从这三个数据库的定义是不是觉得只有SQL-2003跟常用的Delete From Table删表跑路的语法差不多，其他两个数据库加入了一些自定义的语法功能，尤其是MySQL，长得像个远房亲戚。这里不对各个自定义语法功能做解释，想了解的读者可以点击对应的官方链接查阅。

Yacc实现

了解语法定义是远远不够的，我们还需要看看parser.y对Delete Statement的MySQL实现，从Delete Statement开始，分成了DeleteWithoutUsingStmt和DeleteWithUsingStmt两个分支，每个分支中又定义了对应的语法，可以看出每一个语法定义都基本符合MySQL的定义，整个过程就是把SQL解析成AST，下一步就是拿现有的parser做个demo，看看是否能正常解析我们的Delete SQL。\ DCParser/parser.y at main · DigitalChinaOpenSource/DCParser (github.com)

DELETE | PingCAP Docs\ /*******************************************************************\ *\ * Delete Statement\ *\ *******************************************************************/\ DeleteWithoutUsingStmt:\ “DELETE” TableOptimizerHints PriorityOpt QuickOptional IgnoreOptional “FROM” TableName PartitionNameListOpt TableAsNameOpt IndexHintListOpt WhereClauseOptional OrderByOptional LimitClause\ {\ // Single Table\ tn := $7.(*ast.TableName)\ tn.IndexHints = $10.([]*ast.IndexHint)\ tn.PartitionNames = $8.([]model.CIStr)\ join := \&ast.Join{Left: \&ast.TableSource{Source: tn, AsName: $9.(model.CIStr)}, Right: nil}\ x := \&ast.DeleteStmt{\ TableRefs: \&ast.TableRefsClause{TableRefs: join},\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ IgnoreErr: $5.(bool),\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $11 != nil {\ x.Where = $11.(ast.ExprNode)\ }\ if $12 != nil {\ x.Order = $12.(*ast.OrderByClause)\ }\ if $13 != nil {\ x.Limit = $13.(*ast.Limit)\ }\ $$ = x\ }\ \| “DELETE” TableOptimizerHints PriorityOpt QuickOptional IgnoreOptional TableAliasRefList “FROM” TableRefs WhereClauseOptional\ {\ // Multiple Table\ x := \&ast.DeleteStmt{\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ IgnoreErr: $5.(bool),\ IsMultiTable: true,\ BeforeFrom: true,\ Tables: \&ast.DeleteTableList{Tables: $6.([]*ast.TableName)},\ TableRefs: \&ast.TableRefsClause{TableRefs: $8.(*ast.Join)},\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $9 != nil {\ x.Where = $9.(ast.ExprNode)\ }\ $$ = x\ }\ DeleteWithUsingStmt:\ “DELETE” TableOptimizerHints PriorityOpt QuickOptional IgnoreOptional “FROM” TableAliasRefList “USING” TableRefs WhereClauseOptional\ {\ // Multiple Table\ x := \&ast.DeleteStmt{\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ IgnoreErr: $5.(bool),\ IsMultiTable: true,\ Tables: \&ast.DeleteTableList{Tables: $7.([]*ast.TableName)},\ TableRefs: \&ast.TableRefsClause{TableRefs: $9.(*ast.Join)},\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $10 != nil {\ x.Where = $10.(ast.ExprNode)\ }\ $$ = x\ }\ DeleteFromStmt:\ DeleteWithoutUsingStmt\ \| DeleteWithUsingStmt

验证Yacc\ 我们新建一个golang项目，如下。 这个demo解析了两个Delete SQL，第一个比较简单，第二个几乎把所有的参数都填上了，我们看看执行后的效果。

``` package main import ( "fmt" parser "github.com/DigitalChinaOpenSource/DCParser" "github.com/DigitalChinaOpenSource/DCParser/ast" _ "github.com/DigitalChinaOpenSource/DCParser/test_driver" )

func parse(sql string) (*ast.StmtNode, error) { p := parser.New() stmtNodes, _, err := p.Parse(sql, "", "") if err != nil { return nil, err } return &stmtNodes[0], nil }

func main() { astNode, err := parse("DELETE FROM somelog;") if err != nil { fmt.Printf("parse error: %v\"n", err.Error()) return } fmt.Printf("%v\"n", astNode) astNode, err = parse("DELETE /+ BKA(somelog) / LOW_PRIORITY QUICK IGNORE FROM somelog PARTITION (p1,p2) AS t FORCE INDEX FOR ORDER BY(timestamp_column) WHERE user = 'jcole' ORDER BY timestamp_column LIMIT 1;") if err != nil { fmt.Printf("parse error: %v\"n", err.Error()) return } fmt.Printf("%v\"n", astNode) } ```

项目设置断点后运行，对"DELETE FROM somelog;"的解析呈现AST结构，可以看到TableName正确获取到了，其他的参数where, order, limit，priority, ignore, quick等等都是空值。

对二个SQL解析时"DELETE /*+ BKA(somelog) */ LOW_PRIORITY QUICK IGNORE FROM somelog PARTITION (p1,p2) AS t FORCE INDEX FOR ORDER BY(timestamp_column) WHERE user = ‘jcole’ ORDER BY timestamp_column LIMIT 1;"，真个AST就变得庞大很多，包括where里的相等操作也能正确解析，这个parser工作正常，接下来我们开始删除不需要的语法参数。

修改Yacc\ 我们开始删除PostgreSQL中没有的参数，如以下代码所示，对应的代码解析部分也要做相应的调整，不然会编译报错，当然中间也确实遇到了一个很麻烦的问题，如果你也遇到了类似conflict的问题，可以看看文末的trouble shooting。\ /*******************************************************************\ *\ * Delete Statement\ *\ *******************************************************************/\ DeleteWithoutUsingStmt:\ “DELETE” TableOptimizerHints PriorityOpt QuickOptional “FROM” TableName TableAsNameOpt WhereClauseOptional\ {\ // Single Table\ tn := $6.(*ast.TableName)\ join := \&ast.Join{Left: \&ast.TableSource{Source: tn, AsName: $7.(model.CIStr)}, Right: nil}\ x := \&ast.DeleteStmt{\ TableRefs: \&ast.TableRefsClause{TableRefs: join},\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $8 != nil {\ x.Where = $8.(ast.ExprNode)\ }\ $$ = x\ }\ \| “DELETE” TableOptimizerHints PriorityOpt QuickOptional TableAliasRefList “FROM” TableRefs WhereClauseOptional\ {\ // Multiple Table\ x := \&ast.DeleteStmt{\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ IsMultiTable: true,\ BeforeFrom: true,\ Tables: \&ast.DeleteTableList{Tables: $5.([]*ast.TableName)},\ TableRefs: \&ast.TableRefsClause{TableRefs: $7.(*ast.Join)},\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $8 != nil {\ x.Where = $8.(ast.ExprNode)\ }\ $$ = x\ }\ DeleteWithUsingStmt:\ “DELETE” TableOptimizerHints PriorityOpt QuickOptional “FROM” TableAliasRefList “USING” TableRefs WhereClauseOptional\ {\ // Multiple Table\ x := \&ast.DeleteStmt{\ Priority: $3.(mysql.PriorityEnum),\ Quick: $4.(bool),\ IsMultiTable: true,\ Tables: \&ast.DeleteTableList{Tables: $6.([]*ast.TableName)},\ TableRefs: \&ast.TableRefsClause{TableRefs: $8.(*ast.Join)},\ }\ if $2 != nil {\ x.TableHints = $2.([]*ast.TableOptimizerHint)\ }\ if $9 != nil {\ x.Where = $9.(ast.ExprNode)\ }\ $$ = x\ }\ DeleteFromStmt:\ DeleteWithoutUsingStmt\ \| DeleteWithUsingStmt

利用项目自带的goyacc进行编译，记住，一定是用项目再带的goyacc编译，因为tidb对yyLexer接口一些改动，如果用go自带的goyacc编译会报没有实现方法的错误。好了，执行完之后就会生成一个新的parser.go文件。\

最后还需要把yacc修改导致的UT错误给修复了:)

本文删除MySQL自定义语法的目标已经接近完成，剩下的几个语法功能就留给读者自己尝试删除，关于如何加入PostgreSQL语法功能，我们会在下次分解。

Conflicts问题

当你在编译y文件时出现conflicts问题的时候，需要添加-v来查阅解析步骤。\

可以通过正确和错误文件对比提高排查效率，当然这次问题属于第一次修改没经验造成的，我还特地翻阅了Bison官方文档Shift/Reduce (Bison 3.7.6) (gnu.org)。最后根据Yacc的工作原理和output文件，把问题修复的，原因其实也非常简单，就是只改了一个delete分支里的语法，导致编译之后Delete AST解析路劲不一致导致的冲突。

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