClause)
		create_ordered_paths

创建空的最顶层节点

	final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL);

遍历current_rel中所有的path，用add_path加入到最顶层节点中。
其中limit、rowclock的场景需要特殊处理下。

	foreach(lc, current_rel->pathlist)
		if (parse->rowMarks)
			create_lockrows_path
		if (limit_needed(parse))
			create_limit_path
		add_path(final_rel, path);

grouping_planner函数执行结束，最后拼接的final_rel在upper_rels里面记录：

pathlist最上层是投影节点：

1.2 standard_planner→subquery_planner

subquery_planner中后续处理流程：

计划生成步骤	作用
root = subquery_planner	优化器入口，返回PlannerInfo，里面记录了一个最终的RelOptInfo相当于一张逻辑表，每个ROI都记录了多个path，表示不同的计算路径
final_rel = fetch_upper_rel	拿到最终的RelOptInfo
best_path = get_cheapest_fractional_path	在RelOptInfo中选择一个最优的path
top_plan = create_plan→create_plan_recurse	根据最优path生成计划

2 实例：【简单join】【排序非投影列】【投影列无函数】

drop table student;
create table student(sno int primary key, sname varchar(10), ssex int);
insert into student values(1, 'stu1', 0);
insert into student values(2, 'stu2', 1);
insert into student values(3, 'stu3', 1);
insert into student values(4, 'stu4', 0);

drop table course;
create table course(cno int primary key, cname varchar(10), tno int);
insert into course values(20, 'meth', 10);
insert into course values(21, 'english', 11);

drop table teacher;
create table teacher(tno int primary key, tname varchar(10), tsex int);
insert into teacher values(10, 'te1', 1);
insert into teacher values(11, 'te2', 0);

drop table score;
create table score (sno int, cno int, degree int);
create index idx_score_sno on score(sno);
insert into score values (1, 20, 100);
insert into score values (1, 21, 89);
insert into score values (2, 20, 99);
insert into score values (2, 21, 90);
insert into score values (3, 20, 87);
insert into score values (3, 21, 20);
insert into score values (4, 20, 60);
insert into score values (4, 21, 70);


explain verbose
SELECT STUDENT.sname, COURSE.cname, SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno
ORDER BY COURSE.cno;

                                      QUERY PLAN
--------------------------------------------------------------------------------------
 Sort  (cost=3.44..3.46 rows=8 width=19)
   Output: student.sname, course.cname, score.degree, course.cno
   Sort Key: course.cno
   ->  Hash Left Join  (cost=2.14..3.32 rows=8 width=19)
         Output: student.sname, course.cname, score.degree, course.cno
         Inner Unique: true
         Hash Cond: (score.cno = course.cno)
         ->  Hash Right Join  (cost=1.09..2.21 rows=8 width=13)
               Output: student.sname, score.degree, score.cno
               Inner Unique: true
               Hash Cond: (score.sno = student.sno)
               ->  Seq Scan on public.score  (cost=0.00..1.08 rows=8 width=12)
                     Output: score.sno, score.cno, score.degree
               ->  Hash  (cost=1.04..1.04 rows=4 width=9)
                     Output: student.sname, student.sno
                     ->  Seq Scan on public.student  (cost=0.00..1.04 rows=4 width=9)
                           Output: student.sname, student.sno
         ->  Hash  (cost=1.02..1.02 rows=2 width=10)
               Output: course.cname, course.cno
               ->  Seq Scan on public.course  (cost=0.00..1.02 rows=2 width=10)
                     Output: course.cname, course.cno

2.1 grouping_planner

grouping_planner
	current_rel = query_planner(root, standard_qp_callback, &qp_extra);
	final_target = create_pathtarget(root, root->processed_tlist);
	if (parse->sortClause)
		sort_input_target = make_sort_input_target(root, final_target, &have_postponed_srfs);

make_sort_input_target函数的作用：

排序列可能不在最终的投影列里面，需要特殊处理下。
易变函数和成本很高的函数需要再投影列中识别出来，先排序，在计算。
- 因为1：sort limit场景可以少算一些。
- 因为2：易变函数每次算都可能不一样，先排序好了再算有利于结果集稳定，例如current_timestamp这种，期望是排序后给出的每一样的时间都是递增的，如果先排序在计算就能得到这种效果。

生成的final_target和sort_input_target相同，因为没看到srf函数、易变函数。

final_target同sort_input_target	Var	指向列	sortgrouprefs
final_target->exprs->elements0	varno = 1, varattno = 2, vartype = 1043	STUDENT.sname	0
final_target->exprs->elements1	varno = 4, varattno = 2, vartype = 1043	COURSE.cname	0
final_target->exprs->elements2	varno = 2, varattno = 3, vartype = 23	SCORE.degree	0
final_target->exprs->elements3	varno = 4, varattno = 1, vartype = 23	COURSE.cno	1

grouping_planner继续执行，开始生成排序path：

	...
	if (parse->sortClause)
		current_rel = create_ordered_paths(root,
										   current_rel,
										   final_target,
										   final_target_parallel_safe,
										   have_postponed_srfs ? -1.0 :
										   limit_tuples);

grouping_planner→create_ordered_paths

create_ordered_paths
	// 创建一个排序节点
	ordered_rel = fetch_upper_rel(root, UPPERREL_ORDERED, NULL);

	// 拿到path入口，目前顶层是T_ProjectionPath，就一个节点
	foreach(lc, input_rel->pathlist)
		// 判断input_path->pathkeys是不是有序的？
		// 因为现在计划树是hashjoin，每一列都是无序的，所以input_path->pathkeys是空的，需要排序
		is_sorted = pathkeys_count_contained_in(root->sort_pathkeys, input_path->pathkeys, &presorted_keys);
		if (is_sorted)
			sorted_path = input_path;
		else
			sorted_path = (Path *) create_sort_path(root,
														ordered_rel,
														input_path,
														root->sort_pathkeys,
														limit_tuples);

输入的path顶层节点是project本来没有带pathkeys信息，这里创建一个sort节点放在上面，加入pathkey信息。
但生成的sortpath没看到排序列的信息？
排序信息在基类path的pathkeys中。

sorted_path = 
{ path = 
  { type = T_SortPath, 
    pathtype = T_Sort, 
    parent = 0x2334030, 
    pathtarget = 0x2333ef0, 
    param_info = 0x0, 
    parallel_aware = false, parallel_safe = true, parallel_workers = 0, 
    rows = 8, 
    startup_cost = 3.4437500000000005, 
    total_cost = 3.4637500000000006, 
    pathkeys = 0x232e018}, 
  subpath = 0x2333a00}

T_PathKey每个pathkey（排序列）都对应了一个T_EquivalenceClass，T_EquivalenceClass中记录了排序的具体信息。

{ type = T_PathKey, 
  pk_eclass = 0x232bf88, 
  pk_opfamily = 1976, 
  pk_strategy = 1, 
  pk_nulls_first = false}

T_EquivalenceClass中的ec_members记录了排序列信息Var{varno = 4, varattno = 1}

{ type = T_EquivalenceClass, 
  ec_opfamilies = 0x232ddf8,    // List{ 1976 }
  ec_collation = 0, 
  ec_members = 0x232df48,  // List { EquivalenceMember }
                           // EquivalenceMember{
                           //   type = T_EquivalenceMember, 
                           //   em_expr = 0x232de68,  Var{varno = 4, varattno = 1}
                           //   em_relids = 0x232de48, 
                           //   em_is_const = false, 
                           //   em_is_child = false, 
                           //   em_datatype = 23, 
                           //   em_jdomain = 0x2329158, em_parent = 0x0}
  ec_sources = 0x0, 
  ec_derives = 0x0, 
  ec_relids = 0x232df28,
  ec_has_const = false, 
  ec_has_volatile = false, 
  ec_broken = false, 
  ec_sortref = 1, 
  ec_min_security = 4294967295, 
  ec_max_security = 0, 
  ec_merged = 0x0}

生成排序节点后的计划：

sort节点的target是四列，虽然sql只写了三列，但有一列是排序需要的，也会加到pathtarget中。

3 实例：【简单join】【排序非投影列】【投影列中有volatile函数】

drop table student;
create table student(sno int primary key, sname varchar(10), ssex int);
insert into student values(1, 'stu1', 0);
insert into student values(2, 'stu2', 1);
insert into student values(3, 'stu3', 1);
insert into student values(4, 'stu4', 0);

drop table course;
create table course(cno int primary key, cname varchar(10), tno int);
insert into course values(20, 'meth', 10);
insert into course values(21, 'english', 11);

drop table teacher;
create table teacher(tno int primary key, tname varchar(10), tsex int);
insert into teacher values(10, 'te1', 1);
insert into teacher values(11, 'te2', 0);

drop table score;
create table score (sno int, cno int, degree int);
create index idx_score_sno on score(sno);
insert into score values (1, 20, 100);
insert into score values (1, 21, 89);
insert into score values (2, 20, 99);
insert into score values (2, 21, 90);
insert into score values (3, 20, 87);
insert into score values (3, 21, 20);
insert into score values (4, 20, 60);
insert into score values (4, 21, 70);


explain verbose
SELECT STUDENT.sname, random(), SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno
ORDER BY COURSE.cno;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Result  (cost=3.44..3.56 rows=8 width=21)
   Output: student.sname, random(), score.degree, course.cno
   ->  Sort  (cost=3.44..3.46 rows=8 width=13)
         Output: student.sname, score.degree, course.cno
         Sort Key: course.cno
         ->  Hash Left Join  (cost=2.14..3.32 rows=8 width=13)
               Output: student.sname, score.degree, course.cno
               Inner Unique: true
               Hash Cond: (score.cno = course.cno)
               ->  Hash Right Join  (cost=1.09..2.21 rows=8 width=13)
                     Output: student.sname, score.degree, score.cno
                     Inner Unique: true
                     Hash Cond: (score.sno = student.sno)
                     ->  Seq Scan on public.score  (cost=0.00..1.08 rows=8 width=12)
                           Output: score.sno, score.cno, score.degree
                     ->  Hash  (cost=1.04..1.04 rows=4 width=9)
                           Output: student.sname, student.sno
                           ->  Seq Scan on public.student  (cost=0.00..1.04 rows=4 width=9)
                                 Output: student.sname, student.sno
               ->  Hash  (cost=1.02..1.02 rows=2 width=4)
                     Output: course.cno
                     ->  Seq Scan on public.course  (cost=0.00..1.02 rows=2 width=4)
                           Output: course.cno

3.1 grouping_planner→make_one_rel生成的RelOptInfo→reltarget

make_one_rel前：

准备连接的RelOptInfo在simple_rel_array数组中，这里关注下三个RelOptInfo的reltarget：

(gdb) plist root->simple_rel_array[1]->reltarget->exprs
$67 = 2
$68 = {ptr_value = 0x3083218, int_value = 50868760, oid_value = 50868760, xid_value = 50868760}
$69 = {ptr_value = 0x30ab8b8, int_value = 51034296, oid_value = 51034296, xid_value = 51034296}
(gdb) p root->simple_rte_array[1]->relid
$70 = 16564

root→simple_rel_arrayi	simple_rel_arrayi→reltarget->exprs	relid
1	varno = 1, varattno = 2, vartype = 1043	16564 student.sname
1	varno = 1, varattno = 1, vartype = 23	16564 student.sno
2	varno = 2, varattno = 3, vartype = 23	16579 score.degree
2	varno = 2, varattno = 1, vartype = 23	16579 score.cno
2	varno = 2, varattno = 2, vartype = 23	16579 score.sno
4	varno = 4, varattno = 1, vartype = 23	16569 course.cno

SELECT STUDENT.sname, random(), SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno
ORDER BY COURSE.cno;

make_one_rel生成后：

	final_rel->reltarget->exprs	列
1	varno = 1, varattno = 2, vartype = 1043	投影第1列：STUDENT.sname
2	varno = 2, varattno = 3, vartype = 23	投影第3列：SCORE.degree
3	varno = 4, varattno = 1, vartype = 23	排序列：COURSE.cno

3.2 grouping_planner→make_sort_input_target规律v函数生成排序target

final_target = create_pathtarget(root, root->processed_tlist);拿到的final_target：

final_target	Var / FuncExpr	指向列	sortgrouprefs
final_target->exprs->elements0	varno = 1, varattno = 2, vartype = 1043	STUDENT.sname	0
final_target->exprs->elements1	funcid = 1598, funcresulttype = 701	random()	0
final_target->exprs->elements2	varno = 2, varattno = 3, vartype = 23	SCORE.degree	0
final_target->exprs->elements3	varno = 4, varattno = 1, vartype = 23	COURSE.cno	1

make_sort_input_target拿到的sort_input_target，过滤掉了random列：

sort_input_target	Var / FuncExpr	指向列	sortgrouprefs
sort_input_target->exprs->elements0	varno = 1, varattno = 2, vartype = 1043	STUDENT.sname	0
sort_input_target->exprs->elements1	varno = 2, varattno = 3, vartype = 23	SCORE.degree	0
sort_input_target->exprs->elements2	varno = 4, varattno = 1, vartype = 23	COURSE.cno	1

实例2中，apply_scanjoin_target_to_paths会先挂投影节点，后面的create_ordered_paths在创建顶层的排序节点，为什么这里的投影节点在最上层？因为有volatile函数在，需要先排序，在到投影节点上计算random函数

3.3 grouping_planner→apply_scanjoin_target_to_paths

		final_target = create_pathtarget(root, root->processed_tlist);
		...
		sort_input_target = make_sort_input_target(...);
		...
		grouping_target = sort_input_target;
		...
		scanjoin_target = grouping_target;
		...
		scanjoin_targets = list_make1(scanjoin_target);
		...
		scanjoin_target_same_exprs = list_length(scanjoin_targets) == 1
			&& equal(scanjoin_target->exprs, current_rel->reltarget->exprs);
		...
		// 1 确定没有SRF  list_length(scanjoin_targets) == 1
		// 2 这里make_one_rel出来的current_rel和上面make_sort_input_target计算出来的投影列一样，都过滤掉了v函数，剩下三列
		// scanjoin_target_same_exprs == true

		scanjoin_target_same_exprs = list_length(scanjoin_targets) == 1
			&& equal(scanjoin_target->exprs, current_rel->reltarget->exprs);
		apply_scanjoin_target_to_paths(root, current_rel, scanjoin_targets,
									   scanjoin_targets_contain_srfs,
									   scanjoin_target_parallel_safe,

注意：

scanjoin_target->exprs：表示最终结果需要的targetlist。
current_rel->reltarget->exprs：表示当前生成path中带的targetlist。
生成path的路径需要和scanjoin_target一致，所以进入下面函数判断是否生成投影节点。
如果相同，scanjoin_target_same_exprs==true，则不生成投影节点。
如果不同，scanjoin_target_same_exprs==false，则调用create_projection_path传入scanjoin_target，生成投影节点。

在apply_scanjoin_target_to_paths中:

apply_scanjoin_target_to_paths
	...
	...
	foreach(lc, rel->pathlist)
	{
		Path	   *subpath = (Path *) lfirst(lc);

		if (tlist_same_exprs)
			// scanjoin_target->sortgrouprefs = [0, 0, 1] 表示第三列是排序列
			// 因为现在的scanjoin_target（同sort_input_target）中只有三列，投影列1、3和排序列，参考上面sort_input_target表格。
			subpath->pathtarget->sortgrouprefs = scanjoin_target->sortgrouprefs;
		else
		{
			Path	   *newpath;
			newpath = (Path *) create_projection_path(root, rel, subpath,
													  scanjoin_target);
			lfirst(lc) = newpath;
		}
	}

3.4 grouping_planner→create_ordered_paths

继续成成排序node：

grouping_planner
	...
	if (parse->sortClause)
				current_rel = create_ordered_paths(root,
										   current_rel,
										   final_target,
										   final_target_parallel_safe,
										   have_postponed_srfs ? -1.0 :
										   limit_tuples);

create_ordered_paths最重要的入参就是final_target，保存了全部的列信息和排序列的位置sortgrouprefs。
注意前面生成path中的reltarget已经过滤了random列，但这里没有过滤，需要全量的信息。

final_target	Var / FuncExpr	指向列	sortgrouprefs
final_target->exprs->elements0	varno = 1, varattno = 2, vartype = 1043	STUDENT.sname	0
final_target->exprs->elements1	funcid = 1598, funcresulttype = 701	random()	0
final_target->exprs->elements2	varno = 2, varattno = 3, vartype = 23	SCORE.degree	0
final_target->exprs->elements3	varno = 4, varattno = 1, vartype = 23	COURSE.cno	1

注意：这里create_sort_path为hashjoin节点上面加了一层sort节点，sort节点的pathtarget继承了hash节点的pathtarget，也就是三列（没有random函数列）。
注意：这里的target是上面表格中的final_target，也就是四列（带random函数）。
加了sort节点后，发现这里不相同，所以开始增加投影列apply_projection_to_path。

create_ordered_paths
	ordered_rel = fetch_upper_rel(root, UPPERREL_ORDERED, NULL);
	
	foreach(lc, input_rel->pathlist)
		is_sorted = pathkeys_count_contained_in
		if (is_sorted)
			sorted_path = input_path;
		else
			sorted_path = (Path *) create_sort_path(...)

		// 生成sorted_path
		// {type = T_SortPath, pathtype = T_Sort, pathtarget = 三列 }
		
		if (sorted_path->pathtarget != target)
			sorted_path = apply_projection_to_path(root, ordered_rel, sorted_path, target);
		
		// 生成投影列
		// {type = T_ProjectionPath, pathtype = T_Result, pathtarget = 四列 }

最终生成PATH：

SELECT STUDENT.sname, random(), SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno
ORDER BY COURSE.cno;

最终效果：

4 实例：【简单join】【排序volatile函数】【投影列中有volatile函数】

drop table student;
create table student(sno int primary key, sname varchar(10), ssex int);
insert into student values(1, 'stu1', 0);
insert into student values(2, 'stu2', 1);
insert into student values(3, 'stu3', 1);
insert into student values(4, 'stu4', 0);

drop table course;
create table course(cno int primary key, cname varchar(10), tno int);
insert into course values(20, 'meth', 10);
insert into course values(21, 'english', 11);

drop table teacher;
create table teacher(tno int primary key, tname varchar(10), tsex int);
insert into teacher values(10, 'te1', 1);
insert into teacher values(11, 'te2', 0);

drop table score;
create table score (sno int, cno int, degree int);
create index idx_score_sno on score(sno);
insert into score values (1, 20, 100);
insert into score values (1, 21, 89);
insert into score values (2, 20, 99);
insert into score values (2, 21, 90);
insert into score values (3, 20, 87);
insert into score values (3, 21, 20);
insert into score values (4, 20, 60);
insert into score values (4, 21, 70);


explain verbose
SELECT STUDENT.sname, random(), SCORE.degree
FROM STUDENT
LEFT JOIN SCORE ON STUDENT.sno = SCORE.sno
LEFT JOIN COURSE ON SCORE.cno = COURSE.cno
ORDER BY random();

                                   QUERY PLAN
--------------------------------------------------------------------------------
 Sort  (cost=2.35..2.37 rows=8 width=17)
   Output: student.sname, (random()), score.degree
   Sort Key: (random())
   ->  Hash Right Join  (cost=1.09..2.23 rows=8 width=17)
         Output: student.sname, random(), score.degree
         Inner Unique: true
         Hash Cond: (score.sno = student.sno)
         ->  Seq Scan on public.score  (cost=0.00..1.08 rows=8 width=12)
               Output: score.sno, score.cno, score.degree
         ->  Hash  (cost=1.04..1.04 rows=4 width=9)
               Output: student.sname, student.sno
               ->  Seq Scan on public.student  (cost=0.00..1.04 rows=4 width=9)
                     Output: student.sname, student.sno

4.1 make_one_rel结果

第一步：拿到RelOptInfo

current_rel = query_planner(root, standard_qp_callback, &qp_extra);

current_rel->reltarget中忽略了random函数：

{ 
  type = T_PathTarget, 
  exprs = 
    {
    	Var{varno = 1, varattno = 2, vartype = 1043}, // STUDENT.sname
    	Var{varno = 2, varattno = 3, vartype = 23}    // SCORE.degree
    }, 
  sortgrouprefs = 0x0 }

4.2 拿到final_target

final_target = create_pathtarget(root, root->processed_tlist);

{
 	type = T_PathTarget, 
 	exprs = 
 	{
 		Var{varno = 1, varattno = 2, vartype = 1043},         // STUDENT.sname
 		FuncExpr {xpr = {type = T_FuncExpr}, funcid = 1598},  // random()
 		Var{varno = 2, varattno = 3, vartype = 23}            // SCORE.degree
 	}, 
 	sortgrouprefs = [0, 1, 0]
}

4.3 构造排序target：make_sort_input_target

sort_input_target = make_sort_input_target(root, final_target, &have_postponed_srfs);

{
	type = T_PathTarget,
	 exprs = 
	 {
 		Var{varno = 1, varattno = 2, vartype = 1043},         // STUDENT.sname
 		FuncExpr {xpr = {type = T_FuncExpr}, funcid = 1598},  // random()
 		Var{varno = 2, varattno = 3, vartype = 23}            // SCORE.degree
	 }, 
	 sortgrouprefs = [0, 1, 0]
}

4.4 apply_scanjoin_target_to_paths增加投影

apply_scanjoin_target_to_paths执行后，增加投影节点：

{ path = {type = T_ProjectionPath, pathtype = T_Result }

4.5 create_ordered_paths后增加排序节点在最顶层

{ path = {type = T_SortPath, pathtype = T_Sort }

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原始发表：2024-05-28，如有侵权请联系 cloudcommunity@tencent.com 删除

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