本節簡單介紹了PostgreSQL隱式型別轉換中使用哪些運算子(pg_operator)的實現函式func_match_argtypes.

### 一、資料結構
**FuncCandidateList**
該結構體儲存檢索得到的所有可能選中的函式或運算子連結串列.
```

/*
 *  This structure holds a list of possible functions or operators
 *  found by namespace lookup.  Each function/operator is identified
 *  by OID and by argument types; the list must be pruned by type
 *  resolution rules that are embodied in the parser, not here.
 *  See FuncnameGetCandidates's comments for more info.
 *  該結構體儲存檢索得到的所有可能選中的函式或運算子連結串列.
 *  每一個函式/運算子透過OID和引數型別唯一確定,
 *  透過整合到分析器中的type resolution rules來確定裁剪該連結串列(但不是在這裡實現)
 *  詳細可參考FuncnameGetCandidates函式.
 */
typedef struct _FuncCandidateList
{
    struct _FuncCandidateList *next;
    //用於namespace檢索內部使用
    int         pathpos;        /* for internal use of namespace lookup */
    //OID
    Oid         oid;            /* the function or operator's OID */
    //引數個數 
    int         nargs;          /* number of arg types returned */
    //variadic array的引數個數
    int         nvargs;         /* number of args to become variadic array */
    //預設引數個數
    int         ndargs;         /* number of defaulted args */
    //引數位置索引
    int        *argnumbers;     /* args' positional indexes, if named call */
    //引數型別
    Oid         args[FLEXIBLE_ARRAY_MEMBER];    /* arg types */
}          *FuncCandidateList;

```

### 二、原始碼解讀
**func_match_argtypes**
給定候選函式列表(正確的函式名稱/引數個數匹配)和輸入資料型別OIDs陣列,生成實際可匹配輸入資料型別(完全匹配或可轉換)的候選函式連結串列,然後符合條件的候選函式個數.
```

/* func_match_argtypes()
 *
 * Given a list of candidate functions (having the right name and number
 * of arguments) and an array of input datatype OIDs, produce a shortlist of
 * those candidates that actually accept the input datatypes (either exactly
 * or by coercion), and return the number of such candidates.
 * 給定候選函式列表(正確的函式名稱/引數個數匹配)和輸入資料型別OIDs陣列,
 * 生成實際可匹配輸入資料型別(完全匹配或可轉換)的候選函式連結串列,然後符合條件的候選函式個數
 *
 * Note that can_coerce_type will assume that UNKNOWN inputs are coercible to
 * anything, so candidates will not be eliminated on that basis.
 * can_coerce_type函式假定UNKNOWN輸入可轉換為任意型別.
 *
 * NB: okay to modify input list structure, as long as we find at least
 * one match.  If no match at all, the list must remain unmodified.
 * 注意:如果只是找到一個匹配的候選函式,修改輸入連結串列結構是OK的.如無匹配,則連結串列保持不變.
 */
int
func_match_argtypes(int nargs,
                    Oid *input_typeids,
                    FuncCandidateList raw_candidates,
                    FuncCandidateList *candidates)  /* return value */
{
    FuncCandidateList current_candidate;//當前候選
    FuncCandidateList next_candidate;//下一候選
    int         ncandidates = 0;

*candidates = NULL;

for (current_candidate = raw_candidates;
         current_candidate != NULL;
         current_candidate = next_candidate)//遍歷候選函式
    {
        next_candidate = current_candidate->next;
        if (can_coerce_type(nargs, input_typeids, current_candidate->args,
                            COERCION_IMPLICIT))//可匹配輸入資料型別(完全匹配或可轉換)
        {
            current_candidate->next = *candidates;
            *candidates = current_candidate;
            ncandidates++;
        }
    }

return ncandidates;
}                               /* func_match_argtypes() */

```
在pg_operator中,輸入引數型別與operator的引數型別匹配或可轉換,可進入候選函式連結串列.

### 三、跟蹤分析
測試指令碼
```
create cast(integer as text) with inout as implicit;

select id||'X' from t_cast;

```
跟蹤分析
```
(gdb) c
Continuing.

Breakpoint 2, oper_select_candidate (nargs=2, input_typeids=0x7ffeb9cca190, candidates=0x13db8a0, operOid=0x7ffeb9cca22c)
    at parse_oper.c:330
330     ncandidates = func_match_argtypes(nargs, input_typeids,
(gdb) p *candidates
$1 = {next = 0x13db870, pathpos = 0, oid = 3284, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db8c8}
(gdb) p *candidates->next
$2 = {next = 0x13db840, pathpos = 0, oid = 3681, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db898}
(gdb) p *candidates->next->next
$3 = {next = 0x13db810, pathpos = 0, oid = 3633, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db868}
(gdb) p *candidates->next->next->next
$4 = {next = 0x13db7e0, pathpos = 0, oid = 2780, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db838}
(gdb) p *candidates->next->next->next->next
$5 = {next = 0x13db7b0, pathpos = 0, oid = 374, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db808}
(gdb) p *candidates->next->next->next->next->next
$6 = {next = 0x13db780, pathpos = 0, oid = 349, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db7d8}
(gdb) p *candidates->next->next->next->next->next->next
$7 = {next = 0x13db750, pathpos = 0, oid = 375, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db7a8}
(gdb) p *candidates->next->next->next->next->next->next->next
$8 = {next = 0x13db720, pathpos = 0, oid = 1797, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db778}
(gdb) p *candidates->next->next->next->next->next->next->next->next
$9 = {next = 0x13db6f0, pathpos = 0, oid = 2779, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db748}
(gdb) p *candidates->next->next->next->next->next->next->next->next->next
$10 = {next = 0x13db6c0, pathpos = 0, oid = 654, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db718}
(gdb) p *candidates->next->next->next->next->next->next->next->next->next->next
$11 = {next = 0x0, pathpos = 0, oid = 2018, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db6e8}
(gdb) p *candidates->next->next->next->next->next->next->next->next->next->next->next
Cannot access memory at address 0x0
(gdb) n
334     if (ncandidates == 0)
(gdb) 
339     if (ncandidates == 1)
(gdb) 
349     candidates = func_select_candidate(nargs, input_typeids, candidates);
(gdb) p ncandidates
$12 = 2
(gdb) p *candidates
$13 = {next = 0x13db810, pathpos = 0, oid = 374, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db808}
(gdb) p *candidates->next
$14 = {next = 0x0, pathpos = 0, oid = 2780, nargs = 2, nvargs = 0, ndargs = 0, argnumbers = 0x0, args = 0x13db838}
(gdb) p *candidates->next->next
Cannot access memory at address 0x0
(gdb)

```
DONE!

### 四、參考資料
[PostgreSQL Type Conversion](https://www.postgresql.org/docs/11/typeconv-overview.html)

PostgreSQL 原始碼解讀（210）- 隱式型別轉換(func_match_argtypes)

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