gistbuildbuffers.c

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/*-------------------------------------------------------------------------
 *
 * gistbuildbuffers.c
 *    node buffer management functions for GiST buffering build algorithm.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/gist/gistbuildbuffers.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/gist_private.h"
#include "catalog/index.h"
#include "miscadmin.h"
#include "storage/buffile.h"
#include "storage/bufmgr.h"
#include "utils/memutils.h"
#include "utils/rel.h"

static GISTNodeBufferPage *gistAllocateNewPageBuffer(GISTBuildBuffers *gfbb);
static void gistAddLoadedBuffer(GISTBuildBuffers *gfbb,
                    GISTNodeBuffer *nodeBuffer);
static void gistLoadNodeBuffer(GISTBuildBuffers *gfbb,
                   GISTNodeBuffer *nodeBuffer);
static void gistUnloadNodeBuffer(GISTBuildBuffers *gfbb,
                     GISTNodeBuffer *nodeBuffer);
static void gistPlaceItupToPage(GISTNodeBufferPage *pageBuffer,
                    IndexTuple item);
static void gistGetItupFromPage(GISTNodeBufferPage *pageBuffer,
                    IndexTuple *item);
static long gistBuffersGetFreeBlock(GISTBuildBuffers *gfbb);
static void gistBuffersReleaseBlock(GISTBuildBuffers *gfbb, long blocknum);

static void ReadTempFileBlock(BufFile *file, long blknum, void *ptr);
static void WriteTempFileBlock(BufFile *file, long blknum, void *ptr);


/*
 * Initialize GiST build buffers.
 */
GISTBuildBuffers *
gistInitBuildBuffers(int pagesPerBuffer, int levelStep, int maxLevel)
{
    GISTBuildBuffers *gfbb;
    HASHCTL     hashCtl;

    gfbb = palloc(sizeof(GISTBuildBuffers));
    gfbb->pagesPerBuffer = pagesPerBuffer;
    gfbb->levelStep = levelStep;

    /*
     * Create a temporary file to hold buffer pages that are swapped out of
     * memory.
     */
    gfbb->pfile = BufFileCreateTemp(false);
    gfbb->nFileBlocks = 0;

    /* Initialize free page management. */
    gfbb->nFreeBlocks = 0;
    gfbb->freeBlocksLen = 32;
    gfbb->freeBlocks = (long *) palloc(gfbb->freeBlocksLen * sizeof(long));

    /*
     * Current memory context will be used for all in-memory data structures
     * of buffers which are persistent during buffering build.
     */
    gfbb->context = CurrentMemoryContext;

    /*
     * nodeBuffersTab hash is association between index blocks and it's
     * buffers.
     */
    hashCtl.keysize = sizeof(BlockNumber);
    hashCtl.entrysize = sizeof(GISTNodeBuffer);
    hashCtl.hcxt = CurrentMemoryContext;
    hashCtl.hash = tag_hash;
    hashCtl.match = memcmp;
    gfbb->nodeBuffersTab = hash_create("gistbuildbuffers",
                                       1024,
                                       &hashCtl,
                                       HASH_ELEM | HASH_CONTEXT
                                       | HASH_FUNCTION | HASH_COMPARE);

    gfbb->bufferEmptyingQueue = NIL;

    /*
     * Per-level node buffers lists for final buffers emptying process. Node
     * buffers are inserted here when they are created.
     */
    gfbb->buffersOnLevelsLen = 1;
    gfbb->buffersOnLevels = (List **) palloc(sizeof(List *) *
                                             gfbb->buffersOnLevelsLen);
    gfbb->buffersOnLevels[0] = NIL;

    /*
     * Block numbers of node buffers which last pages are currently loaded
     * into main memory.
     */
    gfbb->loadedBuffersLen = 32;
    gfbb->loadedBuffers = (GISTNodeBuffer **) palloc(gfbb->loadedBuffersLen *
                                                   sizeof(GISTNodeBuffer *));
    gfbb->loadedBuffersCount = 0;

    gfbb->rootlevel = maxLevel;

    return gfbb;
}

/*
 * Returns a node buffer for given block. The buffer is created if it
 * doesn't exist yet.
 */
GISTNodeBuffer *
gistGetNodeBuffer(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
                  BlockNumber nodeBlocknum, int level)
{
    GISTNodeBuffer *nodeBuffer;
    bool        found;

    /* Find node buffer in hash table */
    nodeBuffer = (GISTNodeBuffer *) hash_search(gfbb->nodeBuffersTab,
                                                (const void *) &nodeBlocknum,
                                                HASH_ENTER,
                                                &found);
    if (!found)
    {
        /*
         * Node buffer wasn't found. Initialize the new buffer as empty.
         */
        MemoryContext oldcxt = MemoryContextSwitchTo(gfbb->context);

        /* nodeBuffer->nodeBlocknum is the hash key and was filled in already */
        nodeBuffer->blocksCount = 0;
        nodeBuffer->pageBlocknum = InvalidBlockNumber;
        nodeBuffer->pageBuffer = NULL;
        nodeBuffer->queuedForEmptying = false;
        nodeBuffer->level = level;

        /*
         * Add this buffer to the list of buffers on this level. Enlarge
         * buffersOnLevels array if needed.
         */
        if (level >= gfbb->buffersOnLevelsLen)
        {
            int         i;

            gfbb->buffersOnLevels =
                (List **) repalloc(gfbb->buffersOnLevels,
                                   (level + 1) * sizeof(List *));

            /* initialize the enlarged portion */
            for (i = gfbb->buffersOnLevelsLen; i <= level; i++)
                gfbb->buffersOnLevels[i] = NIL;
            gfbb->buffersOnLevelsLen = level + 1;
        }

        /*
         * Prepend the new buffer to the list of buffers on this level. It's
         * not arbitrary that the new buffer is put to the beginning of the
         * list: in the final emptying phase we loop through all buffers at
         * each level, and flush them. If a page is split during the emptying,
         * it's more efficient to flush the new splitted pages first, before
         * moving on to pre-existing pages on the level. The buffers just
         * created during the page split are likely still in cache, so
         * flushing them immediately is more efficient than putting them to
         * the end of the queue.
         */
        gfbb->buffersOnLevels[level] = lcons(nodeBuffer,
                                             gfbb->buffersOnLevels[level]);

        MemoryContextSwitchTo(oldcxt);
    }

    return nodeBuffer;
}

/*
 * Allocate memory for a buffer page.
 */
static GISTNodeBufferPage *
gistAllocateNewPageBuffer(GISTBuildBuffers *gfbb)
{
    GISTNodeBufferPage *pageBuffer;

    pageBuffer = (GISTNodeBufferPage *) MemoryContextAlloc(gfbb->context,
                                                           BLCKSZ);
    pageBuffer->prev = InvalidBlockNumber;

    /* Set page free space */
    PAGE_FREE_SPACE(pageBuffer) = BLCKSZ - BUFFER_PAGE_DATA_OFFSET;
    return pageBuffer;
}

/*
 * Add specified buffer into loadedBuffers array.
 */
static void
gistAddLoadedBuffer(GISTBuildBuffers *gfbb, GISTNodeBuffer *nodeBuffer)
{
    /* Never add a temporary buffer to the array */
    if (nodeBuffer->isTemp)
        return;

    /* Enlarge the array if needed */
    if (gfbb->loadedBuffersCount >= gfbb->loadedBuffersLen)
    {
        gfbb->loadedBuffersLen *= 2;
        gfbb->loadedBuffers = (GISTNodeBuffer **)
            repalloc(gfbb->loadedBuffers,
                     gfbb->loadedBuffersLen * sizeof(GISTNodeBuffer *));
    }

    gfbb->loadedBuffers[gfbb->loadedBuffersCount] = nodeBuffer;
    gfbb->loadedBuffersCount++;
}

/*
 * Load last page of node buffer into main memory.
 */
static void
gistLoadNodeBuffer(GISTBuildBuffers *gfbb, GISTNodeBuffer *nodeBuffer)
{
    /* Check if we really should load something */
    if (!nodeBuffer->pageBuffer && nodeBuffer->blocksCount > 0)
    {
        /* Allocate memory for page */
        nodeBuffer->pageBuffer = gistAllocateNewPageBuffer(gfbb);

        /* Read block from temporary file */
        ReadTempFileBlock(gfbb->pfile, nodeBuffer->pageBlocknum,
                          nodeBuffer->pageBuffer);

        /* Mark file block as free */
        gistBuffersReleaseBlock(gfbb, nodeBuffer->pageBlocknum);

        /* Mark node buffer as loaded */
        gistAddLoadedBuffer(gfbb, nodeBuffer);
        nodeBuffer->pageBlocknum = InvalidBlockNumber;
    }
}

/*
 * Write last page of node buffer to the disk.
 */
static void
gistUnloadNodeBuffer(GISTBuildBuffers *gfbb, GISTNodeBuffer *nodeBuffer)
{
    /* Check if we have something to write */
    if (nodeBuffer->pageBuffer)
    {
        BlockNumber blkno;

        /* Get free file block */
        blkno = gistBuffersGetFreeBlock(gfbb);

        /* Write block to the temporary file */
        WriteTempFileBlock(gfbb->pfile, blkno, nodeBuffer->pageBuffer);

        /* Free memory of that page */
        pfree(nodeBuffer->pageBuffer);
        nodeBuffer->pageBuffer = NULL;

        /* Save block number */
        nodeBuffer->pageBlocknum = blkno;
    }
}

/*
 * Write last pages of all node buffers to the disk.
 */
void
gistUnloadNodeBuffers(GISTBuildBuffers *gfbb)
{
    int         i;

    /* Unload all the buffers that have a page loaded in memory. */
    for (i = 0; i < gfbb->loadedBuffersCount; i++)
        gistUnloadNodeBuffer(gfbb, gfbb->loadedBuffers[i]);

    /* Now there are no node buffers with loaded last page */
    gfbb->loadedBuffersCount = 0;
}

/*
 * Add index tuple to buffer page.
 */
static void
gistPlaceItupToPage(GISTNodeBufferPage *pageBuffer, IndexTuple itup)
{
    Size        itupsz = IndexTupleSize(itup);
    char       *ptr;

    /* There should be enough of space. */
    Assert(PAGE_FREE_SPACE(pageBuffer) >= MAXALIGN(itupsz));

    /* Reduce free space value of page to reserve a spot for the tuple. */
    PAGE_FREE_SPACE(pageBuffer) -= MAXALIGN(itupsz);

    /* Get pointer to the spot we reserved (ie. end of free space). */
    ptr = (char *) pageBuffer + BUFFER_PAGE_DATA_OFFSET
        + PAGE_FREE_SPACE(pageBuffer);

    /* Copy the index tuple there. */
    memcpy(ptr, itup, itupsz);
}

/*
 * Get last item from buffer page and remove it from page.
 */
static void
gistGetItupFromPage(GISTNodeBufferPage *pageBuffer, IndexTuple *itup)
{
    IndexTuple  ptr;
    Size        itupsz;

    Assert(!PAGE_IS_EMPTY(pageBuffer)); /* Page shouldn't be empty */

    /* Get pointer to last index tuple */
    ptr = (IndexTuple) ((char *) pageBuffer
                        + BUFFER_PAGE_DATA_OFFSET
                        + PAGE_FREE_SPACE(pageBuffer));
    itupsz = IndexTupleSize(ptr);

    /* Make a copy of the tuple */
    *itup = (IndexTuple) palloc(itupsz);
    memcpy(*itup, ptr, itupsz);

    /* Mark the space used by the tuple as free */
    PAGE_FREE_SPACE(pageBuffer) += MAXALIGN(itupsz);
}

/*
 * Push an index tuple to node buffer.
 */
void
gistPushItupToNodeBuffer(GISTBuildBuffers *gfbb, GISTNodeBuffer *nodeBuffer,
                         IndexTuple itup)
{
    /*
     * Most part of memory operations will be in buffering build persistent
     * context. So, let's switch to it.
     */
    MemoryContext oldcxt = MemoryContextSwitchTo(gfbb->context);

    /*
     * If the buffer is currently empty, create the first page.
     */
    if (nodeBuffer->blocksCount == 0)
    {
        nodeBuffer->pageBuffer = gistAllocateNewPageBuffer(gfbb);
        nodeBuffer->blocksCount = 1;
        gistAddLoadedBuffer(gfbb, nodeBuffer);
    }

    /* Load last page of node buffer if it wasn't in memory already */
    if (!nodeBuffer->pageBuffer)
        gistLoadNodeBuffer(gfbb, nodeBuffer);

    /*
     * Check if there is enough space on the last page for the tuple.
     */
    if (PAGE_NO_SPACE(nodeBuffer->pageBuffer, itup))
    {
        /*
         * Nope. Swap previous block to disk and allocate a new one.
         */
        BlockNumber blkno;

        /* Write filled page to the disk */
        blkno = gistBuffersGetFreeBlock(gfbb);
        WriteTempFileBlock(gfbb->pfile, blkno, nodeBuffer->pageBuffer);

        /*
         * Reset the in-memory page as empty, and link the previous block to
         * the new page by storing its block number in the prev-link.
         */
        PAGE_FREE_SPACE(nodeBuffer->pageBuffer) =
            BLCKSZ - MAXALIGN(offsetof(GISTNodeBufferPage, tupledata));
        nodeBuffer->pageBuffer->prev = blkno;

        /* We've just added one more page */
        nodeBuffer->blocksCount++;
    }

    gistPlaceItupToPage(nodeBuffer->pageBuffer, itup);

    /*
     * If the buffer just overflowed, add it to the emptying queue.
     */
    if (BUFFER_HALF_FILLED(nodeBuffer, gfbb) && !nodeBuffer->queuedForEmptying)
    {
        gfbb->bufferEmptyingQueue = lcons(nodeBuffer,
                                          gfbb->bufferEmptyingQueue);
        nodeBuffer->queuedForEmptying = true;
    }

    /* Restore memory context */
    MemoryContextSwitchTo(oldcxt);
}

/*
 * Removes one index tuple from node buffer. Returns true if success and false
 * if node buffer is empty.
 */
bool
gistPopItupFromNodeBuffer(GISTBuildBuffers *gfbb, GISTNodeBuffer *nodeBuffer,
                          IndexTuple *itup)
{
    /*
     * If node buffer is empty then return false.
     */
    if (nodeBuffer->blocksCount <= 0)
        return false;

    /* Load last page of node buffer if needed */
    if (!nodeBuffer->pageBuffer)
        gistLoadNodeBuffer(gfbb, nodeBuffer);

    /*
     * Get index tuple from last non-empty page.
     */
    gistGetItupFromPage(nodeBuffer->pageBuffer, itup);

    /*
     * If we just removed the last tuple from the page, fetch previous page on
     * this node buffer (if any).
     */
    if (PAGE_IS_EMPTY(nodeBuffer->pageBuffer))
    {
        BlockNumber prevblkno;

        /*
         * blocksCount includes the page in pageBuffer, so decrease it now.
         */
        nodeBuffer->blocksCount--;

        /*
         * If there's more pages, fetch previous one.
         */
        prevblkno = nodeBuffer->pageBuffer->prev;
        if (prevblkno != InvalidBlockNumber)
        {
            /* There is a previous page. Fetch it. */
            Assert(nodeBuffer->blocksCount > 0);
            ReadTempFileBlock(gfbb->pfile, prevblkno, nodeBuffer->pageBuffer);

            /*
             * Now that we've read the block in memory, we can release its
             * on-disk block for reuse.
             */
            gistBuffersReleaseBlock(gfbb, prevblkno);
        }
        else
        {
            /* No more pages. Free memory. */
            Assert(nodeBuffer->blocksCount == 0);
            pfree(nodeBuffer->pageBuffer);
            nodeBuffer->pageBuffer = NULL;
        }
    }
    return true;
}

/*
 * Select a currently unused block for writing to.
 */
static long
gistBuffersGetFreeBlock(GISTBuildBuffers *gfbb)
{
    /*
     * If there are multiple free blocks, we select the one appearing last in
     * freeBlocks[].  If there are none, assign the next block at the end of
     * the file (causing the file to be extended).
     */
    if (gfbb->nFreeBlocks > 0)
        return gfbb->freeBlocks[--gfbb->nFreeBlocks];
    else
        return gfbb->nFileBlocks++;
}

/*
 * Return a block# to the freelist.
 */
static void
gistBuffersReleaseBlock(GISTBuildBuffers *gfbb, long blocknum)
{
    int         ndx;

    /* Enlarge freeBlocks array if full. */
    if (gfbb->nFreeBlocks >= gfbb->freeBlocksLen)
    {
        gfbb->freeBlocksLen *= 2;
        gfbb->freeBlocks = (long *) repalloc(gfbb->freeBlocks,
                                             gfbb->freeBlocksLen *
                                             sizeof(long));
    }

    /* Add blocknum to array */
    ndx = gfbb->nFreeBlocks++;
    gfbb->freeBlocks[ndx] = blocknum;
}

/*
 * Free buffering build data structure.
 */
void
gistFreeBuildBuffers(GISTBuildBuffers *gfbb)
{
    /* Close buffers file. */
    BufFileClose(gfbb->pfile);

    /* All other things will be freed on memory context release */
}

/*
 * Data structure representing information about node buffer for index tuples
 * relocation from splitted node buffer.
 */
typedef struct
{
    GISTENTRY   entry[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    GISTPageSplitInfo *splitinfo;
    GISTNodeBuffer *nodeBuffer;
} RelocationBufferInfo;

/*
 * At page split, distribute tuples from the buffer of the split page to
 * new buffers for the created page halves. This also adjusts the downlinks
 * in 'splitinfo' to include the tuples in the buffers.
 */
void
gistRelocateBuildBuffersOnSplit(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
                                Relation r, int level,
                                Buffer buffer, List *splitinfo)
{
    RelocationBufferInfo *relocationBuffersInfos;
    bool        found;
    GISTNodeBuffer *nodeBuffer;
    BlockNumber blocknum;
    IndexTuple  itup;
    int         splitPagesCount = 0,
                i;
    GISTENTRY   entry[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    GISTNodeBuffer oldBuf;
    ListCell   *lc;

    /* If the splitted page doesn't have buffers, we have nothing to do. */
    if (!LEVEL_HAS_BUFFERS(level, gfbb))
        return;

    /*
     * Get the node buffer of the splitted page.
     */
    blocknum = BufferGetBlockNumber(buffer);
    nodeBuffer = hash_search(gfbb->nodeBuffersTab, &blocknum,
                             HASH_FIND, &found);
    if (!found)
    {
        /* The page has no buffer, so we have nothing to do. */
        return;
    }

    /*
     * Make a copy of the old buffer, as we're going reuse it as the buffer
     * for the new left page, which is on the same block as the old page.
     * That's not true for the root page, but that's fine because we never
     * have a buffer on the root page anyway. The original algorithm as
     * described by Arge et al did, but it's of no use, as you might as well
     * read the tuples straight from the heap instead of the root buffer.
     */
    Assert(blocknum != GIST_ROOT_BLKNO);
    memcpy(&oldBuf, nodeBuffer, sizeof(GISTNodeBuffer));
    oldBuf.isTemp = true;

    /* Reset the old buffer, used for the new left page from now on */
    nodeBuffer->blocksCount = 0;
    nodeBuffer->pageBuffer = NULL;
    nodeBuffer->pageBlocknum = InvalidBlockNumber;

    /*
     * Allocate memory for information about relocation buffers.
     */
    splitPagesCount = list_length(splitinfo);
    relocationBuffersInfos =
        (RelocationBufferInfo *) palloc(sizeof(RelocationBufferInfo) *
                                        splitPagesCount);

    /*
     * Fill relocation buffers information for node buffers of pages produced
     * by split.
     */
    i = 0;
    foreach(lc, splitinfo)
    {
        GISTPageSplitInfo *si = (GISTPageSplitInfo *) lfirst(lc);
        GISTNodeBuffer *newNodeBuffer;

        /* Decompress parent index tuple of node buffer page. */
        gistDeCompressAtt(giststate, r,
                          si->downlink, NULL, (OffsetNumber) 0,
                          relocationBuffersInfos[i].entry,
                          relocationBuffersInfos[i].isnull);

        /*
         * Create a node buffer for the page. The leftmost half is on the same
         * block as the old page before split, so for the leftmost half this
         * will return the original buffer. The tuples on the original buffer
         * were relinked to the temporary buffer, so the original one is now
         * empty.
         */
        newNodeBuffer = gistGetNodeBuffer(gfbb, giststate, BufferGetBlockNumber(si->buf), level);

        relocationBuffersInfos[i].nodeBuffer = newNodeBuffer;
        relocationBuffersInfos[i].splitinfo = si;

        i++;
    }

    /*
     * Loop through all index tuples in the buffer of the page being split,
     * moving them to buffers for the new pages.  We try to move each tuple to
     * the page that will result in the lowest penalty for the leading column
     * or, in the case of a tie, the lowest penalty for the earliest column
     * that is not tied.
     *
     * The page searching logic is very similar to gistchoose().
     */
    while (gistPopItupFromNodeBuffer(gfbb, &oldBuf, &itup))
    {
        float       best_penalty[INDEX_MAX_KEYS];
        int         i,
                    which;
        IndexTuple  newtup;
        RelocationBufferInfo *targetBufferInfo;

        gistDeCompressAtt(giststate, r,
                          itup, NULL, (OffsetNumber) 0, entry, isnull);

        /* default to using first page (shouldn't matter) */
        which = 0;

        /*
         * best_penalty[j] is the best penalty we have seen so far for column
         * j, or -1 when we haven't yet examined column j.  Array entries to
         * the right of the first -1 are undefined.
         */
        best_penalty[0] = -1;

        /*
         * Loop over possible target pages, looking for one to move this tuple
         * to.
         */
        for (i = 0; i < splitPagesCount; i++)
        {
            RelocationBufferInfo *splitPageInfo = &relocationBuffersInfos[i];
            bool        zero_penalty;
            int         j;

            zero_penalty = true;

            /* Loop over index attributes. */
            for (j = 0; j < r->rd_att->natts; j++)
            {
                float       usize;

                /* Compute penalty for this column. */
                usize = gistpenalty(giststate, j,
                                    &splitPageInfo->entry[j],
                                    splitPageInfo->isnull[j],
                                    &entry[j], isnull[j]);
                if (usize > 0)
                    zero_penalty = false;

                if (best_penalty[j] < 0 || usize < best_penalty[j])
                {
                    /*
                     * New best penalty for column.  Tentatively select this
                     * page as the target, and record the best penalty.  Then
                     * reset the next column's penalty to "unknown" (and
                     * indirectly, the same for all the ones to its right).
                     * This will force us to adopt this page's penalty values
                     * as the best for all the remaining columns during
                     * subsequent loop iterations.
                     */
                    which = i;
                    best_penalty[j] = usize;

                    if (j < r->rd_att->natts - 1)
                        best_penalty[j + 1] = -1;
                }
                else if (best_penalty[j] == usize)
                {
                    /*
                     * The current page is exactly as good for this column as
                     * the best page seen so far.  The next iteration of this
                     * loop will compare the next column.
                     */
                }
                else
                {
                    /*
                     * The current page is worse for this column than the best
                     * page seen so far.  Skip the remaining columns and move
                     * on to the next page, if any.
                     */
                    zero_penalty = false;       /* so outer loop won't exit */
                    break;
                }
            }

            /*
             * If we find a page with zero penalty for all columns, there's no
             * need to examine remaining pages; just break out of the loop and
             * return it.
             */
            if (zero_penalty)
                break;
        }

        /* OK, "which" is the page index to push the tuple to */
        targetBufferInfo = &relocationBuffersInfos[which];

        /* Push item to selected node buffer */
        gistPushItupToNodeBuffer(gfbb, targetBufferInfo->nodeBuffer, itup);

        /* Adjust the downlink for this page, if needed. */
        newtup = gistgetadjusted(r, targetBufferInfo->splitinfo->downlink,
                                 itup, giststate);
        if (newtup)
        {
            gistDeCompressAtt(giststate, r,
                              newtup, NULL, (OffsetNumber) 0,
                              targetBufferInfo->entry,
                              targetBufferInfo->isnull);

            targetBufferInfo->splitinfo->downlink = newtup;
        }
    }

    pfree(relocationBuffersInfos);
}


/*
 * Wrappers around BufFile operations. The main difference is that these
 * wrappers report errors with ereport(), so that the callers don't need
 * to check the return code.
 */

static void
ReadTempFileBlock(BufFile *file, long blknum, void *ptr)
{
    if (BufFileSeekBlock(file, blknum) != 0)
        elog(ERROR, "could not seek temporary file: %m");
    if (BufFileRead(file, ptr, BLCKSZ) != BLCKSZ)
        elog(ERROR, "could not read temporary file: %m");
}

static void
WriteTempFileBlock(BufFile *file, long blknum, void *ptr)
{
    if (BufFileSeekBlock(file, blknum) != 0)
        elog(ERROR, "could not seek temporary file: %m");
    if (BufFileWrite(file, ptr, BLCKSZ) != BLCKSZ)
    {
        /*
         * the other errors in Read/WriteTempFileBlock shouldn't happen, but
         * an error at write can easily happen if you run out of disk space.
         */
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write block %ld of temporary file: %m",
                        blknum)));
    }
}