openGauss 内核分析（三)：SQL 解析
酷哥 [openGauss](javascript:void(0);) 2022-11-26 17:30 发表于广东
在传统数据库中 SQL 引擎一般指对用户输入的 SQL 语句进行解析、优化的软件模块。
SQL 的解析过程主要分为：
词法分析 Lexical Analysis：将用户输入的 SQL 语句拆解成单词(Token)序列，并识别出关键字、标识、常量等。
语法分析 Syntax Analysis：分析器对词法分析器解析出来的单词(Token)序列在语法上是否满足 SQL 语法规则。
语义分析 Semantic Analysis：语义分析是 SQL 解析过程的一个逻辑阶段，主要任务是在语法正确的基础上进行上下文有关性质的审查，在 SQL 解析过程中该阶段完成表名、操作符、类型等元素的合法性判断，同时检测语义上的二义性。
openGauss 在 pg_parse_query 中调用 raw_parser 函数对用户输入的 SQL 命令进行词法分析和语法分析，生成语法树添加到链表 parsetree_list 中。完成语法分析后，对于 parsetree_list 中的每一颗语法树 parsetree，会调用 parse_analyze 函数进行语义分析，根据 SQL 命令的不同，执行对应的入口函数，最终生成查询树。
词法分析 Lexical Analysis
openGauss 使用 flex 工具进行词法分析。flex 工具通过对已经定义好的词法文件进行编译，生成词法分析的代码。词法文件是 scan.l，它根据 SQL 语言标准对 SQL 语言中的关键字、标识符、操作符、常量、终结符进行了定义和识别。在 kwlist.h 中定义了大量的关键字，按照字母的顺序排列，方便在查找关键字时通过二分法进行查找。在 scan.l 中处理“标识符”时，会到关键字列表中进行匹配，如果一个标识符匹配到关键字，则认为是关键字，否则才是标识符，即关键字优先. 以“select a, b from item”为例说明词法分析结果。
语法分析 Syntax Analysis
openGauss 中定义了 bison 工具能够识别的语法文件 gram.y，根据 SQL 语言的不同定义了一系列表达 Statement 的结构体（这些结构体通常以 Stmt 作为命名后缀），用来保存语法分析结果。以 SELECT 查询为例，它对应的 Statement 结构体如下。
typedef struct SelectStmt{NodeTag type;List *distinctClause; /* NULL, list of DISTINCT ON exprs, oro * lcons(NIL,NIL) for all (SELECT DISTINCT) */IntoClause *intoClause; /* target for SELECT INTO */List *targetList; /* the target list (of ResTarget) */List *fromClause; /* the FROM clause */Node *whereClause; /* WHERE qualification */List *groupClause; /* GROUP BY clauses */Node *havingClause; /* HAVING conditional-expression */List *windowClause; /* WINDOW window_name AS (...), ... */WithClause *withClause; /* WITH clause */List *valuesLists; /* untransformed list of expression lists */List *sortClause; /* sort clause (a list of SortBy's) */Node *limitOffset; /* # of result tuples to skip */Node *limitCount; /* # of result tuples to return */ ……} SelectStmt;
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这个结构体可以看作一个多叉树，每个叶子节点都表达了 SELECT 查询语句中的一个语法结构，对应到 gram.y 中，它会有一个 SelectStmt。代码如下：
从 simple_select 语法分析结构可以看出，一条简单的查询语句由以下子句组成：去除行重复的 distinctClause、目标属性 targetList、SELECT INTO 子句 intoClause、FROM 子句 fromClause、WHERE 子句 whereClause、GROUP BY 子句 groupClause、HAVING 子句 havingClause、窗口子句 windowClause 和 plan_hint 子句。在成功匹配 simple_select 语法结构后，将会创建一个 Statement 结构体，将各个子句进行相应的赋值。对 simple_select 而言，目标属性、FROM 子句、WHERE 子句是最重要的组成部分。SelectStmt 与其他结构体的关系如下：
下面以“select a, b from item”为例说明简单 select 语句的解析过程，函数 exec_simple_query 调用 pg_parse_query 执行解析，解析树中只有一个元素。
(gdb) p *parsetree_list$47 = {type = T_List, length = 1, head = 0x7f5ff986c8f0, tail = 0x7f5ff986c8f0}
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List 中的节点类型为 T_SelectStmt
(gdb) p *(Node *)(parsetree_list->head.data->ptr_value)$45 = {type = T_SelectStmt}
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查看 SelectStmt 结构体，targetList 和 fromClause 非空
(gdb) set $stmt = (SelectStmt *)(parsetree_list->head.data->ptr_value)(gdb) p *$stmt$50 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0, targetList = 0x7f5ffa43d588, fromClause = 0x7f5ff986c888, startWithClause = 0x0, whereClause = 0x0, groupClause = 0x0, havingClause = 0x0, windowClause = 0x0, withClause = 0x0, valuesLists = 0x0, sortClause = 0x0, limitOffset = 0x0, limitCount = 0x0, lockingClause = 0x0, hintState = 0x0, op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0, hasPlus = false}
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查看 SelectStmt 的 targetlist，有两个 ResTarget
(gdb) p *($stmt->targetList)$55 = {type = T_List, length = 2, head = 0x7f5ffa43d540, tail = 0x7f5ffa43d800}(gdb) p *(Node *)($stmt->targetList->head.data->ptr_value)$57 = {type = T_ResTarget}
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(gdb) set $restarget1=(ResTarget *)($stmt->targetList->head.data->ptr_value)(gdb) p *$restarget1$60 = {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x7f5ffa43d378, location = 7}(gdb) p *$restarget1->val$63 = {type = T_ColumnRef}(gdb) p *(ColumnRef *)$restarget1->val$64 = {type = T_ColumnRef, fields = 0x7f5ffa43d470, prior = false, indnum = 0, location = 7}(gdb) p *((ColumnRef *)$restarget1->val)->fields$66 = {type = T_List, length = 1, head = 0x7f5ffa43d428, tail = 0x7f5ffa43d428}(gdb) p *(Node *)(((ColumnRef *)$restarget1->val)->fields)->head.data->ptr_value$67 = {type = T_String}(gdb) p *(Value *)(((ColumnRef *)$restarget1->val)->fields)->head.data->ptr_value$77 = {type = T_String, val = {ival = 140050197369648, str = 0x7f5ffa43d330 "a"}}
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(gdb) set $restarget2=(ResTarget *)($stmt->targetList->tail.data->ptr_value)(gdb) p *$restarget2$89 = {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x7f5ffa43d638, location = 10}(gdb) p *$restarget2->val$90 = {type = T_ColumnRef}(gdb) p *(ColumnRef *)$restarget2->val$91 = {type = T_ColumnRef, fields = 0x7f5ffa43d730, prior = false, indnum = 0, location = 10}(gdb) p *((ColumnRef *)$restarget2->val)->fields$92 = {type = T_List, length = 1, head = 0x7f5ffa43d6e8, tail = 0x7f5ffa43d6e8}(gdb) p *(Node *)(((ColumnRef *)$restarget2->val)->fields)->head.data->ptr_value$93 = {type = T_String}(gdb) p *(Value *)(((ColumnRef *)$restarget2->val)->fields)->head.data->ptr_value$94 = {type = T_String, val = {ival = 140050197370352, str = 0x7f5ffa43d5f0 "b"}
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查看 SelectStmt 的 fromClause，有一个 RangeVar
(gdb) p *$stmt->fromClause$102 = {type = T_List, length = 1, head = 0x7f5ffa43dfe0, tail = 0x7f5ffa43dfe0}(gdb) set $fromclause=(RangeVar*)($stmt->fromClause->head.data->ptr_value)(gdb) p *$fromclause$103 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x7f5ffa43d848 "item", partitionname = 0x0, subpartitionname = 0x0, inhOpt = INH_DEFAULT, relpersistence = 112 'p', alias = 0x0, location = 17, ispartition = false, issubpartition = false, partitionKeyValuesList = 0x0, isbucket = false, buckets = 0x0, length = 0, foreignOid = 0, withVerExpr = false}
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综合以上分析可以得到语法树结构
语义分析 Semantic Analysis
在完成词法分析和语法分析后，parse_analyze 函数会根据语法树的类型，调用 transformSelectStmt 将 parseTree 改写为查询树。
(gdb) p *result$3 = {type = T_Query, commandType = CMD_SELECT, querySource = QSRC_ORIGINAL, queryId = 0, canSetTag = false, utilityStmt = 0x0, resultRelation = 0, hasAggs = false, hasWindowFuncs = false, hasSubLinks = false, hasDistinctOn = false, hasRecursive = false, hasModifyingCTE = false, hasForUpdate = false, hasRowSecurity = false, hasSynonyms = false, cteList = 0x0, rtable = 0x7f5ff5eb8c88, jointree = 0x7f5ff5eb9310, targetList = 0x7f5ff5eb9110,…} (gdb) p *result->targetList$13 = {type = T_List, length = 2, head = 0x7f5ff5eb90c8, tail = 0x7f5ff5eb92c8} (gdb) p *(Node *)(result->targetList->head.data->ptr_value)$8 = {type = T_TargetEntry}(gdb) p *(TargetEntry*)(result->targetList->head.data->ptr_value)$9 = {xpr = {type = T_TargetEntry, selec = 0}, expr = 0x7f5ff636ff48, resno = 1, resname = 0x7f5ff5caf330 "a", ressortgroupref = 0, resorigtbl = 24576, resorigcol = 1, resjunk = false}(gdb) p *(TargetEntry*)(result->targetList->tail.data->ptr_value)$10 = {xpr = {type = T_TargetEntry, selec = 0}, expr = 0x7f5ff5eb9178, resno = 2, resname = 0x7f5ff5caf5f0 "b", ressortgroupref = 0, resorigtbl = 24576, resorigcol = 2, resjunk = false}(gdb)
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(gdb) p *result->rtable$14 = {type = T_List, length = 1, head = 0x7f5ff5eb8c40, tail = 0x7f5ff5eb8c40}(gdb) p *(Node *)(result->rtable->head.data->ptr_value)$15 = {type = T_RangeTblEntry}(gdb) p *(RangeTblEntry*)(result->rtable->head.data->ptr_value)$16 = {type = T_RangeTblEntry, rtekind = RTE_RELATION, relname = 0x7f5ff636efb0 "item", partAttrNum = 0x0, relid = 24576, partitionOid = 0, isContainPartition = false, subpartitionOid = 0……}
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得到的查询树结构如下：
完成词法、语法和语义分析后，SQL 解析过程完成，SQL 引擎开始执行查询优化。
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