PostgreSQL 源码解读（176）- 查询#94(语法分析：gram.y)#3

本节继续介绍PostgreSQL的语法分析定义文件gram.y的第三部分Productions(产生式).
Bison输入文件的组成:

%{
Declarations
%}
Definitions
%%
Productions
%%
User subroutines

一、Productions

Productions即产生式,这是用户编写的语法产生式,产生式的书写格式如下:

S -> X \n
X -> X + X | X - X | T_NUMBER

S -> X \n成为产生式,第一条产生式的最左边的符号成为起始符号,在这里是符号S.
为了避免出现递归解析,Bison因此会在最前面多添加一条产生式S’ -> S,S’为起始符号.
在Bison中,符号”:”表示一条”->”,同一个非终结符的不同产生式用”|”隔开,用”;”结束.每条产生式的后面花括号内是一段C代码,这些代码在该产生式被应用时执行,成为Action(动作),产生式的右边是ε(空集合)时,用注释/* empty */代替.
产生式中的非终结符不需要预先定义,Bison会自动根据所有产生式的左边符号来确定哪些符号是非终结符;终结符中,单字符token(token type值和字符的ASCII码相同)也不需要预先定义,在产生式内部直接用单引号括起来即可,其他类型的token则需要预先在 Definitions段中定义好,如%token ABORT_P ABSOLUTE_P ACCESS ACTION ADD_P ADMIN AFTER等,Bison会自动为这种token分配一个编号，再写到gram.h　文件中去,打开该文件,可以看到如下代码:

[root@localhost src]# vim ./include/parser/gram.h
...
/* Token type.  */
 44 #ifndef YYTOKENTYPE
 45 # define YYTOKENTYPE
 46   enum yytokentype
 47   {
 48     IDENT = 258,
 49     FCONST = 259,
 50     SCONST = 260,
 51     BCONST = 261,
 52     XCONST = 262,
 53     Op = 263,
 54     ICONST = 264,
 55     PARAM = 265,
 ....

编号从258开始,根据gram.y中的顺序逐个定义.

...
%token <str>    IDENT FCONST SCONST BCONST XCONST Op
%token <ival>    ICONST PARAM
%token            TYPECAST DOT_DOT COLON_EQUALS EQUALS_GREATER
%token            LESS_EQUALS GREATER_EQUALS NOT_EQUALS
%token <keyword> ABORT_P ABSOLUTE_P ACCESS ACTION ADD_P ADMIN AFTER
    AGGREGATE ALL ALSO ALTER ALWAYS ANALYSE ANALYZE AND ANY ARRAY AS ASC
    ASSERTION ASSIGNMENT ASYMMETRIC AT ATTACH ATTRIBUTE AUTHORIZATION
...

这些token定义在scan.l中可直接使用.

#include "parser/gramparse.h" --> #include "parser/gram.h"

Bison会根据产生式以及符号优先级转化为LALR(1)动作表输出到gram.c文件中去.在gram.c文件中,PG根据自定义语法文件生成一个函数int base_yyparse (core_yyscan_t yyscanner);该函数按LR(1)解析流程对词法分析得到的token流进行解析,每当它需要读入下一个符号时,它就执行一次s = yylex() ,每当它要执行一个折叠(reduce)动作时,这个reduce所应用的产生式后面C代码将被执行,执行完后才将相应的状态出栈。
下面是gram.c中yyparse的部分代码:

/*----------.
| yyparse.  |
`----------*/
int
yyparse (core_yyscan_t yyscanner)
{
/* The lookahead symbol.  */
int yychar;
/* The semantic value of the lookahead symbol.  */
/* Default value used for initialization, for pacifying older GCCs
   or non-GCC compilers.  */
YY_INITIAL_VALUE (static YYSTYPE yyval_default;)
YYSTYPE yylval YY_INITIAL_VALUE (= yyval_default);
/* Location data for the lookahead symbol.  */
static YYLTYPE yyloc_default
# if defined YYLTYPE_IS_TRIVIAL && YYLTYPE_IS_TRIVIAL
  = { 1, 1, 1, 1 }
# endif
;
YYLTYPE yylloc = yyloc_default;
    /* Number of syntax errors so far.  */
    int yynerrs;
    int yystate;
    /* Number of tokens to shift before error messages enabled.  */
    int yyerrstatus;
    /* The stacks and their tools:
       'yyss': related to states.
       'yyvs': related to semantic values.
       'yyls': related to locations.
       Refer to the stacks through separate pointers, to allow yyoverflow
       to reallocate them elsewhere.  */
    /* The state stack.  */
    yytype_int16 yyssa[YYINITDEPTH];
    yytype_int16 *yyss;
    yytype_int16 *yyssp;
    /* The semantic value stack.  */
    YYSTYPE yyvsa[YYINITDEPTH];
    YYSTYPE *yyvs;
    YYSTYPE *yyvsp;
    /* The location stack.  */
    YYLTYPE yylsa[YYINITDEPTH];
    YYLTYPE *yyls;
    YYLTYPE *yylsp;
    /* The locations where the error started and ended.  */
    YYLTYPE yyerror_range[3];
    YYSIZE_T yystacksize;
  int yyn;
  int yyresult;
  /* Lookahead token as an internal (translated) token number.  */
  int yytoken = 0;
  /* The variables used to return semantic value and location from the
     action routines.  */
  YYSTYPE yyval;
  YYLTYPE yyloc;
#if YYERROR_VERBOSE
  /* Buffer for error messages, and its allocated size.  */
  char yymsgbuf[128];
  char *yymsg = yymsgbuf;
  YYSIZE_T yymsg_alloc = sizeof yymsgbuf;
#endif
#define YYPOPSTACK(N)   (yyvsp -= (N), yyssp -= (N), yylsp -= (N))
  /* The number of symbols on the RHS of the reduced rule.
     Keep to zero when no symbol should be popped.  */
  int yylen = 0;
  yyssp = yyss = yyssa;
  yyvsp = yyvs = yyvsa;
  yylsp = yyls = yylsa;
  yystacksize = YYINITDEPTH;
...

二、源码

下面是gram.y产生式定义的部分源码

/*
 *    The target production for the whole parse.
 */
stmtblock:    stmtmulti
            {
                pg_yyget_extra(yyscanner)->parsetree = $1;
            }
        ;
/*
 * At top level, we wrap each stmt with a RawStmt node carrying start location
 * and length of the stmt's text.  Notice that the start loc/len are driven
 * entirely from semicolon locations (@2).  It would seem natural to use
 * @1 or @3 to get the true start location of a stmt, but that doesn't work
 * for statements that can start with empty nonterminals (opt_with_clause is
 * the main offender here); as noted in the comments for YYLLOC_DEFAULT,
 * we'd get -1 for the location in such cases.
 * We also take care to discard empty statements entirely.
 */
stmtmulti:    stmtmulti ';' stmt
                {
                    if ($1 != NIL)
                    {
                        /* update length of previous stmt */
                        updateRawStmtEnd(llast_node(RawStmt, $1), @2);
                    }
                    if ($3 != NULL)
                        $$ = lappend($1, makeRawStmt($3, @2 + 1));
                    else
                        $$ = $1;
                }
            | stmt
                {
                    if ($1 != NULL)
                        $$ = list_make1(makeRawStmt($1, 0));
                    else
                        $$ = NIL;
                }
        ;
stmt :
            AlterEventTrigStmt
            | AlterCollationStmt
            | AlterDatabaseStmt
            | AlterDatabaseSetStmt
            | AlterDefaultPrivilegesStmt
            | AlterDomainStmt
            | AlterEnumStmt
            | AlterExtensionStmt
            | AlterExtensionContentsStmt
            | AlterFdwStmt
            | AlterForeignServerStmt
            | AlterForeignTableStmt
            | AlterFunctionStmt
            | AlterGroupStmt
            | AlterObjectDependsStmt
            | AlterObjectSchemaStmt
            | AlterOwnerStmt
            | AlterOperatorStmt
            | AlterPolicyStmt
            | AlterSeqStmt
            | AlterSystemStmt
            | AlterTableStmt
            | AlterTblSpcStmt
            | AlterCompositeTypeStmt
            | AlterPublicationStmt
            | AlterRoleSetStmt
            | AlterRoleStmt
            | AlterSubscriptionStmt
            | AlterTSConfigurationStmt
            | AlterTSDictionaryStmt
            | AlterUserMappingStmt
            | AnalyzeStmt
            | CallStmt
            | CheckPointStmt
            | ClosePortalStmt
            | ClusterStmt
            | CommentStmt
            | ConstraintsSetStmt
            | CopyStmt
            | CreateAmStmt
            | CreateAsStmt
            | CreateAssertStmt
            | CreateCastStmt
            | CreateConversionStmt
            | CreateDomainStmt
            | CreateExtensionStmt
            | CreateFdwStmt
            | CreateForeignServerStmt
            | CreateForeignTableStmt
            | CreateFunctionStmt
            | CreateGroupStmt
            | CreateMatViewStmt
            | CreateOpClassStmt
            | CreateOpFamilyStmt
            | CreatePublicationStmt
            | AlterOpFamilyStmt
            | CreatePolicyStmt
            | CreatePLangStmt
            | CreateSchemaStmt
            | CreateSeqStmt
            | CreateStmt
            | CreateSubscriptionStmt
            | CreateStatsStmt
            | CreateTableSpaceStmt
            | CreateTransformStmt
            | CreateTrigStmt
            | CreateEventTrigStmt
            | CreateRoleStmt
            | CreateUserStmt
            | CreateUserMappingStmt
            | CreatedbStmt
            | DeallocateStmt
            | DeclareCursorStmt
            | DefineStmt
            | DeleteStmt
            | DiscardStmt
            | DoStmt
            | DropAssertStmt
            | DropCastStmt
            | DropOpClassStmt
            | DropOpFamilyStmt
            | DropOwnedStmt
            | DropPLangStmt
            | DropStmt
            | DropSubscriptionStmt
            | DropTableSpaceStmt
            | DropTransformStmt
            | DropRoleStmt
            | DropUserMappingStmt
            | DropdbStmt
            | ExecuteStmt
            | ExplainStmt
            | FetchStmt
            | GrantStmt
            | GrantRoleStmt
            | ImportForeignSchemaStmt
            | IndexStmt
            | InsertStmt
            | ListenStmt
            | RefreshMatViewStmt
            | LoadStmt
            | LockStmt
            | NotifyStmt
            | PrepareStmt
            | ReassignOwnedStmt
            | ReindexStmt
            | RemoveAggrStmt
            | RemoveFuncStmt
            | RemoveOperStmt
            | RenameStmt
            | RevokeStmt
            | RevokeRoleStmt
            | RuleStmt
            | SecLabelStmt
            | SelectStmt
            | TransactionStmt
            | TruncateStmt
            | UnlistenStmt
            | UpdateStmt
            | VacuumStmt
            | VariableResetStmt
            | VariableSetStmt
            | VariableShowStmt
            | ViewStmt
            | /*EMPTY*/
                { $$ = NULL; }
        ;
/*****************************************************************************
 *
 * CALL statement
 *
 *****************************************************************************/
CallStmt:    CALL func_application
                {
                    CallStmt *n = makeNode(CallStmt);
                    n->funccall = castNode(FuncCall, $2);
                    $$ = (Node *)n;
                }
        ;
...

简单解析如下:
1.stmtblock
stmtblock: stmtmulti
stmtblock为起始符号,最终应折叠(reduce)为该符号,否则会有语法错误.
执行的逻辑是:pg_yyget_extra(yyscanner)->parsetree = $1;
亦即完成语法解析,生成语法解析树parsetree.

2.stmtmulti
tmtmulti: stmtmulti ‘;’ stmt
左递归产生式,PG可接受多个以分号”;”分隔的语句,每个语句的定义为stmt

3.stmt

stmt :
            AlterEventTrigStmt
            | AlterCollationStmt
            ...
            | SelectStmt
            ...

stmt包括N多种语句,我们看最常见的SelectStmt语句

4.SelectStmt

SelectStmt: select_no_parens            %prec UMINUS
            | select_with_parens        %prec UMINUS
        ;
...
select_no_parens:
            simple_select                        { $$ = $1; }
            | select_clause sort_clause
                {
                    insertSelectOptions((SelectStmt \*) $1, $2, NIL,
                                        NULL, NULL, NULL,
                                        yyscanner);
                    $$ = $1;
                }
...
simple_select:
            SELECT opt_all_clause opt_target_list
            into_clause from_clause where_clause
            group_clause having_clause window_clause
                {
                    SelectStmt \*n = makeNode(SelectStmt);
                    n->targetList = $3;
                    n->intoClause = $4;
                    n->fromClause = $5;
                    n->whereClause = $6;
                    n->groupClause = $7;
                    n->havingClause = $8;
                    n->windowClause = $9;
                    $$ = (Node \*)n;
                }
            | SELECT distinct_clause target_list
...

三、参考资料

Flex&Bison