PostgreSQL在何处处理 sql查询之三

前面已经说过，在 exec_simple_query中，完成sql文的执行。

具体地说，是要构造portal，然后运行 PortalStart , PortalRun...

下面就先看看 portal如何构造：

在 exec_simple_query中，有这么一段：

1         /*

2          * Create unnamed portal to run the query or queries in. If there

3          * already is one, silently drop it.

4          */

5         portal = CreatePortal("", true, true);

6         /* Don't display the portal in pg_cursors */

7         portal->visible = false;

具体的CreatePortal，是如何作的(portalmem.c)：

 1 /*

 2  * CreatePortal

 3  *        Returns a new portal given a name.

 4  *

 5  * allowDup: if true, automatically drop any pre-existing portal of the

 6  * same name (if false, an error is raised).

 7  *

 8  * dupSilent: if true, don't even emit a WARNING.

 9  */

10 Portal

11 CreatePortal(const char *name, bool allowDup, bool dupSilent)

12 {

13     Portal        portal;

14 

15     AssertArg(PointerIsValid(name));

16 

17     portal = GetPortalByName(name);

18 

19 

20     if (PortalIsValid(portal))

21     {

22         if (!allowDup)

23             ereport(ERROR,

24                     (errcode(ERRCODE_DUPLICATE_CURSOR),

25                      errmsg("cursor \"%s\" already exists", name)));

26         if (!dupSilent)

27             ereport(WARNING,

28                     (errcode(ERRCODE_DUPLICATE_CURSOR),

29                      errmsg("closing existing cursor \"%s\"",

30                             name)));

31         PortalDrop(portal, false);

32     }

33 

34     /* make new portal structure */

35     portal = (Portal) MemoryContextAllocZero(PortalMemory, sizeof *portal);

36 

37     /* initialize portal heap context; typically it won't store much */

38     portal->heap = AllocSetContextCreate(PortalMemory,

39                                          "PortalHeapMemory",

40                                          ALLOCSET_SMALL_MINSIZE,

41                                          ALLOCSET_SMALL_INITSIZE,

42                                          ALLOCSET_SMALL_MAXSIZE);

43 

44     /* create a resource owner for the portal */

45     portal->resowner = ResourceOwnerCreate(CurTransactionResourceOwner,

46                                            "Portal");

47 

48     /* initialize portal fields that don't start off zero */

49     portal->status = PORTAL_NEW;

50     portal->cleanup = PortalCleanup;

51     portal->createSubid = GetCurrentSubTransactionId();

52     portal->strategy = PORTAL_MULTI_QUERY;

53     portal->cursorOptions = CURSOR_OPT_NO_SCROLL;

54     portal->atStart = true;

55     portal->atEnd = true;        /* disallow fetches until query is set */

56     portal->visible = true;

57     portal->creation_time = GetCurrentStatementStartTimestamp();

58 

59     /* put portal in table (sets portal->name) */

60     PortalHashTableInsert(portal, name);

61 

62     return portal;

63 }

由于调用 CreatePortal 的时候，传递name是空值，所以 portal = GetPortalByName(name); 返回NULL。

通过  MemoryContextAllocZero  建立一个初始的portal，然后再赋予各种值，最后压入哈希表中。此时name依然为空。