本節繼續處理上一節給出的任務,重點介紹第二部分:輸出,實現結果投影列名稱命名為sysdate

### 一、輸出
參照current_date函式,首先透過日誌跟蹤其parse tree,重點關注targetList
```
 2019-04-19 17:06:15.591 CST,"xdb","testdb",1693,"[local]",5cb98b16.69d,3,"SELECT",2019-04-19 16:47:18 CST,3/4,0,LOG,00000,"parse tree:","   {QUERY 
   :commandType 1 
   ...
   :targetList (
      {TARGETENTRY 
      :expr 
         {SQLVALUEFUNCTION 
         :op 0 
         :type 1082 
         :typmod -1 
         :location 7
         }
      :resno 1 
      :resname current_date 
      :ressortgroupref 0 
      :resorigtbl 0 
      :resorigcol 0 
      :resjunk false
      }
   )
   ...
   :stmt_len 27
   }

```

分析原始碼,找到與輸出相關的檔案是src/backend/parser/parse_target.c,相關的函式為transformTargetEntry,跟蹤此函式,列印呼叫棧:
```
(gdb) bt
#0  transformTargetEntry (pstate=0x16afae8, node=0x16af770, expr=0x0, exprKind=EXPR_KIND_SELECT_TARGET, colname=0x0, 
    resjunk=false) at parse_target.c:93
#1  0x000000000060b1f9 in transformTargetList (pstate=0x16afae8, targetlist=0x16af828, exprKind=EXPR_KIND_SELECT_TARGET)
    at parse_target.c:191
#2  0x00000000005b1e3a in transformSelectStmt (pstate=0x16afae8, stmt=0x16af938) at analyze.c:1243
#3  0x00000000005b03db in transformStmt (pstate=0x16afae8, parseTree=0x16af938) at analyze.c:301
#4  0x00000000005b02b6 in transformOptionalSelectInto (pstate=0x16afae8, parseTree=0x16af938) at analyze.c:246
#5  0x00000000005b0174 in transformTopLevelStmt (pstate=0x16afae8, parseTree=0x16afa50) at analyze.c:196
#6  0x00000000005affcc in parse_analyze (parseTree=0x16afa50, sourceText=0x16aed78 "select current_date from t1;", 
    paramTypes=0x0, numParams=0, queryEnv=0x0) at analyze.c:116
#7  0x00000000008bb78c in pg_analyze_and_rewrite (parsetree=0x16afa50, 
    query_string=0x16aed78 "select current_date from t1;", paramTypes=0x0, numParams=0, queryEnv=0x0) at postgres.c:689
#8  0x00000000008bbddd in exec_simple_query (query_string=0x16aed78 "select current_date from t1;") at postgres.c:1070
#9  0x00000000008c01f3 in PostgresMain (argc=1, argv=0x16dcd28, dbname=0x16dcb90 "testdb", username=0x16aba98 "xdb")
    at postgres.c:4182
#10 0x000000000081e0ce in BackendRun (port=0x16d0b50) at postmaster.c:4361
#11 0x000000000081d841 in BackendStartup (port=0x16d0b50) at postmaster.c:4033
#12 0x0000000000819c3b in ServerLoop () at postmaster.c:1706
#13 0x00000000008194f1 in PostmasterMain (argc=1, argv=0x16a9a50) at postmaster.c:1379
#14 0x0000000000742993 in main (argc=1, argv=0x16a9a50) at main.c:228
```
其中FigureColnameInternal函式為current_date設定輸出列名,新增以下程式碼,則實現輸出sysdate:
```
//Hacker : 新增系統列
case SVFOP_ZZ_SYSDATE:
					*name = "sysdate";//zz_sysdate -> sysdate
					return 2;
```

**相關原始碼解讀**
```
/*
 * transformTargetEntry()
 *	Transform any ordinary "expression-type" node into a targetlist entry.
 *	This is exported so that parse_clause.c can generate targetlist entries
 *	for ORDER/GROUP BY items that are not already in the targetlist.
 *	轉換所有普通的"表示式型別expression-type"節點為tagetlist條目.
 *	TargetEntry作為輸出引數以便parse_clause.c
 *    可以為ORDER/GROUP BY等未在targetlist中的items生成targetlist條目
 *
 * node		the (untransformed) parse tree for the value expression.
 * node 	未經變換的值表示式
 *
 * expr		the transformed expression, or NULL if caller didn't do it yet.
 * expr     已轉換的表示式,如呼叫者未轉換則為NULL
 *
 * exprKind expression kind (EXPR_KIND_SELECT_TARGET, etc)
 * exprKind 表示式型別(如EXPR_KIND_SELECT_TARGET等)
 *
 * colname	the column name to be assigned, or NULL if none yet set.
 * colname  分配的列名或者為NULL
 *
 * resjunk	true if the target should be marked resjunk, ie, it is not
 *			wanted in the final projected tuple.
 * resjunk  如目標應標記為resjunk,則為T,比如該列不希望投影為最終的元組
 */
TargetEntry *
transformTargetEntry(ParseState *pstate,
					 Node *node,
					 Node *expr,
					 ParseExprKind exprKind,
					 char *colname,
					 bool resjunk)
{
	/* Transform the node if caller didn't do it already */
	//expr為NULL,則轉換之
	if (expr == NULL)
	{
		/*
		 * If it's a SetToDefault node and we should allow that, pass it
		 * through unmodified.  (transformExpr will throw the appropriate
		 * error if we're disallowing it.)
		 */
		if (exprKind == EXPR_KIND_UPDATE_SOURCE && IsA(node, SetToDefault))
			expr = node;
		else
			expr = transformExpr(pstate, node, exprKind);
	}

if (colname == NULL && !resjunk)
	{
		/*
		 * Generate a suitable column name for a column without any explicit
		 * 'AS ColumnName' clause.
		 * 如非顯式指定列名(AS ColumnName),產生一個合適的列名
		 */
		colname = FigureColname(node);
	}

//返回TargetEntry
	return makeTargetEntry((Expr *) expr,
						   (AttrNumber) pstate->p_next_resno++,
						   colname,
						   resjunk);
}

/*
 * FigureColname -
 *	  if the name of the resulting column is not specified in the target
 *	  list, we have to guess a suitable name.  The SQL spec provides some
 *	  guidance, but not much...
 *
 * Note that the argument is the *untransformed* parse tree for the target
 * item.  This is a shade easier to work with than the transformed tree.
 */
char *
FigureColname(Node *node)
{
	char	   *name = NULL;

(void) FigureColnameInternal(node, &name);
	if (name != NULL)
		return name;
	/* default result if we can't guess anything */
	return "?column?";
}

/*
 * FigureColnameInternal -
 *	  internal workhorse for FigureColname
 *
 * Return value indicates strength of confidence in result:
 *		0 - no information
 *		1 - second-best name choice
 *		2 - good name choice
 * The return value is actually only used internally.
 * If the result isn't zero, *name is set to the chosen name.
 */
static int
FigureColnameInternal(Node *node, char **name)
{
	int			strength = 0;

if (node == NULL)
		return strength;

switch (nodeTag(node))
	{
		...
		case T_SQLValueFunction:
			/* make these act like a function or variable */
			switch (((SQLValueFunction *) node)->op)
			{
				case SVFOP_CURRENT_DATE:
					*name = "current_date";
					return 2;
				...
				//Hacker : 新增系統列
				case SVFOP_ZZ_SYSDATE:
					*name = "sysdate";//zz_sysdate -> sysdate
					return 2;
			}
			break;
		...
	}
}

```

### 二、參考資料
[Flex&Bison](http://web.iitd.ac.in/~sumeet/flex__bison.pdf)

PostgreSQL 原始碼解讀（174）- 核心研發#2(如何新增系統欄位)#2

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