ginpostinglist.c

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/*-------------------------------------------------------------------------
 *
 * ginpostinglist.c
 *    routines for dealing with posting lists.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *          src/backend/access/gin/ginpostinglist.c
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/gin_private.h"

#ifdef USE_ASSERT_CHECKING
#define CHECK_ENCODING_ROUNDTRIP
#endif

/*
 * For encoding purposes, item pointers are represented as 64-bit unsigned
 * integers. The lowest 11 bits represent the offset number, and the next
 * lowest 32 bits are the block number. That leaves 21 bits unused, i.e.
 * only 43 low bits are used.
 *
 * 11 bits is enough for the offset number, because MaxHeapTuplesPerPage <
 * 2^11 on all supported block sizes. We are frugal with the bits, because
 * smaller integers use fewer bytes in the varbyte encoding, saving disk
 * space. (If we get a new table AM in the future that wants to use the full
 * range of possible offset numbers, we'll need to change this.)
 *
 * These 43-bit integers are encoded using varbyte encoding. In each byte,
 * the 7 low bits contain data, while the highest bit is a continuation bit.
 * When the continuation bit is set, the next byte is part of the same
 * integer, otherwise this is the last byte of this integer. 43 bits need
 * at most 7 bytes in this encoding:
 *
 * 0XXXXXXX
 * 1XXXXXXX 0XXXXYYY
 * 1XXXXXXX 1XXXXYYY 0YYYYYYY
 * 1XXXXXXX 1XXXXYYY 1YYYYYYY 0YYYYYYY
 * 1XXXXXXX 1XXXXYYY 1YYYYYYY 1YYYYYYY 0YYYYYYY
 * 1XXXXXXX 1XXXXYYY 1YYYYYYY 1YYYYYYY 1YYYYYYY 0YYYYYYY
 * 1XXXXXXX 1XXXXYYY 1YYYYYYY 1YYYYYYY 1YYYYYYY 1YYYYYYY 0uuuuuuY
 *
 * X = bits used for offset number
 * Y = bits used for block number
 * u = unused bit
 *
 * The bytes are in stored in little-endian order.
 *
 * An important property of this encoding is that removing an item from list
 * never increases the size of the resulting compressed posting list. Proof:
 *
 * Removing number is actually replacement of two numbers with their sum. We
 * have to prove that varbyte encoding of a sum can't be longer than varbyte
 * encoding of its summands. Sum of two numbers is at most one bit wider than
 * the larger of the summands. Widening a number by one bit enlarges its length
 * in varbyte encoding by at most one byte. Therefore, varbyte encoding of sum
 * is at most one byte longer than varbyte encoding of larger summand. Lesser
 * summand is at least one byte, so the sum cannot take more space than the
 * summands, Q.E.D.
 *
 * This property greatly simplifies VACUUM, which can assume that posting
 * lists always fit on the same page after vacuuming. Note that even though
 * that holds for removing items from a posting list, you must also be
 * careful to not cause expansion e.g. when merging uncompressed items on the
 * page into the compressed lists, when vacuuming.
 */

/*
 * How many bits do you need to encode offset number? OffsetNumber is a 16-bit
 * integer, but you can't fit that many items on a page. 11 ought to be more
 * than enough. It's tempting to derive this from MaxHeapTuplesPerPage, and
 * use the minimum number of bits, but that would require changing the on-disk
 * format if MaxHeapTuplesPerPage changes. Better to leave some slack.
 */
#define MaxHeapTuplesPerPageBits        11

/* Max. number of bytes needed to encode the largest supported integer. */
#define MaxBytesPerInteger              7

static inline uint64
itemptr_to_uint64(const ItemPointer iptr)
{
    uint64      val;

    Assert(ItemPointerIsValid(iptr));
    Assert(GinItemPointerGetOffsetNumber(iptr) < (1 << MaxHeapTuplesPerPageBits));

    val = GinItemPointerGetBlockNumber(iptr);
    val <<= MaxHeapTuplesPerPageBits;
    val |= GinItemPointerGetOffsetNumber(iptr);

    return val;
}

static inline void
uint64_to_itemptr(uint64 val, ItemPointer iptr)
{
    GinItemPointerSetOffsetNumber(iptr, val & ((1 << MaxHeapTuplesPerPageBits) - 1));
    val = val >> MaxHeapTuplesPerPageBits;
    GinItemPointerSetBlockNumber(iptr, val);

    Assert(ItemPointerIsValid(iptr));
}

/*
 * Varbyte-encode 'val' into *ptr. *ptr is incremented to next integer.
 */
static void
encode_varbyte(uint64 val, unsigned char **ptr)
{
    unsigned char *p = *ptr;

    while (val > 0x7F)
    {
        *(p++) = 0x80 | (val & 0x7F);
        val >>= 7;
    }
    *(p++) = (unsigned char) val;

    *ptr = p;
}

/*
 * Decode varbyte-encoded integer at *ptr. *ptr is incremented to next integer.
 */
static uint64
decode_varbyte(unsigned char **ptr)
{
    uint64      val;
    unsigned char *p = *ptr;
    uint64      c;

    /* 1st byte */
    c = *(p++);
    val = c & 0x7F;
    if (c & 0x80)
    {
        /* 2nd byte */
        c = *(p++);
        val |= (c & 0x7F) << 7;
        if (c & 0x80)
        {
            /* 3rd byte */
            c = *(p++);
            val |= (c & 0x7F) << 14;
            if (c & 0x80)
            {
                /* 4th byte */
                c = *(p++);
                val |= (c & 0x7F) << 21;
                if (c & 0x80)
                {
                    /* 5th byte */
                    c = *(p++);
                    val |= (c & 0x7F) << 28;
                    if (c & 0x80)
                    {
                        /* 6th byte */
                        c = *(p++);
                        val |= (c & 0x7F) << 35;
                        if (c & 0x80)
                        {
                            /* 7th byte, should not have continuation bit */
                            c = *(p++);
                            val |= c << 42;
                            Assert((c & 0x80) == 0);
                        }
                    }
                }
            }
        }
    }

    *ptr = p;

    return val;
}

/*
 * Encode a posting list.
 *
 * The encoded list is returned in a palloc'd struct, which will be at most
 * 'maxsize' bytes in size.  The number items in the returned segment is
 * returned in *nwritten. If it's not equal to nipd, not all the items fit
 * in 'maxsize', and only the first *nwritten were encoded.
 *
 * The allocated size of the returned struct is short-aligned, and the padding
 * byte at the end, if any, is zero.
 */
GinPostingList *
ginCompressPostingList(const ItemPointer ipd, int nipd, int maxsize,
                       int *nwritten)
{
    uint64      prev;
    int         totalpacked = 0;
    int         maxbytes;
    GinPostingList *result;
    unsigned char *ptr;
    unsigned char *endptr;

    maxsize = SHORTALIGN_DOWN(maxsize);

    result = palloc(maxsize);

    maxbytes = maxsize - offsetof(GinPostingList, bytes);
    Assert(maxbytes > 0);

    /* Store the first special item */
    result->first = ipd[0];

    prev = itemptr_to_uint64(&result->first);

    ptr = result->bytes;
    endptr = result->bytes + maxbytes;
    for (totalpacked = 1; totalpacked < nipd; totalpacked++)
    {
        uint64      val = itemptr_to_uint64(&ipd[totalpacked]);
        uint64      delta = val - prev;

        Assert(val > prev);

        if (endptr - ptr >= MaxBytesPerInteger)
            encode_varbyte(delta, &ptr);
        else
        {
            /*
             * There are less than 7 bytes left. Have to check if the next
             * item fits in that space before writing it out.
             */
            unsigned char buf[MaxBytesPerInteger];
            unsigned char *p = buf;

            encode_varbyte(delta, &p);
            if (p - buf > (endptr - ptr))
                break;          /* output is full */

            memcpy(ptr, buf, p - buf);
            ptr += (p - buf);
        }
        prev = val;
    }
    result->nbytes = ptr - result->bytes;

    /*
     * If we wrote an odd number of bytes, zero out the padding byte at the
     * end.
     */
    if (result->nbytes != SHORTALIGN(result->nbytes))
        result->bytes[result->nbytes] = 0;

    if (nwritten)
        *nwritten = totalpacked;

    Assert(SizeOfGinPostingList(result) <= maxsize);

    /*
     * Check that the encoded segment decodes back to the original items.
     */
#if defined (CHECK_ENCODING_ROUNDTRIP)
    {
        int         ndecoded;
        ItemPointer tmp = ginPostingListDecode(result, &ndecoded);

        Assert(ndecoded == totalpacked);
        Assert(memcmp(tmp, ipd, ndecoded * sizeof(ItemPointerData)) == 0);
        pfree(tmp);
    }
#endif

    return result;
}

/*
 * Decode a compressed posting list into an array of item pointers.
 * The number of items is returned in *ndecoded.
 */
ItemPointer
ginPostingListDecode(GinPostingList *plist, int *ndecoded)
{
    return ginPostingListDecodeAllSegments(plist,
                                           SizeOfGinPostingList(plist),
                                           ndecoded);
}

/*
 * Decode multiple posting list segments into an array of item pointers.
 * The number of items is returned in *ndecoded_out. The segments are stored
 * one after each other, with total size 'len' bytes.
 */
ItemPointer
ginPostingListDecodeAllSegments(GinPostingList *segment, int len, int *ndecoded_out)
{
    ItemPointer result;
    int         nallocated;
    uint64      val;
    char       *endseg = ((char *) segment) + len;
    int         ndecoded;
    unsigned char *ptr;
    unsigned char *endptr;

    /*
     * Guess an initial size of the array.
     */
    nallocated = segment->nbytes * 2 + 1;
    result = palloc(nallocated * sizeof(ItemPointerData));

    ndecoded = 0;
    while ((char *) segment < endseg)
    {
        /* enlarge output array if needed */
        if (ndecoded >= nallocated)
        {
            nallocated *= 2;
            result = repalloc(result, nallocated * sizeof(ItemPointerData));
        }

        /* copy the first item */
        Assert(OffsetNumberIsValid(ItemPointerGetOffsetNumber(&segment->first)));
        Assert(ndecoded == 0 || ginCompareItemPointers(&segment->first, &result[ndecoded - 1]) > 0);
        result[ndecoded] = segment->first;
        ndecoded++;

        val = itemptr_to_uint64(&segment->first);
        ptr = segment->bytes;
        endptr = segment->bytes + segment->nbytes;
        while (ptr < endptr)
        {
            /* enlarge output array if needed */
            if (ndecoded >= nallocated)
            {
                nallocated *= 2;
                result = repalloc(result, nallocated * sizeof(ItemPointerData));
            }

            val += decode_varbyte(&ptr);

            uint64_to_itemptr(val, &result[ndecoded]);
            ndecoded++;
        }
        segment = GinNextPostingListSegment(segment);
    }

    if (ndecoded_out)
        *ndecoded_out = ndecoded;
    return result;
}

/*
 * Add all item pointers from a bunch of posting lists to a TIDBitmap.
 */
int
ginPostingListDecodeAllSegmentsToTbm(GinPostingList *ptr, int len,
                                     TIDBitmap *tbm)
{
    int         ndecoded;
    ItemPointer items;

    items = ginPostingListDecodeAllSegments(ptr, len, &ndecoded);
    tbm_add_tuples(tbm, items, ndecoded, false);
    pfree(items);

    return ndecoded;
}

/*
 * Merge two ordered arrays of itempointers, eliminating any duplicates.
 *
 * Returns a palloc'd array, and *nmerged is set to the number of items in
 * the result, after eliminating duplicates.
 */
ItemPointer
ginMergeItemPointers(ItemPointerData *a, uint32 na,
                     ItemPointerData *b, uint32 nb,
                     int *nmerged)
{
    ItemPointerData *dst;

    dst = (ItemPointer) palloc((na + nb) * sizeof(ItemPointerData));

    /*
     * If the argument arrays don't overlap, we can just append them to each
     * other.
     */
    if (na == 0 || nb == 0 || ginCompareItemPointers(&a[na - 1], &b[0]) < 0)
    {
        memcpy(dst, a, na * sizeof(ItemPointerData));
        memcpy(&dst[na], b, nb * sizeof(ItemPointerData));
        *nmerged = na + nb;
    }
    else if (ginCompareItemPointers(&b[nb - 1], &a[0]) < 0)
    {
        memcpy(dst, b, nb * sizeof(ItemPointerData));
        memcpy(&dst[nb], a, na * sizeof(ItemPointerData));
        *nmerged = na + nb;
    }
    else
    {
        ItemPointerData *dptr = dst;
        ItemPointerData *aptr = a;
        ItemPointerData *bptr = b;

        while (aptr - a < na && bptr - b < nb)
        {
            int         cmp = ginCompareItemPointers(aptr, bptr);

            if (cmp > 0)
                *dptr++ = *bptr++;
            else if (cmp == 0)
            {
                /* only keep one copy of the identical items */
                *dptr++ = *bptr++;
                aptr++;
            }
            else
                *dptr++ = *aptr++;
        }

        while (aptr - a < na)
            *dptr++ = *aptr++;

        while (bptr - b < nb)
            *dptr++ = *bptr++;

        *nmerged = dptr - dst;
    }

    return dst;
}