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
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。
名称词性内容说明
关键字keywordSELECT,FROM如SELECT/FROM/WHERE等，对大小写不敏感
标识符IDENTa,b,item用户自己定义的名字、常量名、变量名和过程名，若无括号修饰则对大小写不敏感
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, or
* 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;
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}
List中的节点类型为T_SelectStmt
(gdb) p *(Node *)(parsetree_list->head.data->ptr_value)
$45 = {type = T_SelectStmt}
查看SelectStmt结构体，targetList 和fromClause非空
(gdb)set$stmt = (SelectStmt *)(parsetree_list->head.data->ptr_value)(gdb) p *$stmt$50= {type= T_SelectStmt, distinctClause =0x0, intoClause =0x0, targetList =0x7f5ffa43d588, fromClause =0x7f5ff986c888, startWithClause =0x0, whereClause =0x0, groupClause =0x0, havingClause =0x0, windowClause =0x0, withClause =0x0, valuesLists =0x0, sortClause =0x0, limitOffset =0x0, limitCount =0x0, lockingClause =0x0, hintState =0x0, op = SETOP_NONE,all=false, larg =0x0, rarg =0x0, hasPlus =false}
查看SelectStmt的targetlist，有两个ResTarget
(gdb) p *($stmt->targetList)
$55 = {type = T_List, length = 2, head = 0x7f5ffa43d540, tail = 0x7f5ffa43d800}
(gdb) p *(Node *)($stmt->targetList->head.data->ptr_value)
$57 = {type = T_ResTarget}
(gdb)set$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$77= {type= T_String, val = {ival =140050197369648,str=0x7f5ffa43d330"a"}}
(gdb)set$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$94= {type= T_String, val = {ival =140050197370352,str=0x7f5ffa43d5f0"b"}}
查看SelectStmt的fromClause，有一个RangeVar
(gdb) p *$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}
综合以上分析可以得到语法树结构
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)
(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……}
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。
