datum.c

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/*-------------------------------------------------------------------------
 *
 * datum.c
 *    POSTGRES Datum (abstract data type) manipulation routines.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/adt/datum.c
 *
 *-------------------------------------------------------------------------
 */

/*
 * In the implementation of these routines we assume the following:
 *
 * A) if a type is "byVal" then all the information is stored in the
 * Datum itself (i.e. no pointers involved!). In this case the
 * length of the type is always greater than zero and not more than
 * "sizeof(Datum)"
 *
 * B) if a type is not "byVal" and it has a fixed length (typlen > 0),
 * then the "Datum" always contains a pointer to a stream of bytes.
 * The number of significant bytes are always equal to the typlen.
 *
 * C) if a type is not "byVal" and has typlen == -1,
 * then the "Datum" always points to a "struct varlena".
 * This varlena structure has information about the actual length of this
 * particular instance of the type and about its value.
 *
 * D) if a type is not "byVal" and has typlen == -2,
 * then the "Datum" always points to a null-terminated C string.
 *
 * Note that we do not treat "toasted" datums specially; therefore what
 * will be copied or compared is the compressed data or toast reference.
 * An exception is made for datumCopy() of an expanded object, however,
 * because most callers expect to get a simple contiguous (and pfree'able)
 * result from datumCopy().  See also datumTransfer().
 */

#include "postgres.h"

#include "utils/datum.h"
#include "utils/expandeddatum.h"


/*-------------------------------------------------------------------------
 * datumGetSize
 *
 * Find the "real" size of a datum, given the datum value,
 * whether it is a "by value", and the declared type length.
 * (For TOAST pointer datums, this is the size of the pointer datum.)
 *
 * This is essentially an out-of-line version of the att_addlength_datum()
 * macro in access/tupmacs.h.  We do a tad more error checking though.
 *-------------------------------------------------------------------------
 */
Size
datumGetSize(Datum value, bool typByVal, int typLen)
{
    Size        size;

    if (typByVal)
    {
        /* Pass-by-value types are always fixed-length */
        Assert(typLen > 0 && typLen <= sizeof(Datum));
        size = (Size) typLen;
    }
    else
    {
        if (typLen > 0)
        {
            /* Fixed-length pass-by-ref type */
            size = (Size) typLen;
        }
        else if (typLen == -1)
        {
            /* It is a varlena datatype */
            struct varlena *s = (struct varlena *) DatumGetPointer(value);

            if (!PointerIsValid(s))
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_EXCEPTION),
                         errmsg("invalid Datum pointer")));

            size = (Size) VARSIZE_ANY(s);
        }
        else if (typLen == -2)
        {
            /* It is a cstring datatype */
            char       *s = (char *) DatumGetPointer(value);

            if (!PointerIsValid(s))
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_EXCEPTION),
                         errmsg("invalid Datum pointer")));

            size = (Size) (strlen(s) + 1);
        }
        else
        {
            elog(ERROR, "invalid typLen: %d", typLen);
            size = 0;           /* keep compiler quiet */
        }
    }

    return size;
}

/*-------------------------------------------------------------------------
 * datumCopy
 *
 * Make a copy of a non-NULL datum.
 *
 * If the datatype is pass-by-reference, memory is obtained with palloc().
 *
 * If the value is a reference to an expanded object, we flatten into memory
 * obtained with palloc().  We need to copy because one of the main uses of
 * this function is to copy a datum out of a transient memory context that's
 * about to be destroyed, and the expanded object is probably in a child
 * context that will also go away.  Moreover, many callers assume that the
 * result is a single pfree-able chunk.
 *-------------------------------------------------------------------------
 */
Datum
datumCopy(Datum value, bool typByVal, int typLen)
{
    Datum       res;

    if (typByVal)
        res = value;
    else if (typLen == -1)
    {
        /* It is a varlena datatype */
        struct varlena *vl = (struct varlena *) DatumGetPointer(value);

        if (VARATT_IS_EXTERNAL_EXPANDED(vl))
        {
            /* Flatten into the caller's memory context */
            ExpandedObjectHeader *eoh = DatumGetEOHP(value);
            Size        resultsize;
            char       *resultptr;

            resultsize = EOH_get_flat_size(eoh);
            resultptr = (char *) palloc(resultsize);
            EOH_flatten_into(eoh, (void *) resultptr, resultsize);
            res = PointerGetDatum(resultptr);
        }
        else
        {
            /* Otherwise, just copy the varlena datum verbatim */
            Size        realSize;
            char       *resultptr;

            realSize = (Size) VARSIZE_ANY(vl);
            resultptr = (char *) palloc(realSize);
            memcpy(resultptr, vl, realSize);
            res = PointerGetDatum(resultptr);
        }
    }
    else
    {
        /* Pass by reference, but not varlena, so not toasted */
        Size        realSize;
        char       *resultptr;

        realSize = datumGetSize(value, typByVal, typLen);

        resultptr = (char *) palloc(realSize);
        memcpy(resultptr, DatumGetPointer(value), realSize);
        res = PointerGetDatum(resultptr);
    }
    return res;
}

/*-------------------------------------------------------------------------
 * datumTransfer
 *
 * Transfer a non-NULL datum into the current memory context.
 *
 * This is equivalent to datumCopy() except when the datum is a read-write
 * pointer to an expanded object.  In that case we merely reparent the object
 * into the current context, and return its standard R/W pointer (in case the
 * given one is a transient pointer of shorter lifespan).
 *-------------------------------------------------------------------------
 */
Datum
datumTransfer(Datum value, bool typByVal, int typLen)
{
    if (!typByVal && typLen == -1 &&
        VARATT_IS_EXTERNAL_EXPANDED_RW(DatumGetPointer(value)))
        value = TransferExpandedObject(value, CurrentMemoryContext);
    else
        value = datumCopy(value, typByVal, typLen);
    return value;
}

/*-------------------------------------------------------------------------
 * datumIsEqual
 *
 * Return true if two datums are equal, false otherwise
 *
 * NOTE: XXX!
 * We just compare the bytes of the two values, one by one.
 * This routine will return false if there are 2 different
 * representations of the same value (something along the lines
 * of say the representation of zero in one's complement arithmetic).
 * Also, it will probably not give the answer you want if either
 * datum has been "toasted".
 *
 * Do not try to make this any smarter than it currently is with respect
 * to "toasted" datums, because some of the callers could be working in the
 * context of an aborted transaction.
 *-------------------------------------------------------------------------
 */
bool
datumIsEqual(Datum value1, Datum value2, bool typByVal, int typLen)
{
    bool        res;

    if (typByVal)
    {
        /*
         * just compare the two datums. NOTE: just comparing "len" bytes will
         * not do the work, because we do not know how these bytes are aligned
         * inside the "Datum".  We assume instead that any given datatype is
         * consistent about how it fills extraneous bits in the Datum.
         */
        res = (value1 == value2);
    }
    else
    {
        Size        size1,
                    size2;
        char       *s1,
                   *s2;

        /*
         * Compare the bytes pointed by the pointers stored in the datums.
         */
        size1 = datumGetSize(value1, typByVal, typLen);
        size2 = datumGetSize(value2, typByVal, typLen);
        if (size1 != size2)
            return false;
        s1 = (char *) DatumGetPointer(value1);
        s2 = (char *) DatumGetPointer(value2);
        res = (memcmp(s1, s2, size1) == 0);
    }
    return res;
}

/*-------------------------------------------------------------------------
 * datumEstimateSpace
 *
 * Compute the amount of space that datumSerialize will require for a
 * particular Datum.
 *-------------------------------------------------------------------------
 */
Size
datumEstimateSpace(Datum value, bool isnull, bool typByVal, int typLen)
{
    Size        sz = sizeof(int);

    if (!isnull)
    {
        /* no need to use add_size, can't overflow */
        if (typByVal)
            sz += sizeof(Datum);
        else if (typLen == -1 &&
                 VARATT_IS_EXTERNAL_EXPANDED(DatumGetPointer(value)))
        {
            /* Expanded objects need to be flattened, see comment below */
            sz += EOH_get_flat_size(DatumGetEOHP(value));
        }
        else
            sz += datumGetSize(value, typByVal, typLen);
    }

    return sz;
}

/*-------------------------------------------------------------------------
 * datumSerialize
 *
 * Serialize a possibly-NULL datum into caller-provided storage.
 *
 * Note: "expanded" objects are flattened so as to produce a self-contained
 * representation, but other sorts of toast pointers are transferred as-is.
 * This is because the intended use of this function is to pass the value
 * to another process within the same database server.  The other process
 * could not access an "expanded" object within this process's memory, but
 * we assume it can dereference the same TOAST pointers this one can.
 *
 * The format is as follows: first, we write a 4-byte header word, which
 * is either the length of a pass-by-reference datum, -1 for a
 * pass-by-value datum, or -2 for a NULL.  If the value is NULL, nothing
 * further is written.  If it is pass-by-value, sizeof(Datum) bytes
 * follow.  Otherwise, the number of bytes indicated by the header word
 * follow.  The caller is responsible for ensuring that there is enough
 * storage to store the number of bytes that will be written; use
 * datumEstimateSpace() to find out how many will be needed.
 * *start_address is updated to point to the byte immediately following
 * those written.
 *-------------------------------------------------------------------------
 */
void
datumSerialize(Datum value, bool isnull, bool typByVal, int typLen,
               char **start_address)
{
    ExpandedObjectHeader *eoh = NULL;
    int         header;

    /* Write header word. */
    if (isnull)
        header = -2;
    else if (typByVal)
        header = -1;
    else if (typLen == -1 &&
             VARATT_IS_EXTERNAL_EXPANDED(DatumGetPointer(value)))
    {
        eoh = DatumGetEOHP(value);
        header = EOH_get_flat_size(eoh);
    }
    else
        header = datumGetSize(value, typByVal, typLen);
    memcpy(*start_address, &header, sizeof(int));
    *start_address += sizeof(int);

    /* If not null, write payload bytes. */
    if (!isnull)
    {
        if (typByVal)
        {
            memcpy(*start_address, &value, sizeof(Datum));
            *start_address += sizeof(Datum);
        }
        else if (eoh)
        {
            EOH_flatten_into(eoh, (void *) *start_address, header);
            *start_address += header;
        }
        else
        {
            memcpy(*start_address, DatumGetPointer(value), header);
            *start_address += header;
        }
    }
}

/*-------------------------------------------------------------------------
 * datumRestore
 *
 * Restore a possibly-NULL datum previously serialized by datumSerialize.
 * *start_address is updated according to the number of bytes consumed.
 *-------------------------------------------------------------------------
 */
Datum
datumRestore(char **start_address, bool *isnull)
{
    int         header;
    void       *d;

    /* Read header word. */
    memcpy(&header, *start_address, sizeof(int));
    *start_address += sizeof(int);

    /* If this datum is NULL, we can stop here. */
    if (header == -2)
    {
        *isnull = true;
        return (Datum) 0;
    }

    /* OK, datum is not null. */
    *isnull = false;

    /* If this datum is pass-by-value, sizeof(Datum) bytes follow. */
    if (header == -1)
    {
        Datum       val;

        memcpy(&val, *start_address, sizeof(Datum));
        *start_address += sizeof(Datum);
        return val;
    }

    /* Pass-by-reference case; copy indicated number of bytes. */
    Assert(header > 0);
    d = palloc(header);
    memcpy(d, *start_address, header);
    *start_address += header;
    return PointerGetDatum(d);
}