本節簡單介紹了PostgreSQL在執行parse中重要的兩個資料結構:SelectStmt&Value.

### 一、資料結構
**SelectStmt**
"simple" SELECT可轉換為SelectStmt節點.包含集合操作(UNION, INTERSECT, EXCEPT)的查詢透過SelectStmt節點樹來表示,在這棵樹中,葉子節點是SELECTs元件,而內部節點表示UNION, INTERSECT, or EXCEPT運算子.內部節點和葉子節點是相同的節點型別.
```

/* ----------------------
 *    Select Statement
 *
 * A "simple" SELECT is represented in the output of gram.y by a single
 * SelectStmt node; so is a VALUES construct.  A query containing set
 * operators (UNION, INTERSECT, EXCEPT) is represented by a tree of SelectStmt
 * nodes, in which the leaf nodes are component SELECTs and the internal nodes
 * represent UNION, INTERSECT, or EXCEPT operators.  Using the same node
 * type for both leaf and internal nodes allows gram.y to stick ORDER BY,
 * LIMIT, etc, clause values into a SELECT statement without worrying
 * whether it is a simple or compound SELECT.
 *
 * "simple" SELECT可轉換為SelectStmt節點.
 * 包含集合操作(UNION, INTERSECT, EXCEPT)的查詢透過SelectStmt節點樹來表示,
 *   在這棵樹中,葉子節點是SELECTs元件,而內部節點表示UNION, INTERSECT, or EXCEPT運算子.
 * 內部節點和葉子節點是相同的節點型別.
 * ----------------------
 */
typedef enum SetOperation
{
  SETOP_NONE = 0,
  SETOP_UNION,
  SETOP_INTERSECT,
  SETOP_EXCEPT
} SetOperation;

typedef struct SelectStmt
{
  NodeTag   type;

/*
   * These fields are used only in "leaf" SelectStmts.
   * "葉子"SelectStmts節點使用的欄位
   */
  //NULL,DISTINCT ON表示式連結串列,或者所有(SELECT DISTINCT)的lcons(NIL,NIL)
  List     *distinctClause; /* NULL, list of DISTINCT ON exprs, or
                 * lcons(NIL,NIL) for all (SELECT DISTINCT) */
  //SELECT INTO的target
  IntoClause *intoClause;   /* target for SELECT INTO */
  //目標連結串列(元素為ResTarget)
  List     *targetList;   /* the target list (of ResTarget) */
  //FROM子句
  List     *fromClause;   /* the FROM clause */
  //WHERE
  Node     *whereClause;  /* WHERE qualification */
  //GROUP BY 子句
  List     *groupClause;  /* GROUP BY clauses */
  //HAVING條件表示式
  Node     *havingClause; /* HAVING conditional-expression */
  //視窗函式連結串列
  List     *windowClause; /* WINDOW window_name AS (...), ... */

/*
   * In a "leaf" node representing a VALUES list, the above fields are all
   * null, and instead this field is set.  Note that the elements of the
   * sublists are just expressions, without ResTarget decoration. Also note
   * that a list element can be DEFAULT (represented as a SetToDefault
   * node), regardless of the context of the VALUES list. It's up to parse
   * analysis to reject that where not valid.
   * 如果"葉子"節點表示VALUES連結串列,則上述欄位均為NULL,而該欄位會儲存相關資訊.
   */
  List     *valuesLists;  /* untransformed list of expression lists */

/*
   * These fields are used in both "leaf" SelectStmts and upper-level
   * SelectStmts.
   * 下面這些欄位用於"left"節點和上層節點
   */
  //排序子句
  List     *sortClause;   /* sort clause (a list of SortBy's) */
  //limit偏移
  Node     *limitOffset;  /* # of result tuples to skip */
  //limit個數
  Node     *limitCount;   /* # of result tuples to return */
  //FOR UPDATE
  List     *lockingClause;  /* FOR UPDATE (list of LockingClause's) */
  //CTE
  WithClause *withClause;   /* WITH clause */

/*
   * These fields are used only in upper-level SelectStmts.
   * 下面這些欄位用於高層SelectStmts
   */
  //操作型別
  SetOperation op;      /* type of set op */
  bool    all;      /* ALL specified? */
  //左邊樹
  struct SelectStmt *larg;  /* left child */
  //右邊樹
  struct SelectStmt *rarg;  /* right child */
  /* Eventually add fields for CORRESPONDING spec here */
} SelectStmt;

```

**Value**
相同的Value結構體用於5中節點型別:T_Integer, T_Float, T_String, T_BitString, T_Null
```
/*-------------------------------------------------------------------------
 *
 * value.h
 *    interface for Value nodes
 *
 *
 * Copyright (c) 2003-2019, PostgreSQL Global Development Group
 *
 * src/include/nodes/value.h
 *
 *-------------------------------------------------------------------------
 */

#ifndef VALUE_H
#define VALUE_H

#include "nodes/nodes.h"

/*----------------------
 *    Value node
 *
 * The same Value struct is used for five node types: T_Integer,
 * T_Float, T_String, T_BitString, T_Null.
 * 相同的Value結構體用於5中節點型別:T_Integer, T_Float, T_String, T_BitString, T_Null
 *
 * Integral values are actually represented by a machine integer,
 * but both floats and strings are represented as strings.
 * Using T_Float as the node type simply indicates that
 * the contents of the string look like a valid numeric literal.
 * 整型值實際上透過機器integer來表示,但floats和strings均使用字串來表示.
 * 使用T_Float作為節點型別只是簡單的表示字串中的記憶體看似有效的數值字面量.
 *
 * (Before Postgres 7.0, we used a double to represent T_Float,
 * but that creates loss-of-precision problems when the value is
 * ultimately destined to be converted to NUMERIC.  Since Value nodes
 * are only used in the parsing process, not for runtime data, it's
 * better to use the more general representation.)
 * (在PG 7.0以前,使用double來表示T_Float,但會出現精度丟失的問題)
 *
 * Note that an integer-looking string will get lexed as T_Float if
 * the value is too large to fit in an 'int'.
 * 注意 : 看似整型字串在不能使用int儲存的情況下會在語義上視為T_Float
 *
 * Nulls, of course, don't need the value part at all.
 * Nulls,不需要值
 *----------------------
 */
typedef struct Value
{
  NodeTag   type;     /* tag appropriately (eg. T_String) */
  union ValUnion
  {
    int     ival;   /* machine integer */
    char     *str;    /* string */
  }     val;
} Value;

#define intVal(v)   (((Value *)(v))->val.ival)
#define floatVal(v)   atof(((Value *)(v))->val.str)
#define strVal(v)   (((Value *)(v))->val.str)

extern Value *makeInteger(int i);
extern Value *makeFloat(char *numericStr);
extern Value *makeString(char *str);
extern Value *makeBitString(char *str);

#endif              /* VALUE_H */

```

### 二、原始碼解讀
N/A

### 三、跟蹤分析
測試SQL語句:
```
-- 用於測試的查詢語句
testdb=# select t_dwxx.dwmc,t_grxx.grbh,t_grxx.xm,t_jfxx.ny,t_jfxx.je
testdb-# from t_dwxx,t_grxx,t_jfxx
testdb-# where t_dwxx.dwbh = t_grxx.dwbh 
testdb-# and t_grxx.grbh = t_jfxx.grbh
testdb-# and t_dwxx.dwbh IN ('1001','1002')
testdb-# order by t_grxx.grbh
testdb-# limit 8;
   dwmc    | grbh |  xm  |   ny   |   je   
-----------+------+------+--------+--------
 X有限公司 | 901  | 張三 | 201801 |  401.3
 X有限公司 | 901  | 張三 | 201802 |  401.3
 X有限公司 | 901  | 張三 | 201803 |  401.3
 Y有限公司 | 902  | 李四 | 201801 |  513.1
 Y有限公司 | 902  | 李四 | 201802 |  513.3
 Y有限公司 | 902  | 李四 | 201804 |  513.3
 Y有限公司 | 903  | 王五 | 201801 | 372.22
 Y有限公司 | 903  | 王五 | 201804 | 372.22
(8 rows)
```
樣例資料如下:
```
...
(gdb) p *(RawStmt *)(raw_parsetree_list->head.data->ptr_value)
$7 = {type = T_RawStmt, stmt = 0x1a48c00, stmt_location = 0, stmt_len = 232}
(gdb) p *((RawStmt *)(raw_parsetree_list->head.data->ptr_value))->stmt
$8 = {type = T_SelectStmt}
###### 實際型別SelectStmt 
(gdb)  p *(SelectStmt *)((RawStmt *)(raw_parsetree_list->head.data->ptr_value))->stmt
$16 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0, targetList = 0x1a47b18, 
  fromClause = 0x1a48900, whereClause = 0x1a48b40, groupClause = 0x0, havingClause = 0x0, windowClause = 0x0, 
  valuesLists = 0x0, sortClause = 0x1afd858, limitOffset = 0x0, limitCount = 0x1afd888, lockingClause = 0x0, 
  withClause = 0x0, op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0}
```

### 四、參考資料
N/A

PostgreSQL 原始碼解讀（204）- 查詢#117(資料結構SelectStmt&Value)

相關文章