tuptable.h

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/*-------------------------------------------------------------------------
 *
 * tuptable.h
 *    tuple table support stuff
 *
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/executor/tuptable.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef TUPTABLE_H
#define TUPTABLE_H

#include "access/htup.h"
#include "access/tupdesc.h"
#include "storage/buf.h"

/*----------
 * The executor stores tuples in a "tuple table" which is a List of
 * independent TupleTableSlots.  There are several cases we need to handle:
 *      1. physical tuple in a disk buffer page
 *      2. physical tuple constructed in palloc'ed memory
 *      3. "minimal" physical tuple constructed in palloc'ed memory
 *      4. "virtual" tuple consisting of Datum/isnull arrays
 *
 * The first two cases are similar in that they both deal with "materialized"
 * tuples, but resource management is different.  For a tuple in a disk page
 * we need to hold a pin on the buffer until the TupleTableSlot's reference
 * to the tuple is dropped; while for a palloc'd tuple we usually want the
 * tuple pfree'd when the TupleTableSlot's reference is dropped.
 *
 * A "minimal" tuple is handled similarly to a palloc'd regular tuple.
 * At present, minimal tuples never are stored in buffers, so there is no
 * parallel to case 1.  Note that a minimal tuple has no "system columns".
 * (Actually, it could have an OID, but we have no need to access the OID.)
 *
 * A "virtual" tuple is an optimization used to minimize physical data
 * copying in a nest of plan nodes.  Any pass-by-reference Datums in the
 * tuple point to storage that is not directly associated with the
 * TupleTableSlot; generally they will point to part of a tuple stored in
 * a lower plan node's output TupleTableSlot, or to a function result
 * constructed in a plan node's per-tuple econtext.  It is the responsibility
 * of the generating plan node to be sure these resources are not released
 * for as long as the virtual tuple needs to be valid.  We only use virtual
 * tuples in the result slots of plan nodes --- tuples to be copied anywhere
 * else need to be "materialized" into physical tuples.  Note also that a
 * virtual tuple does not have any "system columns".
 *
 * It is also possible for a TupleTableSlot to hold both physical and minimal
 * copies of a tuple.  This is done when the slot is requested to provide
 * the format other than the one it currently holds.  (Originally we attempted
 * to handle such requests by replacing one format with the other, but that
 * had the fatal defect of invalidating any pass-by-reference Datums pointing
 * into the existing slot contents.)  Both copies must contain identical data
 * payloads when this is the case.
 *
 * The Datum/isnull arrays of a TupleTableSlot serve double duty.  When the
 * slot contains a virtual tuple, they are the authoritative data.  When the
 * slot contains a physical tuple, the arrays contain data extracted from
 * the tuple.  (In this state, any pass-by-reference Datums point into
 * the physical tuple.)  The extracted information is built "lazily",
 * ie, only as needed.  This serves to avoid repeated extraction of data
 * from the physical tuple.
 *
 * A TupleTableSlot can also be "empty", holding no valid data.  This is
 * the only valid state for a freshly-created slot that has not yet had a
 * tuple descriptor assigned to it.  In this state, tts_isempty must be
 * TRUE, tts_shouldFree FALSE, tts_tuple NULL, tts_buffer InvalidBuffer,
 * and tts_nvalid zero.
 *
 * The tupleDescriptor is simply referenced, not copied, by the TupleTableSlot
 * code.  The caller of ExecSetSlotDescriptor() is responsible for providing
 * a descriptor that will live as long as the slot does.  (Typically, both
 * slots and descriptors are in per-query memory and are freed by memory
 * context deallocation at query end; so it's not worth providing any extra
 * mechanism to do more.  However, the slot will increment the tupdesc
 * reference count if a reference-counted tupdesc is supplied.)
 *
 * When tts_shouldFree is true, the physical tuple is "owned" by the slot
 * and should be freed when the slot's reference to the tuple is dropped.
 *
 * If tts_buffer is not InvalidBuffer, then the slot is holding a pin
 * on the indicated buffer page; drop the pin when we release the
 * slot's reference to that buffer.  (tts_shouldFree should always be
 * false in such a case, since presumably tts_tuple is pointing at the
 * buffer page.)
 *
 * tts_nvalid indicates the number of valid columns in the tts_values/isnull
 * arrays.  When the slot is holding a "virtual" tuple this must be equal
 * to the descriptor's natts.  When the slot is holding a physical tuple
 * this is equal to the number of columns we have extracted (we always
 * extract columns from left to right, so there are no holes).
 *
 * tts_values/tts_isnull are allocated when a descriptor is assigned to the
 * slot; they are of length equal to the descriptor's natts.
 *
 * tts_mintuple must always be NULL if the slot does not hold a "minimal"
 * tuple.  When it does, tts_mintuple points to the actual MinimalTupleData
 * object (the thing to be pfree'd if tts_shouldFreeMin is true).  If the slot
 * has only a minimal and not also a regular physical tuple, then tts_tuple
 * points at tts_minhdr and the fields of that struct are set correctly
 * for access to the minimal tuple; in particular, tts_minhdr.t_data points
 * MINIMAL_TUPLE_OFFSET bytes before tts_mintuple.  This allows column
 * extraction to treat the case identically to regular physical tuples.
 *
 * tts_slow/tts_off are saved state for slot_deform_tuple, and should not
 * be touched by any other code.
 *----------
 */
typedef struct TupleTableSlot
{
    NodeTag     type;
    bool        tts_isempty;    /* true = slot is empty */
    bool        tts_shouldFree; /* should pfree tts_tuple? */
    bool        tts_shouldFreeMin;  /* should pfree tts_mintuple? */
    bool        tts_slow;       /* saved state for slot_deform_tuple */
    HeapTuple   tts_tuple;      /* physical tuple, or NULL if virtual */
    TupleDesc   tts_tupleDescriptor;    /* slot's tuple descriptor */
    MemoryContext tts_mcxt;     /* slot itself is in this context */
    Buffer      tts_buffer;     /* tuple's buffer, or InvalidBuffer */
    int         tts_nvalid;     /* # of valid values in tts_values */
    Datum      *tts_values;     /* current per-attribute values */
    bool       *tts_isnull;     /* current per-attribute isnull flags */
    MinimalTuple tts_mintuple;  /* minimal tuple, or NULL if none */
    HeapTupleData tts_minhdr;   /* workspace for minimal-tuple-only case */
    long        tts_off;        /* saved state for slot_deform_tuple */
} TupleTableSlot;

#define TTS_HAS_PHYSICAL_TUPLE(slot)  \
    ((slot)->tts_tuple != NULL && (slot)->tts_tuple != &((slot)->tts_minhdr))

/*
 * TupIsNull -- is a TupleTableSlot empty?
 */
#define TupIsNull(slot) \
    ((slot) == NULL || (slot)->tts_isempty)

/* in executor/execTuples.c */
extern TupleTableSlot *MakeTupleTableSlot(void);
extern TupleTableSlot *ExecAllocTableSlot(List **tupleTable);
extern void ExecResetTupleTable(List *tupleTable, bool shouldFree);
extern TupleTableSlot *MakeSingleTupleTableSlot(TupleDesc tupdesc);
extern void ExecDropSingleTupleTableSlot(TupleTableSlot *slot);
extern void ExecSetSlotDescriptor(TupleTableSlot *slot, TupleDesc tupdesc);
extern TupleTableSlot *ExecStoreTuple(HeapTuple tuple,
               TupleTableSlot *slot,
               Buffer buffer,
               bool shouldFree);
extern TupleTableSlot *ExecStoreMinimalTuple(MinimalTuple mtup,
                      TupleTableSlot *slot,
                      bool shouldFree);
extern TupleTableSlot *ExecClearTuple(TupleTableSlot *slot);
extern TupleTableSlot *ExecStoreVirtualTuple(TupleTableSlot *slot);
extern TupleTableSlot *ExecStoreAllNullTuple(TupleTableSlot *slot);
extern HeapTuple ExecCopySlotTuple(TupleTableSlot *slot);
extern MinimalTuple ExecCopySlotMinimalTuple(TupleTableSlot *slot);
extern HeapTuple ExecFetchSlotTuple(TupleTableSlot *slot);
extern MinimalTuple ExecFetchSlotMinimalTuple(TupleTableSlot *slot);
extern Datum ExecFetchSlotTupleDatum(TupleTableSlot *slot);
extern HeapTuple ExecMaterializeSlot(TupleTableSlot *slot);
extern TupleTableSlot *ExecCopySlot(TupleTableSlot *dstslot,
             TupleTableSlot *srcslot);

/* in access/common/heaptuple.c */
extern Datum slot_getattr(TupleTableSlot *slot, int attnum, bool *isnull);
extern void slot_getallattrs(TupleTableSlot *slot);
extern void slot_getsomeattrs(TupleTableSlot *slot, int attnum);
extern bool slot_attisnull(TupleTableSlot *slot, int attnum);

#endif                          /* TUPTABLE_H */