本節簡單介紹了PostgreSQL在執行邏輯最佳化中相關的資料結構,包括RangeSubselect/Alias/RangeVar/ResTarget/ColumnRef.

### 一、資料結構
**RangeSubselect**
出現在FROM語句中的子查詢
```
/*
 * RangeSubselect - subquery appearing in a FROM clause
 * 出現在FROM語句中的子查詢
 */
typedef struct RangeSubselect
{
  NodeTag   type;
  //是否有LATERAL?
  bool    lateral;    /* does it have LATERAL prefix? */
  //未轉換的子查詢語句
  Node     *subquery;   /* the untransformed sub-select clause */
  //表別名和可選的列別名
  Alias    *alias;      /* table alias & optional column aliases */
} RangeSubselect;

```

**Alias**
為RangeVar指定別名.別名可能同時重新命名了表列.
```
/*
 * Alias -
 *    specifies an alias for a range variable; the alias might also
 *    specify renaming of columns within the table.
 *    為RangeVar指定別名.別名可能同時重新命名了表列.
 *
 * Note: colnames is a list of Value nodes (always strings).  In Alias structs
 * associated with RTEs, there may be entries corresponding to dropped
 * columns; these are normally empty strings ("").  See parsenodes.h for info.
 * 注意:colnames是Value節點(通常是字串)連結串列.
 *      在與RTEs相關的Alias結構體中,可能有跟已刪除的列對應的條目.
 *    這些通常是空字串("").詳細可參考parsenodes.h
 */
typedef struct Alias
{
  NodeTag   type;
  //別名
  char     *aliasname;    /* aliased rel name (never qualified) */
  //列別名連結串列
  List     *colnames;   /* optional list of column aliases */
} Alias;

```

**RangeVar**
range variable,用於FROM語句中.
```

/*
 * RangeVar - range variable, used in FROM clauses
 * range variable,用於FROM語句中.
 *
 * Also used to represent table names in utility statements; there, the alias
 * field is not used, and inh tells whether to apply the operation
 * recursively to child tables.  In some contexts it is also useful to carry
 * a TEMP table indication here.
 * 在工具語句中,用於表示表名,這時alias欄位沒有使用.
 * inh用於判斷是否在子表中遞回應用相關的操作.
 */
typedef struct RangeVar
{
  NodeTag   type;
  char     *catalogname;  /* the catalog (database) name, or NULL */
  char     *schemaname;   /* the schema name, or NULL */
  char     *relname;    /* the relation/sequence name */
  bool    inh;      /* expand rel by inheritance? recursively act
                 * on children? */
  char    relpersistence; /* see RELPERSISTENCE_* in pg_class.h */
  Alias    *alias;      /* table alias & optional column aliases */
  int     location;   /* token location, or -1 if unknown */
} RangeVar;

```

**ResTarget**
結果目標列(用於先前已轉換的解析樹的目標連結串列中)
```

/*
 * ResTarget -
 *    result target (used in target list of pre-transformed parse trees)
 *    結果目標列(用於先前已轉換的解析樹的目標連結串列中)
 *
 * In a SELECT target list, 'name' is the column label from an
 * 'AS ColumnLabel' clause, or NULL if there was none, and 'val' is the
 * value expression itself.  The 'indirection' field is not used.
 * 在SELECT的目標連結串列中,
 *   'name'是'AS ColumnLabel'語句中的列標籤,如無則為NULL,
 *   'val'是value表示式本身.
 *   'indirection'未使用.
 *
 * INSERT uses ResTarget in its target-column-names list.  Here, 'name' is
 * the name of the destination column, 'indirection' stores any subscripts
 * attached to the destination, and 'val' is not used.
 * INSERT在target-column-names連結串列中使用ResTarget.
 * 這裡'name'是目標列名稱,'indirection'儲存了所有與目標相關的子指令碼,'val'未使用.
 *
 * In an UPDATE target list, 'name' is the name of the destination column,
 * 'indirection' stores any subscripts attached to the destination, and
 * 'val' is the expression to assign.
 * 在UPDATE目標連結串列中,'name'是目標列名稱,'indirection'儲存了所有與目標相關的子指令碼,
 * 'val'是與賦值相關的表示式.
 *
 * See A_Indirection for more info about what can appear in 'indirection'.
 * 詳細參見A_Indirection('indirection')
 */
typedef struct ResTarget
{
  NodeTag   type;
  //列名或NULL
  char     *name;     /* column name or NULL */
  //子指令碼,欄位名稱,'*'或NIL
  List     *indirection;  /* subscripts, field names, and '*', or NIL */
  //需要計算或賦值的值表示式
  Node     *val;      /* the value expression to compute or assign */
  //token的位置,-1表示未知
  int     location;   /* token location, or -1 if unknown */
} ResTarget;

```

**ColumnRef**
 指定對列或整個元組的引用
```

/*
 * ColumnRef - specifies a reference to a column, or possibly a whole tuple
 * 指定對列或整個元組的引用
 *
 * The "fields" list must be nonempty.  It can contain string Value nodes
 * (representing names) and A_Star nodes (representing occurrence of a '*').
 * Currently, A_Star must appear only as the last list element --- the grammar
 * is responsible for enforcing this!
 * "fields"連結串列不能為空.可能包含字串Value節點和A_Star節點(表示出現了*).
 * 截止到目前為止,A_Star必須出現在最後一個連結串列元素中.
 *
 * Note: any container subscripting or selection of fields from composite columns
 * is represented by an A_Indirection node above the ColumnRef.  However,
 * for simplicity in the normal case, initial field selection from a table
 * name is represented within ColumnRef and not by adding A_Indirection.
 */
typedef struct ColumnRef
{
  NodeTag   type;
  //欄位名稱(字串值)連結串列或A_Star
  List     *fields;     /* field names (Value strings) or A_Star */
  //token位置
  int     location;   /* token location, or -1 if unknown */
} ColumnRef;

```

### 二、原始碼解讀
N/A

### 三、跟蹤分析
**RangeSubselect/Alias**
```
(gdb) p *(Node *)($stmt->fromClause->head.data->ptr_value)
$15 = {type = T_RangeSubselect} #實際型別是範圍子查詢RangeSubselect
(gdb) set $fromclause=(RangeSubselect *)($stmt->fromClause->head.data->ptr_value)
(gdb) p *$fromclause
$16 = {type = T_RangeSubselect, lateral = false, subquery = 0x1666c18, alias = 0x1666d40}
(gdb) p *($fromclause->subquery) #subquery,子查詢是SelectStmt型別的節點
$17 = {type = T_SelectStmt}
(gdb) p *($fromclause->alias) #alias,別名,實際值是字串ret
$18 = {type = T_Alias, aliasname = 0x1666d28 "ret", colnames = 0x0}
```

**RangeVar**
```
...
$43 = {type = T_RangeVar}
(gdb) p *(RangeVar *)((JoinExpr *)($joinexpr->larg))->larg
$44 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x1643380 "t_dwxx", inh = true, 
  relpersistence = 112 'p', alias = 0x0, location = 82}
...
```

**ResTarget**
```
...
(gdb) p *(Node *)($subquerylarg->targetList->head.data->ptr_value)
$26 = {type = T_ResTarget}
(gdb) set $subvar=(ResTarget *)($subquerylarg->targetList->head.data->ptr_value)
(gdb) p *$subvar 
{type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x1642c70, location = 23}
...
```

**ColumnRef**
```
...
(gdb) p *$restarget->val
$25 = {type = T_ColumnRef}
(gdb) p *(ColumnRef *)$restarget->val
$26 = {type = T_ColumnRef, fields = 0x1a47a08, location = 7}
(gdb) p *((ColumnRef *)$restarget->val)->fields
$27 = {type = T_List, length = 2, head = 0x1a47a88, tail = 0x1a479e8}
(gdb) p *(Node *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value
$32 = {type = T_String}
#fields連結串列的第1個元素是資料表,第2個元素是資料列
(gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value
$37 = {type = T_String, val = {ival = 27556248, str = 0x1a47998 "t_dwxx"}}
(gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->tail.data->ptr_value
$38 = {type = T_String, val = {ival = 27556272, str = 0x1a479b0 "dwmc"}}
...
```

### 四、參考資料
N/A

PostgreSQL 原始碼解讀（206）- 查詢#119(資料結構RangSubselect等)

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