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/*-------------------------------------------------------------------------
*
* dest.h
* support for communication destinations
*
* Whenever the backend executes a query that returns tuples, the results
* have to go someplace. For example:
*
* - stdout is the destination only when we are running a
* standalone backend (no postmaster) and are returning results
* back to an interactive user.
*
* - a remote process is the destination when we are
* running a backend with a frontend and the frontend executes
* PQexec() or PQfn(). In this case, the results are sent
* to the frontend via the functions in backend/libpq.
*
* - DestNone is the destination when the system executes
* a query internally. The results are discarded.
*
* dest.c defines three functions that implement destination management:
*
* BeginCommand: initialize the destination at start of command.
* CreateDestReceiver: return a pointer to a struct of destination-specific
* receiver functions.
* EndCommand: clean up the destination at end of command.
*
* BeginCommand/EndCommand are executed once per received SQL query.
*
* CreateDestReceiver returns a receiver object appropriate to the specified
* destination. The executor, as well as utility statements that can return
* tuples, are passed the resulting DestReceiver* pointer. Each executor run
* or utility execution calls the receiver's rStartup method, then the
* receiveSlot method (zero or more times), then the rShutdown method.
* The same receiver object may be re-used multiple times; eventually it is
* destroyed by calling its rDestroy method.
*
* In some cases, receiver objects require additional parameters that must
* be passed to them after calling CreateDestReceiver. Since the set of
* parameters varies for different receiver types, this is not handled by
* this module, but by direct calls from the calling code to receiver type
* specific functions.
*
* The DestReceiver object returned by CreateDestReceiver may be a statically
* allocated object (for destination types that require no local state),
* in which case rDestroy is a no-op. Alternatively it can be a palloc'd
* object that has DestReceiver as its first field and contains additional
* fields (see printtup.c for an example). These additional fields are then
* accessible to the DestReceiver functions by casting the DestReceiver*
* pointer passed to them. The palloc'd object is pfree'd by the rDestroy
* method. Note that the caller of CreateDestReceiver should take care to
* do so in a memory context that is long-lived enough for the receiver
* object not to disappear while still needed.
*
* Special provision: None_Receiver is a permanently available receiver
* object for the DestNone destination. This avoids useless creation/destroy
* calls in portal and cursor manipulations.
*
*
* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/tcop/dest.h
*
*-------------------------------------------------------------------------
*/
#ifndef DEST_H
#define DEST_H
#include "executor/tuptable.h"
#include "tcop/cmdtag.h"
/* buffer size to use for command completion tags */
#define COMPLETION_TAG_BUFSIZE 64
/* ----------------
* CommandDest is a simplistic means of identifying the desired
* destination. Someday this will probably need to be improved.
*
* Note: only the values DestNone, DestDebug, DestRemote are legal for the
* global variable whereToSendOutput. The other values may be used
* as the destination for individual commands.
* ----------------
*/
typedef enum
{
DestNone, /* results are discarded */
DestDebug, /* results go to debugging output */
DestRemote, /* results sent to frontend process */
DestRemoteExecute, /* sent to frontend, in Execute command */
DestRemoteSimple, /* sent to frontend, w/no catalog access */
DestSPI, /* results sent to SPI manager */
DestTuplestore, /* results sent to Tuplestore */
DestIntoRel, /* results sent to relation (SELECT INTO) */
DestCopyOut, /* results sent to COPY TO code */
DestSQLFunction, /* results sent to SQL-language func mgr */
DestTransientRel, /* results sent to transient relation */
DestTupleQueue /* results sent to tuple queue */
} CommandDest;
/* ----------------
* DestReceiver is a base type for destination-specific local state.
* In the simplest cases, there is no state info, just the function
* pointers that the executor must call.
*
* Note: the receiveSlot routine must be passed a slot containing a TupleDesc
* identical to the one given to the rStartup routine. It returns bool where
* a "true" value means "continue processing" and a "false" value means
* "stop early, just as if we'd reached the end of the scan".
* ----------------
*/
typedef struct _DestReceiver DestReceiver;
struct _DestReceiver
{
/* Called for each tuple to be output: */
bool (*receiveSlot) (TupleTableSlot *slot,
DestReceiver *self);
/* Per-executor-run initialization and shutdown: */
void (*rStartup) (DestReceiver *self,
int operation,
TupleDesc typeinfo);
void (*rShutdown) (DestReceiver *self);
/* Destroy the receiver object itself (if dynamically allocated) */
void (*rDestroy) (DestReceiver *self);
/* CommandDest code for this receiver */
CommandDest mydest;
/* Private fields might appear beyond this point... */
};
extern PGDLLIMPORT DestReceiver *None_Receiver; /* permanent receiver for
* DestNone */
/* The primary destination management functions */
extern void BeginCommand(CommandTag commandTag, CommandDest dest);
extern DestReceiver *CreateDestReceiver(CommandDest dest);
extern void EndCommand(const QueryCompletion *qc, CommandDest dest,
bool force_undecorated_output);
extern void EndReplicationCommand(const char *commandTag);
/* Additional functions that go with destination management, more or less. */
extern void NullCommand(CommandDest dest);
extern void ReadyForQuery(CommandDest dest);
#endif /* DEST_H */
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