openGauss核心:SQL解析過程分析

華為雲開發者聯盟 發表於 2022-06-28
SQL
摘要:在傳統資料庫中SQL引擎一般指對使用者輸入的SQL語句進行解析、優化的軟體模組。SQL的解析過程主要分為:詞法、語法和語義分析。

本文分享自華為雲社群《 openGauss核心分析(三):SQL解析》,作者:Gauss松鼠會。

在傳統資料庫中SQL引擎一般指對使用者輸入的SQL語句進行解析、優化的軟體模組。

SQL的解析過程主要分為:

• 詞法分析:將使用者輸入的SQL語句拆解成單詞(Token)序列,並識別出關鍵字、標識、常量等。

• 語法分析:分析器對詞法分析器解析出來的單詞(Token)序列在語法上是否滿足SQL語法規則。

• 語義分析:語義分析是SQL解析過程的一個邏輯階段,主要任務是在語法正確的基礎上進行上下文有關性質的審查,在SQL解析過程中該階段完成表名、操作符、型別等元素的合法性判斷,同時檢測語義上的二義性。

openGauss在pg_parse_query中呼叫raw_parser函式對使用者輸入的SQL命令進行詞法分析和語法分析,生成語法樹新增到連結串列parsetree_list中。完成語法分析後,對於parsetree_list中的每一顆語法樹parsetree,會呼叫parse_**yze函式進行語義分析,根據SQL命令的不同,執行對應的入口函式,最終生成查詢樹。

openGauss核心:SQL解析過程分析

詞法分析

openGauss使用flex工具進行詞法分析。flex工具通過對已經定義好的詞法檔案進行編譯,生成詞法分析的程式碼。詞法檔案是scan.l,它根據SQL語言標準對SQL語言中的關鍵字、識別符號、操作符、常量、終結符進行了定義和識別。在kwlist.h中定義了大量的關鍵字,按照字母的順序排列,方便在查詢關鍵字時通過二分法進行查詢。 在scan.l中處理“識別符號”時,會到關鍵字列表中進行匹配,如果一個識別符號匹配到關鍵字,則認為是關鍵字,否則才是識別符號,即關鍵字優先. 以“select a, b from item”為例說明詞法分析結果。

openGauss核心:SQL解析過程分析

語法分析

openGauss中定義了bison工具能夠識別的語法檔案gram.y,根據SQL語言的不同定義了一系列表達Statement的結構體(這些結構體通常以Stmt作為命名字尾),用來儲存語法分析結果。以SELECT查詢為例,它對應的Statement結構體如下。

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;

這個結構體可以看作一個多叉樹,每個葉子節點都表達了SELECT查詢語句中的一個語法結構,對應到gram.y中,它會有一個SelectStmt。程式碼如下:

openGauss核心:SQL解析過程分析

從simple_select語法分析結構可以看出,一條簡單的查詢語句由以下子句組成:去除行重複的distinctClause、目標屬性targetList、SELECT INTO子句intoClause、FROM子句fromClause、WHERE子句whereClause、GROUP BY子句groupClause、HAVING子句havingClause、視窗子句windowClause和plan_hint子句。在成功匹配simple_select語法結構後,將會建立一個Statement結構體,將各個子句進行相應的賦值。對simple_select而言,目標屬性、FROM子句、WHERE子句是最重要的組成部分。SelectStmt與其他結構體的關係如下:

openGauss核心:SQL解析過程分析

下面以“select a, b from item”為例說明簡單select語句的解析過程,函式exec_simple_query呼叫pg_parse_query執行解析,解析樹中只有一個元素。

openGauss核心:SQL解析過程分析
(gdb) p *parsetree_list
$47 = {type = T_List, length = 1, head = 0x7f5ff986c8f0, tail = 0x7f5ff986c8f0}

List中的節點型別為T_SelectStmt。

(gdb) p *(Node *)(parsetree_list->head.data->ptr_value)
$45 = {type = T_SelectStmt}

檢視SelectStmt結構體,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}

檢視SelectStmt的targetlist,有兩個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"}}

檢視SelectStmt的fromClause,有一個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}

綜合以上分析可以得到語法樹結構。

openGauss核心:SQL解析過程分析

語義分析

在完成詞法分析和語法分析後,parse_Ana lyze函式會根據語法樹的型別,呼叫transformSelectStmt將parseTree改寫為查詢樹。

openGauss核心:SQL解析過程分析
(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……}

得到的查詢樹結構如下:

openGauss核心:SQL解析過程分析

完成詞法、語法和語義分析後,SQL解析過程完成,SQL引擎開始執行查詢優化,在下一期中再具體分析。

 

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