上一小节已介绍了grouping_planner函数,该函数通过调用query_planner函数为扫描/连接部分生成最优的未排序和预排序(presorted)路径,本节主要介绍query_planner函数对简单SQL如SELECT 2+2;的处理逻辑。
一、重要的数据结构
RelOptInfo
查询语句经过查询重写/表达式简化/外连接消除等处理后,查询树Query已完成规范化,查询树中的RangeTblEntry(RTE)数据结构已完成其历史使命,由此可进入逻辑优化处理,在此阶段使用RelOptInfo数据结构.
typedef enum RelOptKind
{
RELOPT_BASEREL,//基本关系(如基表/子查询等)
RELOPT_JOINREL,//连接产生的关系,要注意的是通过连接等方式产生的结果亦可以视为关系
RELOPT_OTHER_MEMBER_REL,
RELOPT_OTHER_JOINREL,
RELOPT_UPPER_REL,//上层的关系
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;//节点标识
RelOptKind reloptkind;//RelOpt类型
/* 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; /*结果元组的估算数量 estimated number of result tuples */
/* per-relation planner control flags */
bool consider_startup; /*是否考虑启动成本?是,需要保留启动成本低的路径 keep cheap-startup-cost paths? */
bool consider_param_startup; /*是否考虑参数化?的路径 ditto, for parameterized paths? */
bool consider_parallel; /*是否考虑并行处理路径 consider parallel paths? */
/* default result targetlist for Paths scanning this relation */
struct PathTarget *reltarget; /*扫描该Relation时默认的结果 list of Vars/Exprs, cost, width */
/* materialization information */
List *pathlist; /*访问路径链表 Path structures */
List *ppilist; /*路径链表中使用参数化路径进行 ParamPathInfos used in pathlist */
List *partial_pathlist; /* partial Paths */
struct Path *cheapest_startup_path;//代价最低的启动路径
struct Path *cheapest_total_path;//代价最低的整体路径
struct Path *cheapest_unique_path;//代价最低的获取唯一值的路径
List *cheapest_parameterized_paths;//代价最低的参数化?路径链表
/* parameterization information needed for both base rels and join rels */
/* (see also lateral_vars and lateral_referencers) */
Relids direct_lateral_relids; /*使用lateral语法,需依赖的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时使用的数据结构
Index relid; /* Relation ID */
Oid reltablespace; /* 表空间 containing tablespace */
RTEKind rtekind; /* 基表?子查询?还是函数等等?RELATION, SUBQUERY, FUNCTION, etc */
AttrNumber min_attr; /* 最小的属性编号 smallest attrno of rel (often <0) */
AttrNumber max_attr; /* 最大的属性编号 largest attrno of rel */
Relids *attr_needed; /* 数组 array indexed [min_attr .. max_attr] */
int32 *attr_widths; /* 属性宽度 array indexed [min_attr .. max_attr] */
List *lateral_vars; /* 关系依赖的Vars/PHVs LATERAL Vars and PHVs referenced by rel */
Relids lateral_referencers; /*依赖该关系的Relids rels that reference me laterally */
List *indexlist; /* 该关系的IndexOptInfo链表 list of IndexOptInfo */
List *statlist; /* 统计信息链表 list of StatisticExtInfo */
BlockNumber pages; /* 块数 size estimates derived from pg_class */
double tuples; /* 元组数 */
double allvisfrac; /* ? */
PlannerInfo *subroot; /* 如为子查询,存储子查询的root if subquery */
List *subplan_params; /* 如为子查询,存储子查询的参数 if subquery */
int rel_parallel_workers; /* 并行执行,需要多少个workers? wanted number of parallel workers */
/* Information about foreign tables and foreign joins */
//FWD相关信息
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,可保证唯一的Relids链表
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链表 known not unique for these set(s) */
/* used by various scans and joins: */
List *baserestrictinfo; /* 如为基本关系,存储约束条件 RestrictInfo structures (if base rel) */
QualCost baserestrictcost; /* 解析约束表达式的成本? cost of evaluating the above */
Index baserestrict_min_security; /* 最低安全等级 min security_level found in
* baserestrictinfo */
List *joininfo; /* 连接语句的约束条件信息 RestrictInfo structures for join clauses
* involving this rel */
bool has_eclass_joins; /* 是否存在等价类连接? T means joininfo is incomplete */
/* used by partitionwise joins: */
bool consider_partitionwise_join; /* 分区? consider partitionwise
* join paths? (if
* partitioned rel) */
Relids top_parent_relids; /* Relids of topmost parents (if "other"
* rel) */
/* used for partitioned relations */
//分区表使用
PartitionScheme part_scheme; /* 分区的schema Partitioning scheme. */
int nparts; /* 分区数 number of partitions */
struct PartitionBoundInfoData *boundinfo; /* 分区边界信息 Partition bounds */
List *partition_qual; /* 分区约束 partition constraint */
struct RelOptInfo **part_rels; /* 分区的RelOptInfo数组 Array of RelOptInfos of partitions,
* stored in the same order of bounds */
List **partexprs; /* 非空分区键表达式 Non-nullable partition key expressions. */
List **nullable_partexprs; /* 可为空的分区键表达式 Nullable partition key expressions. */
List *partitioned_child_rels; /* RT Indexes链表 List of RT indexes. */
} RelOptInfo;
PathCostComparison
typedef enum
{
COSTS_EQUAL, /* 近似相等 path costs are fuzzily equal */
COSTS_BETTER1, /* 第一个路径成本较低 first path is cheaper than second */
COSTS_BETTER2, /* 第二个相对较低 second path is cheaper than first */
COSTS_DIFFERENT /* 不管是哪个路径,成本上都不占优势 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 //表示无基础表的结果计划节点
{
Path path;//扫描路径
List *quals;//where语句表达式,bare clauses, not RestrictInfos
} ResultPath;
二、源码解读
/*
* query_planner
* Generate a path (that is, a simplified plan) for a basic query,
* which may involve joins but not any fancier features.
*
* 为一个基本的查询(可能涉及连接)生成访问路径(也可以视为一个简化的计划).
*
* 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不会处理顶层的处理过程(如最后的分组/排序等操作),因此,不能选择最优的访问路径
* 该函数会返回RelOptInfo给最高层的连接,grouping_planner可以在剩下的路径中进行选择
*
* 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是计划信息/tlist是投影列
* qp_callback是计算query_pathkeys的函数/qp_extra是传递给qp_callback的函数
*
* 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;//查询树
List *joinlist;
RelOptInfo *final_rel;//结果
Index rti;//RTE的index
double total_pages;//总pages数
/*
* 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)//简单SQL,无FROM/WHERE语句
{
/* We need a dummy joinrel to describe the empty set of baserels */
final_rel = build_empty_join_rel(root);//创建返回结果
/*
* 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)//并行模式?
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));//添加访问路径
/* Select cheapest path (pretty easy in this case...) */
set_cheapest(final_rel);//选择最优的访问路径
/*
* 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);//回调函数
return final_rel;//返回
}
//其他代码
...
}
add_path/set_cheapest
后续再行介绍
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);//结果
pathnode->path.pathtype = T_Result;//扫描路径类型
pathnode->path.parent = rel;//路径的partentbuild_empty_join_rel
pathnode->path.pathtarget = target;//目标列
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;//并行workers数目,设置为0
pathnode->path.pathkeys = NIL;//
pathnode->quals = resconstantqual;//表达式
/* Hardly worth defining a cost_result() function ... just do it */
pathnode->path.rows = 1;//行数为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、跟踪分析
(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)
#创建的空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 */
#创建的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!
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