PostgreSQL 源码解读（44）- 查询语句#29（等价类

上一小节介绍了函数query_planner中子函数deconstruct_jointree的主要实现逻辑,调用该函数后,PlannerInfo中生成等价类的相关信息,本节结合实际的内存数据介绍相关数据结构。

一、数据结构

 /*
  * EquivalenceClasses
  *
  * Whenever we can determine that a mergejoinable equality clause A = B is
  * not delayed by any outer join, we create an EquivalenceClass containing
  * the expressions A and B to record this knowledge.  If we later find another
  * equivalence B = C, we add C to the existing EquivalenceClass; this may
  * require merging two existing EquivalenceClasses.  At the end of the qual
  * distribution process, we have sets of values that are known all transitively
  * equal to each other, where "equal" is according to the rules of the btree
  * operator family(s) shown in ec_opfamilies, as well as the collation shown
  * by ec_collation.  (We restrict an EC to contain only equalities whose
  * operators belong to the same set of opfamilies.  This could probably be
  * relaxed, but for now it's not worth the trouble, since nearly all equality
  * operators belong to only one btree opclass anyway.  Similarly, we suppose
  * that all or none of the input datatypes are collatable, so that a single
  * collation value is sufficient.)
  *
  * We also use EquivalenceClasses as the base structure for PathKeys, letting
  * us represent knowledge about different sort orderings being equivalent.
  * Since every PathKey must reference an EquivalenceClass, we will end up
  * with single-member EquivalenceClasses whenever a sort key expression has
  * not been equivalenced to anything else.  It is also possible that such an
  * EquivalenceClass will contain a volatile expression ("ORDER BY random()"),
  * which is a case that can't arise otherwise since clauses containing
  * volatile functions are never considered mergejoinable.  We mark such
  * EquivalenceClasses specially to prevent them from being merged with
  * ordinary EquivalenceClasses.  Also, for volatile expressions we have
  * to be careful to match the EquivalenceClass to the correct targetlist
  * entry: consider SELECT random() AS a, random() AS b ... ORDER BY b,a.
  * So we record the SortGroupRef of the originating sort clause.
  *
  * We allow equality clauses appearing below the nullable side of an outer join
  * to form EquivalenceClasses, but these have a slightly different meaning:
  * the included values might be all NULL rather than all the same non-null
  * values.  See src/backend/optimizer/README for more on that point.
  *
  * NB: if ec_merged isn't NULL, this class has been merged into another, and
  * should be ignored in favor of using the pointed-to class.
  */
 typedef struct EquivalenceClass
 {
     NodeTag     type;
 
     List       *ec_opfamilies;  /* btree操作符族(pg_opfamily)Oids,btree operator family OIDs */
     Oid         ec_collation;   /* 主要用于排序的规则,collation, if datatypes are collatable */
     List       *ec_members;     /* 等价类成员链表,list of EquivalenceMembers */
     List       *ec_sources;     /* 产生等价类的RestrictInfo链表,list of generating RestrictInfos */
     List       *ec_derives;     /* 衍生的RestrictInfo链表,list of derived RestrictInfos */
     Relids      ec_relids;      /* 出现在成员中的所有relids,all relids appearing in ec_members, except
                                  * for child members (see below) */
     bool        ec_has_const;   /* 成员中是否存在常量?any pseudoconstants in ec_members? */
     bool        ec_has_volatile;    /* 成员中是否存在易变表达式(如Random等),the (sole) member is a volatile expr */
     bool        ec_below_outer_join;    /* 等价类是否应用于外连接下层?equivalence applies below an OJ */
     bool        ec_broken;      /* 产生所需要的子句是否失败?failed to generate needed clauses? */
     Index       ec_sortref;     /* 源于排序子句的标志,originating sortclause label, or 0 */
     Index       ec_min_security;    /* 最小安全等级,minimum security_level in ec_sources */
     Index       ec_max_security;    /* 最大安全等级,maximum security_level in ec_sources */
     struct EquivalenceClass *ec_merged; /* 合并后的等价类,set if merged into another EC */
 } EquivalenceClass;
 
 /*
  * If an EC contains a const and isn't below-outer-join, any PathKey depending
  * on it must be redundant, since there's only one possible value of the key.
  */
 #define EC_MUST_BE_REDUNDANT(eclass)  \
     ((eclass)->ec_has_const && !(eclass)->ec_below_outer_join)
 
 /*
  * EquivalenceMember - one member expression of an EquivalenceClass
  *
  * em_is_child signifies that this element was built by transposing a member
  * for an appendrel parent relation to represent the corresponding expression
  * for an appendrel child.  These members are used for determining the
  * pathkeys of scans on the child relation and for explicitly sorting the
  * child when necessary to build a MergeAppend path for the whole appendrel
  * tree.  An em_is_child member has no impact on the properties of the EC as a
  * whole; in particular the EC's ec_relids field does NOT include the child
  * relation.  An em_is_child member should never be marked em_is_const nor
  * cause ec_has_const or ec_has_volatile to be set, either.  Thus, em_is_child
  * members are not really full-fledged members of the EC, but just reflections
  * or doppelgangers of real members.  Most operations on EquivalenceClasses
  * should ignore em_is_child members, and those that don't should test
  * em_relids to make sure they only consider relevant members.
  *
  * em_datatype is usually the same as exprType(em_expr), but can be
  * different when dealing with a binary-compatible opfamily; in particular
  * anyarray_ops would never work without this.  Use em_datatype when
  * looking up a specific btree operator to work with this expression.
  */
 typedef struct EquivalenceMember
 {
     NodeTag     type;
 
     Expr       *em_expr;        /* 该成员所代表的表达式,the expression represented */
     Relids      em_relids;      /* 出现在表达式中的relids,all relids appearing in em_expr */
     Relids      em_nullable_relids; /* 低层外连接nullable端的relids,nullable by lower outer joins */
     bool        em_is_const;    /* 常量?expression is pseudoconstant? */
     bool        em_is_child;    /* 子Relation的衍生版本?derived version for a child relation? */
     Oid         em_datatype;    /* 操作族使用到的数据类型,the "nominal type" used by the opfamily */
 } EquivalenceMember;
 

二、跟踪分析

启动gdb,跟踪:

(gdb) b query_planner
Breakpoint 3 at 0x7693b5: file planmain.c, line 57.

执行函数deconstruct_jointree,查看root结构

156   joinlist = deconstruct_jointree(root);
(gdb) 
163   reconsider_outer_join_clauses(root);
(gdb) p *root
$4 = {type = T_PlannerInfo, parse = 0x2c53ad0, glob = 0x2c8bff8, query_level = 1, parent_root = 0x0, plan_params = 0x0, 
  outer_params = 0x0, simple_rel_array = 0x2c941f8, simple_rel_array_size = 6, simple_rte_array = 0x2c94248, 
  all_baserels = 0x0, nullable_baserels = 0x0, join_rel_list = 0x0, join_rel_hash = 0x0, join_rel_level = 0x0, 
  join_cur_level = 0, init_plans = 0x0, cte_plan_ids = 0x0, multiexpr_params = 0x0, eq_classes = 0x2c960b8, 
  canon_pathkeys = 0x0, left_join_clauses = 0x0, right_join_clauses = 0x0, full_join_clauses = 0x0, join_info_list = 0x0, 
  append_rel_list = 0x0, rowMarks = 0x0, placeholder_list = 0x0, fkey_list = 0x0, query_pathkeys = 0x0, 
  group_pathkeys = 0x0, window_pathkeys = 0x0, distinct_pathkeys = 0x0, sort_pathkeys = 0x0, part_schemes = 0x0, 
  initial_rels = 0x0, upper_rels = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, upper_targets = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 
    0x0}, processed_tlist = 0x2c8e3d0, grouping_map = 0x0, minmax_aggs = 0x0, planner_cxt = 0x2b9fde0, 
  total_table_pages = 0, tuple_fraction = 0, limit_tuples = -1, qual_security_level = 0, inhTargetKind = INHKIND_NONE, 
  hasJoinRTEs = true, hasLateralRTEs = true, hasDeletedRTEs = false, hasHavingQual = false, hasPseudoConstantQuals = false, 
  hasRecursion = false, wt_param_id = -1, non_recursive_path = 0x0, curOuterRels = 0x0, curOuterParams = 0x0, 
  join_search_private = 0x0, partColsUpdated = false}

root->eq_classes是等价类链表,其中的元素是等价类

(gdb) p *root->eq_classes
$1 = {type = T_List, length = 2, head = 0x2c6daf8, tail = 0x2c6ddf8}
(gdb) set $ec1=(EquivalenceClass *)root->eq_classes->head->data.ptr_value
(gdb) set $ec2=(EquivalenceClass *)root->eq_classes->head->next->data.ptr_value
(gdb) p *$ec1
$4 = {type = T_EquivalenceClass, ec_opfamilies = 0x2c6d980, ec_collation = 100, ec_members = 0x2c6da58, 
  ec_sources = 0x2c6d9f0, ec_derives = 0x0, ec_relids = 0x2c6da20, ec_has_const = false, ec_has_volatile = false, 
  ec_below_outer_join = false, ec_broken = false, ec_sortref = 0, ec_min_security = 0, ec_max_security = 0, ec_merged = 0x0}
(gdb) p *$ec2
$5 = {type = T_EquivalenceClass, ec_opfamilies = 0x2c6dc30, ec_collation = 100, ec_members = 0x2c6dd58, 
  ec_sources = 0x2c6dca0, ec_derives = 0x0, ec_relids = 0x2c6dd20, ec_has_const = true, ec_has_volatile = false, 
  ec_below_outer_join = false, ec_broken = false, ec_sortref = 0, ec_min_security = 0, ec_max_security = 0, ec_merged = 0x0}
(gdb) 

第1个等价类信息
ec_opfamilies

(gdb) p *$ec1->ec_opfamilies
$6 = {type = T_OidList, length = 2, head = 0x2c6d960, tail = 0x2c6d9b0}
(gdb) p $ec1->ec_opfamilies->head->data.oid_value
$7 = 1994
(gdb) p $ec1->ec_opfamilies->head->next->data.oid_value
$8 = 2095
(gdb) 

数据字典中相应的记录:

testdb=# select * from pg_opfamily where oid=2095;
 opfmethod |     opfname      | opfnamespace | opfowner 
-----------+------------------+--------------+----------
       403 | text_pattern_ops |           11 |       10
(1 row)

testdb=# select * from pg_opfamily where oid=1994;
 opfmethod | opfname  | opfnamespace | opfowner 
-----------+----------+--------------+----------
       403 | text_ops |           11 |       10
(1 row)

ec_members,共有2个元素
第1个元素,是rtindex=3的RTE,属性编号为2的字段,即t_grxx.grbh

(gdb) p *$ec1->ec_members
$10 = {type = T_List, length = 2, head = 0x2c6da38, tail = 0x2c6dad8}
(gdb) set $ec1_em1=(EquivalenceMember *)$ec1->ec_members->head->data.ptr_value
(gdb) set $ec1_em2=(EquivalenceMember *)$ec1->ec_members->head->next->data.ptr_value
(gdb) p *$ec1_em1
$13 = {type = T_EquivalenceMember, em_expr = 0x2c69f88, em_relids = 0x2c6d770, em_nullable_relids = 0x0, 
  em_is_const = false, em_is_child = false, em_datatype = 25}
(gdb) p *$ec1_em1
$13 = {type = T_EquivalenceMember, em_expr = 0x2c69f88, em_relids = 0x2c6d770, em_nullable_relids = 0x0, 
  em_is_const = false, em_is_child = false, em_datatype = 25}
(gdb) p *$ec1_em1->em_expr
$14 = {type = T_RelabelType}
(gdb) p *(RelabelType *)$ec1_em1->em_expr
$15 = {xpr = {type = T_RelabelType}, arg = 0x2c69f38, resulttype = 25, resulttypmod = -1, resultcollid = 100, 
  relabelformat = COERCE_IMPLICIT_CAST, location = -1}
(gdb) p *((RelabelType *)$ec1_em1->em_expr)->arg
$16 = {type = T_Var}
(gdb) p *(Var *)((RelabelType *)$ec1_em1->em_expr)->arg
$17 = {xpr = {type = T_Var}, varno = 3, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 3, varoattno = 2, location = 136} 

第2个元素,是rtindex=4的RTE,属性编号为1的字段,即t_jfxx.grbh

(gdb) p *$ec1_em2->em_expr
$28 = {type = T_RelabelType}
(gdb) p *(RelabelType *)$ec1_em2->em_expr
$29 = {xpr = {type = T_RelabelType}, arg = 0x2c69fd8, resulttype = 25, resulttypmod = -1, resultcollid = 100, 
  relabelformat = COERCE_IMPLICIT_CAST, location = -1}
(gdb) p *((RelabelType *)$ec1_em2->em_expr)->arg
$30 = {type = T_Var}
(gdb) p *(Var *)((RelabelType *)$ec1_em2->em_expr)->arg
$31 = {xpr = {type = T_Var}, varno = 4, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 4, varoattno = 1, location = 146}

其他信息

(gdb) p *$ec1->ec_sources
$34 = {type = T_List, length = 1, head = 0x2c6d9d0, tail = 0x2c6d9d0}
(gdb) p *(Node *)$ec1->ec_sources->head->data.ptr_value
$35 = {type = T_RestrictInfo}
(gdb) p *(RestrictInfo *)$ec1->ec_sources->head->data.ptr_value
$36 = {type = T_RestrictInfo, clause = 0x2c6a098, is_pushed_down = true, outerjoin_delayed = false, can_join = true, 
  pseudoconstant = false, leakproof = false, security_level = 0, clause_relids = 0x2c6d7a0, required_relids = 0x2c6d758, 
  outer_relids = 0x0, nullable_relids = 0x0, left_relids = 0x2c6d770, right_relids = 0x2c6d788, orclause = 0x0, 
  parent_ec = 0x0, eval_cost = {startup = -1, per_tuple = 0}, norm_selec = -1, outer_selec = -1, 
  mergeopfamilies = 0x2c6d980, left_ec = 0x2c6ce68, right_ec = 0x2c6ce68, left_em = 0x2c6d890, right_em = 0x2c6da88, 
  scansel_cache = 0x0, outer_is_left = false, hashjoinoperator = 0, left_bucketsize = -1, right_bucketsize = -1, 
  left_mcvfreq = -1, right_mcvfreq = -1}
(gdb) p *$ec1->ec_relids
$38 = {nwords = 1, words = 0x2c6da24}
#即3号和4号RTE
(gdb) p $ec1->ec_relids->words[0]
$39 = 24  

第2个等价类信息

(gdb) p *$ec2
$41 = {type = T_EquivalenceClass, ec_opfamilies = 0x2c6dc30, ec_collation = 100, ec_members = 0x2c6dd58, 
  ec_sources = 0x2c6dca0, ec_derives = 0x0, ec_relids = 0x2c6dd20, ec_has_const = true, ec_has_volatile = false, 
  ec_below_outer_join = false, ec_broken = false, ec_sortref = 0, ec_min_security = 0, ec_max_security = 0, ec_merged = 0x0}

ec_opfamilies,与第1个等价类的信息一致

(gdb) p *$ec2->ec_opfamilies
$42 = {type = T_OidList, length = 2, head = 0x2c6dc60, tail = 0x2c6dc10}
(gdb) p $ec2->ec_opfamilies->head->data.oid_value
$43 = 1994
(gdb) p $ec2->ec_opfamilies->head->next->data.oid_value
$44 = 2095

ec_members,有3个元素

(gdb) p *$ec2->ec_members
$46 = {type = T_List, length = 3, head = 0x2c6dd38, tail = 0x2c6df20}
(gdb) set $ec2_em1=(EquivalenceMember *)$ec2->ec_members->head->data.ptr_value
(gdb) set $ec2_em2=(EquivalenceMember *)$ec2->ec_members->head->next->data.ptr_value
(gdb) set $ec2_em3=(EquivalenceMember *)$ec2->ec_members->head->next->next->data.ptr_value

第1个元素,3号RTE,属性编号为1的字段,即t_grxx.dwbh

(gdb) p *$ec2_em1
$47 = {type = T_EquivalenceMember, em_expr = 0x2c69d58, em_relids = 0x2c6dbc8, em_nullable_relids = 0x0, 
  em_is_const = false, em_is_child = false, em_datatype = 25}
(gdb) p *$ec2_em1->em_expr
$48 = {type = T_RelabelType}
(gdb) p *(RelabelType *)$ec2_em1->em_expr
$49 = {xpr = {type = T_RelabelType}, arg = 0x2c69d08, resulttype = 25, resulttypmod = -1, resultcollid = 100, 
  relabelformat = COERCE_IMPLICIT_CAST, location = -1}
(gdb) p *(Var *)((RelabelType *)$ec2_em1->em_expr)->arg
$50 = {xpr = {type = T_Var}, varno = 3, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 3, varoattno = 1, location = 115}

第2个元素

(gdb) p *$ec2_em2,1号RTE,属性编号为2的字段,即t_dwxx.dwbh
$52 = {type = T_EquivalenceMember, em_expr = 0x2c69e28, em_relids = 0x2c6dbe0, em_nullable_relids = 0x0, 
  em_is_const = false, em_is_child = false, em_datatype = 25}
(gdb) p *$ec2_em2->em_expr
$53 = {type = T_RelabelType}
(gdb) p *(Var *)((RelabelType *)$ec2_em2->em_expr)->arg
$54 = {xpr = {type = T_Var}, varno = 1, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 1, varoattno = 2, location = 125}

第3个元素,是一个常量,即'1001'

(gdb) p *$ec2_em3
$55 = {type = T_EquivalenceMember, em_expr = 0x2c6a498, em_relids = 0x0, em_nullable_relids = 0x0, em_is_const = true, 
  em_is_child = false, em_datatype = 25}
(gdb) p *$ec2_em3->em_expr
$56 = {type = T_Const}
(gdb) p *((Const *)$ec2_em2->em_expr)->arg
(gdb) p *(Const *)$ec2_em3->em_expr
$58 = {xpr = {type = T_Const}, consttype = 25, consttypmod = -1, constcollid = 100, constlen = -1, constvalue = 46517720, 
  constisnull = false, constbyval = false, location = 172}

三、参考资料

relation.h