1 /*-------------------------------------------------------------------------
4 * support for communication destinations
6 * Whenever the backend executes a query that returns tuples, the results
7 * have to go someplace. For example:
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.
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.
18 * - DestNone is the destination when the system executes
19 * a query internally. The results are discarded.
21 * dest.c defines three functions that implement destination management:
23 * BeginCommand: initialize the destination at start of command.
24 * CreateDestReceiver: return a pointer to a struct of destination-specific
26 * EndCommand: clean up the destination at end of command.
28 * BeginCommand/EndCommand are executed once per received SQL query.
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.
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
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.
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.
60 * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
61 * Portions Copyright (c) 1994, Regents of the University of California
63 * src/include/tcop/dest.h
65 *-------------------------------------------------------------------------
70 #include "executor/tuptable.h"
73 /* buffer size to use for command completion tags */
74 #define COMPLETION_TAG_BUFSIZE 64
78 * CommandDest is a simplistic means of identifying the desired
79 * destination. Someday this will probably need to be improved.
81 * Note: only the values DestNone, DestDebug, DestRemote are legal for the
82 * global variable whereToSendOutput. The other values may be used
83 * as the destination for individual commands.
88 DestNone, /* results are discarded */
89 DestDebug, /* results go to debugging output */
90 DestRemote, /* results sent to frontend process */
91 DestRemoteExecute, /* sent to frontend, in Execute command */
92 DestSPI, /* results sent to SPI manager */
93 DestTuplestore, /* results sent to Tuplestore */
94 DestIntoRel, /* results sent to relation (SELECT INTO) */
95 DestCopyOut, /* results sent to COPY TO code */
96 DestSQLFunction, /* results sent to SQL-language func mgr */
97 DestTransientRel /* results sent to transient relation */
101 * DestReceiver is a base type for destination-specific local state.
102 * In the simplest cases, there is no state info, just the function
103 * pointers that the executor must call.
105 * Note: the receiveSlot routine must be passed a slot containing a TupleDesc
106 * identical to the one given to the rStartup routine.
109 typedef struct _DestReceiver DestReceiver;
113 /* Called for each tuple to be output: */
114 void (*receiveSlot) (TupleTableSlot *slot,
116 /* Per-executor-run initialization and shutdown: */
117 void (*rStartup) (DestReceiver *self,
120 void (*rShutdown) (DestReceiver *self);
121 /* Destroy the receiver object itself (if dynamically allocated) */
122 void (*rDestroy) (DestReceiver *self);
123 /* CommandDest code for this receiver */
125 /* Private fields might appear beyond this point... */
128 extern DestReceiver *None_Receiver; /* permanent receiver for DestNone */
130 /* The primary destination management functions */
132 extern void BeginCommand(const char *commandTag, CommandDest dest);
133 extern DestReceiver *CreateDestReceiver(CommandDest dest);
134 extern void EndCommand(const char *commandTag, CommandDest dest);
136 /* Additional functions that go with destination management, more or less. */
138 extern void NullCommand(CommandDest dest);
139 extern void ReadyForQuery(CommandDest dest);