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FromExpr&JoinExpr.

### 一、資料結構
**FromExpr**
表示FROM ... WHERE結構
```

/*----------
 * FromExpr - represents a FROM ... WHERE ... construct
 * 表示FROM ... WHERE結構
 *
 * This is both more flexible than a JoinExpr (it can have any number of
 * children, including zero) and less so --- we don't need to deal with
 * aliases and so on.  The output column set is implicitly just the union
 * of the outputs of the children.
 * 該結構比JoinExpr(有0..n個子節點)更為靈活 -- 不需要處理別名等.
 * 輸出列集合是子集輸出的彙總.
 *----------
 */
typedef struct FromExpr
{
  NodeTag   type;
  //連線子樹連結串列
  List     *fromlist;   /* List of join subtrees */
  //join中的表示式
  Node     *quals;      /* qualifiers on join, if any */
} FromExpr;

```

**JoinExpr**
用於SQL JOIN表示式.
```

/*----------
 * JoinExpr - for SQL JOIN expressions
 * 用於SQL JOIN表示式.
 *
 * isNatural, usingClause, and quals are interdependent.  The user can write
 * only one of NATURAL, USING(), or ON() (this is enforced by the grammar).
 * If he writes NATURAL then parse analysis generates the equivalent USING()
 * list, and from that fills in "quals" with the right equality comparisons.
 * If he writes USING() then "quals" is filled with equality comparisons.
 * If he writes ON() then only "quals" is set.  Note that NATURAL/USING
 * are not equivalent to ON() since they also affect the output column list.
 * isNatural, usingClause, and quals是相互依賴的.
 * 使用者只可以使用NATURAL, USING(), or ON()(語法限制).
 * 如果是NATURAL,則解析器會產生相應的USING()連結串列,並使用正確的等值比較表示式填充quals.
 * 如果是USING(),則使用正確的等值比較表示式填充quals.
 * 如果是ON(),則只設定quals欄位.
 * 注意NATURAL/USING與ON()並不相同,因為它們同時影響了輸出列連結串列.
 *
 * alias is an Alias node representing the AS alias-clause attached to the
 * join expression, or NULL if no clause.  NB: presence or absence of the
 * alias has a critical impact on semantics, because a join with an alias
 * restricts visibility of the tables/columns inside it.
 * alias表示與join表示式相關的AS別名子句,如無則為NULL.
 * 注意:別名的存在與否對語義有很大影響,因此有別名的join限制了其中表/列的可見性.
 *
 * During parse analysis, an RTE is created for the Join, and its index
 * is filled into rtindex.  This RTE is present mainly so that Vars can
 * be created that refer to the outputs of the join.  The planner sometimes
 * generates JoinExprs internally; these can have rtindex = 0 if there are
 * no join alias variables referencing such joins.
 * 在解析時,RTE在參與Join時解析,編號填充到rtindex中.
 * 該RTE存在的目的主要是可以建立引用join輸出的Vars.
 * 計劃器有時候會在內部生成JoinExprs;如沒有join別名變數參考這樣的連線,那麼rtindex = 0
 *----------
 */
typedef struct JoinExpr
{
  NodeTag   type;
  //join型別
  JoinType  jointype;   /* type of join */
  //自然連線?
  bool    isNatural;    /* Natural join? Will need to shape table */
  //左樹
  Node     *larg;     /* left subtree */
  //右樹
  Node     *rarg;     /* right subtree */
  //USING語句(String連結串列)
  List     *usingClause;  /* USING clause, if any (list of String) */
  //join限定符
  Node     *quals;      /* qualifiers on join, if any */
  //別名語句
  Alias    *alias;      /* user-written alias clause, if any */
  //分配給join的RT編號,或者為0
  int     rtindex;    /* RT index assigned for join, or 0 */
} JoinExpr;

```

### 二、原始碼解讀
N/A

### 三、跟蹤分析
```
...
(gdb) p *(FromExpr *)($rte_sq_rte->subquery->jointree)
$44 = {type = T_FromExpr, fromlist = 0x16fda18, quals = 0x16fe0f0}
(gdb) set $rtesq2_jointree=(FromExpr *)($rte_sq_rte->subquery->jointree)
(gdb) p *$rtesq2_jointree->fromlist
$48 = {type = T_List, length = 1, head = 0x16fd9f8, tail = 0x16fd9f8}
(gdb) p *(Node *)$rtesq2_jointree->fromlist->head->data.ptr_value
$49 = {type = T_JoinExpr}
(gdb) set $tmpvar=(JoinExpr *)$rtesq2_jointree->fromlist->head->data.ptr_value
(gdb) p *$tmpvar
$3 = {type = T_JoinExpr, jointype = JOIN_INNER, isNatural = false, larg = 0x2b68730, rarg = 0x2c215e8, usingClause = 0x0, quals = 0x2c28130, alias = 0x0, rtindex = 5}
(gdb) p *$tmpvar->larg
$4 = {type = T_JoinExpr}
(gdb) p *(JoinExpr *)$tmpvar->larg
$5 = {type = T_JoinExpr, jointype = JOIN_INNER, isNatural = false, larg = 0x2c1e848, rarg = 0x2c1ebd8, 
  usingClause = 0x0, quals = 0x2c20c48, alias = 0x0, rtindex = 3}
(gdb) p *(JoinExpr *)$tmpvar->rarg
$6 = {type = T_RangeTblRef, jointype = JOIN_SEMI, isNatural = 8, larg = 0x2b66de0, rarg = 0x636d7764, 
  usingClause = 0x10, quals = 0x2b66de0, alias = 0xda, rtindex = 46274048}
(gdb) p *(JoinExpr *)$tmpvar->quals
$7 = {type = T_OpExpr, jointype = 98, isNatural = 67, larg = 0x0, rarg = 0x64, usingClause = 0x2c27fb8, 
  quals = 0xa9, alias = 0x0, rtindex = 0}
...
```

### 四、參考資料
N/A

PostgreSQL 原始碼解讀（207）- 查詢#120(資料結構FromExpr&JoinExpr)

相關文章