本節繼續介紹PostgreSQL的語法分析定義檔案gram.y的第三部分Productions(產生式).
Bison輸入檔案的組成:
```
%{
Declarations
%}
Definitions
%%
Productions
%%
User subroutines
```

### 一、Productions
Productions即產生式,這是使用者編寫的語法產生式,產生式的書寫格式如下:
```
S -> X \n
X -> X + X | X - X | T_NUMBER
```
S -> X \n成為產生式,第一條產生式的最左邊的符號成為起始符號,在這裡是符號S.
為了避免出現遞迴解析,Bison因此會在最前面多新增一條產生式S’ -> S,S’為起始符號.
在Bison中,符號":"表示一條"->",同一個非終結符的不同產生式用"|"隔開,用";"結束.每條產生式的後面花括號內是一段C程式碼,這些程式碼在該產生式被應用時執行,成為Action(動作),產生式的右邊是ε(空集合)時,用註釋/\* empty \*/代替.
產生式中的非終結符不需要預先定義,Bison會自動根據所有產生式的左邊符號來確定哪些符號是非終結符;終結符中,單字元token(token type值和字元的ASCII碼相同)也不需要預先定義,在產生式內部直接用單引號括起來即可,其他型別的token則需要預先在 Definitions段中定義好,如%token <keyword> ABORT_P ABSOLUTE_P ACCESS ACTION ADD_P ADMIN AFTER等,Bison會自動為這種token分配一個編號，再寫到gram.h　檔案中去,開啟該檔案,可以看到如下程式碼:
```
[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,
 ....
```
編號從258開始,根據gram.y中的順序逐個定義.
```
...
%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
...
```
這些token定義在scan.l中可直接使用.
```
#include "parser/gramparse.h" --> #include "parser/gram.h"

```

Bison會根據產生式以及符號優先順序轉化為LALR(1)動作表輸出到gram.c檔案中去.在gram.c檔案中,PG根據自定義語法檔案生成一個函式int base_yyparse (core_yyscan_t yyscanner);該函式按LR(1)解析流程對詞法分析得到的token流進行解析,每當它需要讀入下一個符號時,它就執行一次s = yylex() ,每當它要執行一個摺疊(reduce)動作時,這個reduce所應用的產生式後面C程式碼將被執行,執行完後才將相應的狀態出棧。
下面是gram.c中yyparse的部分程式碼:
```
/*----------.
| 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、原始碼
下面是gram.y產生式定義的部分原始碼
```
/*
 *	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;
				}
		;
...
```

簡單解析如下:
1.stmtblock
stmtblock:	stmtmulti
stmtblock為起始符號,最終應摺疊(reduce)為該符號,否則會有語法錯誤.
執行的邏輯是:pg_yyget_extra(yyscanner)->parsetree = $1;
亦即完成語法解析,生成語法解析樹parsetree.

2.stmtmulti
tmtmulti:	stmtmulti ';' stmt
左遞迴產生式,PG可接受多個以分號";"分隔的語句,每個語句的定義為stmt

3.stmt
```
stmt :
			AlterEventTrigStmt
			| AlterCollationStmt
			...
			| SelectStmt
			...
```
stmt包括N多種語句,我們看最常見的SelectStmt語句

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、參考資料
[Flex&Bison](http://web.iitd.ac.in/~sumeet/flex__bison.pdf)</keyword></ival></str></keyword>

PostgreSQL 原始碼解讀（176）- 查詢#94(語法分析：gram.y)#3

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