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dest.h
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1 /*-------------------------------------------------------------------------
2  *
3  * dest.h
4  * support for communication destinations
5  *
6  * Whenever the backend executes a query that returns tuples, the results
7  * have to go someplace. For example:
8  *
9  * - stdout is the destination only when we are running a
10  * standalone backend (no postmaster) and are returning results
11  * back to an interactive user.
12  *
13  * - a remote process is the destination when we are
14  * running a backend with a frontend and the frontend executes
15  * PQexec() or PQfn(). In this case, the results are sent
16  * to the frontend via the functions in backend/libpq.
17  *
18  * - DestNone is the destination when the system executes
19  * a query internally. The results are discarded.
20  *
21  * dest.c defines three functions that implement destination management:
22  *
23  * BeginCommand: initialize the destination at start of command.
24  * CreateDestReceiver: return a pointer to a struct of destination-specific
25  * receiver functions.
26  * EndCommand: clean up the destination at end of command.
27  *
28  * BeginCommand/EndCommand are executed once per received SQL query.
29  *
30  * CreateDestReceiver returns a receiver object appropriate to the specified
31  * destination. The executor, as well as utility statements that can return
32  * tuples, are passed the resulting DestReceiver* pointer. Each executor run
33  * or utility execution calls the receiver's rStartup method, then the
34  * receiveSlot method (zero or more times), then the rShutdown method.
35  * The same receiver object may be re-used multiple times; eventually it is
36  * destroyed by calling its rDestroy method.
37  *
38  * In some cases, receiver objects require additional parameters that must
39  * be passed to them after calling CreateDestReceiver. Since the set of
40  * parameters varies for different receiver types, this is not handled by
41  * this module, but by direct calls from the calling code to receiver type
42  * specific functions.
43  *
44  * The DestReceiver object returned by CreateDestReceiver may be a statically
45  * allocated object (for destination types that require no local state),
46  * in which case rDestroy is a no-op. Alternatively it can be a palloc'd
47  * object that has DestReceiver as its first field and contains additional
48  * fields (see printtup.c for an example). These additional fields are then
49  * accessible to the DestReceiver functions by casting the DestReceiver*
50  * pointer passed to them. The palloc'd object is pfree'd by the rDestroy
51  * method. Note that the caller of CreateDestReceiver should take care to
52  * do so in a memory context that is long-lived enough for the receiver
53  * object not to disappear while still needed.
54  *
55  * Special provision: None_Receiver is a permanently available receiver
56  * object for the DestNone destination. This avoids useless creation/destroy
57  * calls in portal and cursor manipulations.
58  *
59  *
60  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
61  * Portions Copyright (c) 1994, Regents of the University of California
62  *
63  * src/include/tcop/dest.h
64  *
65  *-------------------------------------------------------------------------
66  */
67 #ifndef DEST_H
68 #define DEST_H
69 
70 #include "executor/tuptable.h"
71 #include "tcop/cmdtag.h"
72 
73 
74 /* buffer size to use for command completion tags */
75 #define COMPLETION_TAG_BUFSIZE 64
76 
77 
78 /* ----------------
79  * CommandDest is a simplistic means of identifying the desired
80  * destination. Someday this will probably need to be improved.
81  *
82  * Note: only the values DestNone, DestDebug, DestRemote are legal for the
83  * global variable whereToSendOutput. The other values may be used
84  * as the destination for individual commands.
85  * ----------------
86  */
87 typedef enum
88 {
89  DestNone, /* results are discarded */
90  DestDebug, /* results go to debugging output */
91  DestRemote, /* results sent to frontend process */
92  DestRemoteExecute, /* sent to frontend, in Execute command */
93  DestRemoteSimple, /* sent to frontend, w/no catalog access */
94  DestSPI, /* results sent to SPI manager */
95  DestTuplestore, /* results sent to Tuplestore */
96  DestIntoRel, /* results sent to relation (SELECT INTO) */
97  DestCopyOut, /* results sent to COPY TO code */
98  DestSQLFunction, /* results sent to SQL-language func mgr */
99  DestTransientRel, /* results sent to transient relation */
100  DestTupleQueue /* results sent to tuple queue */
101 } CommandDest;
102 
103 /* ----------------
104  * DestReceiver is a base type for destination-specific local state.
105  * In the simplest cases, there is no state info, just the function
106  * pointers that the executor must call.
107  *
108  * Note: the receiveSlot routine must be passed a slot containing a TupleDesc
109  * identical to the one given to the rStartup routine. It returns bool where
110  * a "true" value means "continue processing" and a "false" value means
111  * "stop early, just as if we'd reached the end of the scan".
112  * ----------------
113  */
115 
117 {
118  /* Called for each tuple to be output: */
120  DestReceiver *self);
121  /* Per-executor-run initialization and shutdown: */
122  void (*rStartup) (DestReceiver *self,
123  int operation,
124  TupleDesc typeinfo);
125  void (*rShutdown) (DestReceiver *self);
126  /* Destroy the receiver object itself (if dynamically allocated) */
127  void (*rDestroy) (DestReceiver *self);
128  /* CommandDest code for this receiver */
130  /* Private fields might appear beyond this point... */
131 };
132 
133 extern PGDLLIMPORT DestReceiver *None_Receiver; /* permanent receiver for
134  * DestNone */
135 
136 /* The primary destination management functions */
137 
138 extern void BeginCommand(CommandTag commandTag, CommandDest dest);
140 extern void EndCommand(const QueryCompletion *qc, CommandDest dest,
141  bool force_undecorated_output);
142 
143 /* Additional functions that go with destination management, more or less. */
144 
145 extern void NullCommand(CommandDest dest);
146 extern void ReadyForQuery(CommandDest dest);
147 
148 #endif /* DEST_H */
bool(* receiveSlot)(TupleTableSlot *slot, DestReceiver *self)
Definition: dest.h:119
CommandTag
Definition: cmdtag.h:20
CommandDest
Definition: dest.h:87
Definition: dest.h:94
CommandDest mydest
Definition: dest.h:129
void NullCommand(CommandDest dest)
Definition: dest.c:227
#define PGDLLIMPORT
Definition: c.h:1257
void BeginCommand(CommandTag commandTag, CommandDest dest)
Definition: dest.c:103
void ReadyForQuery(CommandDest dest)
Definition: dest.c:270
void EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_output)
Definition: dest.c:166
void(* rStartup)(DestReceiver *self, int operation, TupleDesc typeinfo)
Definition: dest.h:122
Definition: dest.h:89
Definition: dest.h:90
void(* rDestroy)(DestReceiver *self)
Definition: dest.h:127
void(* rShutdown)(DestReceiver *self)
Definition: dest.h:125
DestReceiver * CreateDestReceiver(CommandDest dest)
Definition: dest.c:113
PGDLLIMPORT DestReceiver * None_Receiver
Definition: dest.c:96
unsigned char bool
Definition: c.h:324