tableam.c

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/*----------------------------------------------------------------------
 *
 * tableam.c
 *      Table access method routines too big to be inline functions.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/table/tableam.c
 *
 * NOTES
 *    Note that most function in here are documented in tableam.h, rather than
 *    here. That's because there's a lot of inline functions in tableam.h and
 *    it'd be harder to understand if one constantly had to switch between files.
 *
 *----------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "access/syncscan.h"
#include "access/tableam.h"
#include "access/xact.h"
#include "optimizer/plancat.h"
#include "port/pg_bitutils.h"
#include "storage/bufmgr.h"
#include "storage/shmem.h"
#include "storage/smgr.h"

/*
 * Constants to control the behavior of block allocation to parallel workers
 * during a parallel seqscan.  Technically these values do not need to be
 * powers of 2, but having them as powers of 2 makes the math more optimal
 * and makes the ramp-down stepping more even.
 */

/* The number of I/O chunks we try to break a parallel seqscan down into */
#define PARALLEL_SEQSCAN_NCHUNKS            2048
/* Ramp down size of allocations when we've only this number of chunks left */
#define PARALLEL_SEQSCAN_RAMPDOWN_CHUNKS    64
/* Cap the size of parallel I/O chunks to this number of blocks */
#define PARALLEL_SEQSCAN_MAX_CHUNK_SIZE     8192

/* GUC variables */
char       *default_table_access_method = DEFAULT_TABLE_ACCESS_METHOD;
bool        synchronize_seqscans = true;


/* ----------------------------------------------------------------------------
 * Slot functions.
 * ----------------------------------------------------------------------------
 */

const TupleTableSlotOps *
table_slot_callbacks(Relation relation)
{
    const TupleTableSlotOps *tts_cb;

    if (relation->rd_tableam)
        tts_cb = relation->rd_tableam->slot_callbacks(relation);
    else if (relation->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
    {
        /*
         * Historically FDWs expect to store heap tuples in slots. Continue
         * handing them one, to make it less painful to adapt FDWs to new
         * versions. The cost of a heap slot over a virtual slot is pretty
         * small.
         */
        tts_cb = &TTSOpsHeapTuple;
    }
    else
    {
        /*
         * These need to be supported, as some parts of the code (like COPY)
         * need to create slots for such relations too. It seems better to
         * centralize the knowledge that a heap slot is the right thing in
         * that case here.
         */
        Assert(relation->rd_rel->relkind == RELKIND_VIEW ||
               relation->rd_rel->relkind == RELKIND_PARTITIONED_TABLE);
        tts_cb = &TTSOpsVirtual;
    }

    return tts_cb;
}

TupleTableSlot *
table_slot_create(Relation relation, List **reglist)
{
    const TupleTableSlotOps *tts_cb;
    TupleTableSlot *slot;

    tts_cb = table_slot_callbacks(relation);
    slot = MakeSingleTupleTableSlot(RelationGetDescr(relation), tts_cb);

    if (reglist)
        *reglist = lappend(*reglist, slot);

    return slot;
}


/* ----------------------------------------------------------------------------
 * Table scan functions.
 * ----------------------------------------------------------------------------
 */

TableScanDesc
table_beginscan_catalog(Relation relation, int nkeys, struct ScanKeyData *key)
{
    uint32      flags = SO_TYPE_SEQSCAN |
    SO_ALLOW_STRAT | SO_ALLOW_SYNC | SO_ALLOW_PAGEMODE | SO_TEMP_SNAPSHOT;
    Oid         relid = RelationGetRelid(relation);
    Snapshot    snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));

    return relation->rd_tableam->scan_begin(relation, snapshot, nkeys, key,
                                            NULL, flags);
}

void
table_scan_update_snapshot(TableScanDesc scan, Snapshot snapshot)
{
    Assert(IsMVCCSnapshot(snapshot));

    RegisterSnapshot(snapshot);
    scan->rs_snapshot = snapshot;
    scan->rs_flags |= SO_TEMP_SNAPSHOT;
}


/* ----------------------------------------------------------------------------
 * Parallel table scan related functions.
 * ----------------------------------------------------------------------------
 */

Size
table_parallelscan_estimate(Relation rel, Snapshot snapshot)
{
    Size        sz = 0;

    if (IsMVCCSnapshot(snapshot))
        sz = add_size(sz, EstimateSnapshotSpace(snapshot));
    else
        Assert(snapshot == SnapshotAny);

    sz = add_size(sz, rel->rd_tableam->parallelscan_estimate(rel));

    return sz;
}

void
table_parallelscan_initialize(Relation rel, ParallelTableScanDesc pscan,
                              Snapshot snapshot)
{
    Size        snapshot_off = rel->rd_tableam->parallelscan_initialize(rel, pscan);

    pscan->phs_snapshot_off = snapshot_off;

    if (IsMVCCSnapshot(snapshot))
    {
        SerializeSnapshot(snapshot, (char *) pscan + pscan->phs_snapshot_off);
        pscan->phs_snapshot_any = false;
    }
    else
    {
        Assert(snapshot == SnapshotAny);
        pscan->phs_snapshot_any = true;
    }
}

TableScanDesc
table_beginscan_parallel(Relation relation, ParallelTableScanDesc parallel_scan)
{
    Snapshot    snapshot;
    uint32      flags = SO_TYPE_SEQSCAN |
    SO_ALLOW_STRAT | SO_ALLOW_SYNC | SO_ALLOW_PAGEMODE;

    Assert(RelationGetRelid(relation) == parallel_scan->phs_relid);

    if (!parallel_scan->phs_snapshot_any)
    {
        /* Snapshot was serialized -- restore it */
        snapshot = RestoreSnapshot((char *) parallel_scan +
                                   parallel_scan->phs_snapshot_off);
        RegisterSnapshot(snapshot);
        flags |= SO_TEMP_SNAPSHOT;
    }
    else
    {
        /* SnapshotAny passed by caller (not serialized) */
        snapshot = SnapshotAny;
    }

    return relation->rd_tableam->scan_begin(relation, snapshot, 0, NULL,
                                            parallel_scan, flags);
}


/* ----------------------------------------------------------------------------
 * Index scan related functions.
 * ----------------------------------------------------------------------------
 */

/*
 * To perform that check simply start an index scan, create the necessary
 * slot, do the heap lookup, and shut everything down again. This could be
 * optimized, but is unlikely to matter from a performance POV. If there
 * frequently are live index pointers also matching a unique index key, the
 * CPU overhead of this routine is unlikely to matter.
 *
 * Note that *tid may be modified when we return true if the AM supports
 * storing multiple row versions reachable via a single index entry (like
 * heap's HOT).
 */
bool
table_index_fetch_tuple_check(Relation rel,
                              ItemPointer tid,
                              Snapshot snapshot,
                              bool *all_dead)
{
    IndexFetchTableData *scan;
    TupleTableSlot *slot;
    bool        call_again = false;
    bool        found;

    slot = table_slot_create(rel, NULL);
    scan = table_index_fetch_begin(rel);
    found = table_index_fetch_tuple(scan, tid, snapshot, slot, &call_again,
                                    all_dead);
    table_index_fetch_end(scan);
    ExecDropSingleTupleTableSlot(slot);

    return found;
}


/* ------------------------------------------------------------------------
 * Functions for non-modifying operations on individual tuples
 * ------------------------------------------------------------------------
 */

void
table_tuple_get_latest_tid(TableScanDesc scan, ItemPointer tid)
{
    Relation    rel = scan->rs_rd;
    const TableAmRoutine *tableam = rel->rd_tableam;

    /*
     * We don't expect direct calls to table_tuple_get_latest_tid with valid
     * CheckXidAlive for catalog or regular tables.  See detailed comments in
     * xact.c where these variables are declared.
     */
    if (unlikely(TransactionIdIsValid(CheckXidAlive) && !bsysscan))
        elog(ERROR, "unexpected table_tuple_get_latest_tid call during logical decoding");

    /*
     * Since this can be called with user-supplied TID, don't trust the input
     * too much.
     */
    if (!tableam->tuple_tid_valid(scan, tid))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("tid (%u, %u) is not valid for relation \"%s\"",
                        ItemPointerGetBlockNumberNoCheck(tid),
                        ItemPointerGetOffsetNumberNoCheck(tid),
                        RelationGetRelationName(rel))));

    tableam->tuple_get_latest_tid(scan, tid);
}


/* ----------------------------------------------------------------------------
 * Functions to make modifications a bit simpler.
 * ----------------------------------------------------------------------------
 */

/*
 * simple_table_tuple_insert - insert a tuple
 *
 * Currently, this routine differs from table_tuple_insert only in supplying a
 * default command ID and not allowing access to the speedup options.
 */
void
simple_table_tuple_insert(Relation rel, TupleTableSlot *slot)
{
    table_tuple_insert(rel, slot, GetCurrentCommandId(true), 0, NULL);
}

/*
 * simple_table_tuple_delete - delete a tuple
 *
 * This routine may be used to delete a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_table_tuple_delete(Relation rel, ItemPointer tid, Snapshot snapshot)
{
    TM_Result   result;
    TM_FailureData tmfd;

    result = table_tuple_delete(rel, tid,
                                GetCurrentCommandId(true),
                                snapshot, InvalidSnapshot,
                                true /* wait for commit */ ,
                                &tmfd, false /* changingPart */ );

    switch (result)
    {
        case TM_SelfModified:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case TM_Ok:
            /* done successfully */
            break;

        case TM_Updated:
            elog(ERROR, "tuple concurrently updated");
            break;

        case TM_Deleted:
            elog(ERROR, "tuple concurrently deleted");
            break;

        default:
            elog(ERROR, "unrecognized table_tuple_delete status: %u", result);
            break;
    }
}

/*
 * simple_table_tuple_update - replace a tuple
 *
 * This routine may be used to update a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_table_tuple_update(Relation rel, ItemPointer otid,
                          TupleTableSlot *slot,
                          Snapshot snapshot,
                          bool *update_indexes)
{
    TM_Result   result;
    TM_FailureData tmfd;
    LockTupleMode lockmode;

    result = table_tuple_update(rel, otid, slot,
                                GetCurrentCommandId(true),
                                snapshot, InvalidSnapshot,
                                true /* wait for commit */ ,
                                &tmfd, &lockmode, update_indexes);

    switch (result)
    {
        case TM_SelfModified:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case TM_Ok:
            /* done successfully */
            break;

        case TM_Updated:
            elog(ERROR, "tuple concurrently updated");
            break;

        case TM_Deleted:
            elog(ERROR, "tuple concurrently deleted");
            break;

        default:
            elog(ERROR, "unrecognized table_tuple_update status: %u", result);
            break;
    }

}


/* ----------------------------------------------------------------------------
 * Helper functions to implement parallel scans for block oriented AMs.
 * ----------------------------------------------------------------------------
 */

Size
table_block_parallelscan_estimate(Relation rel)
{
    return sizeof(ParallelBlockTableScanDescData);
}

Size
table_block_parallelscan_initialize(Relation rel, ParallelTableScanDesc pscan)
{
    ParallelBlockTableScanDesc bpscan = (ParallelBlockTableScanDesc) pscan;

    bpscan->base.phs_relid = RelationGetRelid(rel);
    bpscan->phs_nblocks = RelationGetNumberOfBlocks(rel);
    /* compare phs_syncscan initialization to similar logic in initscan */
    bpscan->base.phs_syncscan = synchronize_seqscans &&
        !RelationUsesLocalBuffers(rel) &&
        bpscan->phs_nblocks > NBuffers / 4;
    SpinLockInit(&bpscan->phs_mutex);
    bpscan->phs_startblock = InvalidBlockNumber;
    pg_atomic_init_u64(&bpscan->phs_nallocated, 0);

    return sizeof(ParallelBlockTableScanDescData);
}

void
table_block_parallelscan_reinitialize(Relation rel, ParallelTableScanDesc pscan)
{
    ParallelBlockTableScanDesc bpscan = (ParallelBlockTableScanDesc) pscan;

    pg_atomic_write_u64(&bpscan->phs_nallocated, 0);
}

/*
 * find and set the scan's startblock
 *
 * Determine where the parallel seq scan should start.  This function may be
 * called many times, once by each parallel worker.  We must be careful only
 * to set the startblock once.
 */
void
table_block_parallelscan_startblock_init(Relation rel,
                                         ParallelBlockTableScanWorker pbscanwork,
                                         ParallelBlockTableScanDesc pbscan)
{
    BlockNumber sync_startpage = InvalidBlockNumber;

    /* Reset the state we use for controlling allocation size. */
    memset(pbscanwork, 0, sizeof(*pbscanwork));

    StaticAssertStmt(MaxBlockNumber <= 0xFFFFFFFE,
                     "pg_nextpower2_32 may be too small for non-standard BlockNumber width");

    /*
     * We determine the chunk size based on the size of the relation. First we
     * split the relation into PARALLEL_SEQSCAN_NCHUNKS chunks but we then
     * take the next highest power of 2 number of the chunk size.  This means
     * we split the relation into somewhere between PARALLEL_SEQSCAN_NCHUNKS
     * and PARALLEL_SEQSCAN_NCHUNKS / 2 chunks.
     */
    pbscanwork->phsw_chunk_size = pg_nextpower2_32(Max(pbscan->phs_nblocks /
                                                       PARALLEL_SEQSCAN_NCHUNKS, 1));

    /*
     * Ensure we don't go over the maximum chunk size with larger tables. This
     * means we may get much more than PARALLEL_SEQSCAN_NCHUNKS for larger
     * tables.  Too large a chunk size has been shown to be detrimental to
     * synchronous scan performance.
     */
    pbscanwork->phsw_chunk_size = Min(pbscanwork->phsw_chunk_size,
                                      PARALLEL_SEQSCAN_MAX_CHUNK_SIZE);

retry:
    /* Grab the spinlock. */
    SpinLockAcquire(&pbscan->phs_mutex);

    /*
     * If the scan's startblock has not yet been initialized, we must do so
     * now.  If this is not a synchronized scan, we just start at block 0, but
     * if it is a synchronized scan, we must get the starting position from
     * the synchronized scan machinery.  We can't hold the spinlock while
     * doing that, though, so release the spinlock, get the information we
     * need, and retry.  If nobody else has initialized the scan in the
     * meantime, we'll fill in the value we fetched on the second time
     * through.
     */
    if (pbscan->phs_startblock == InvalidBlockNumber)
    {
        if (!pbscan->base.phs_syncscan)
            pbscan->phs_startblock = 0;
        else if (sync_startpage != InvalidBlockNumber)
            pbscan->phs_startblock = sync_startpage;
        else
        {
            SpinLockRelease(&pbscan->phs_mutex);
            sync_startpage = ss_get_location(rel, pbscan->phs_nblocks);
            goto retry;
        }
    }
    SpinLockRelease(&pbscan->phs_mutex);
}

/*
 * get the next page to scan
 *
 * Get the next page to scan.  Even if there are no pages left to scan,
 * another backend could have grabbed a page to scan and not yet finished
 * looking at it, so it doesn't follow that the scan is done when the first
 * backend gets an InvalidBlockNumber return.
 */
BlockNumber
table_block_parallelscan_nextpage(Relation rel,
                                  ParallelBlockTableScanWorker pbscanwork,
                                  ParallelBlockTableScanDesc pbscan)
{
    BlockNumber page;
    uint64      nallocated;

    /*
     * The logic below allocates block numbers out to parallel workers in a
     * way that each worker will receive a set of consecutive block numbers to
     * scan.  Earlier versions of this would allocate the next highest block
     * number to the next worker to call this function.  This would generally
     * result in workers never receiving consecutive block numbers.  Some
     * operating systems would not detect the sequential I/O pattern due to
     * each backend being a different process which could result in poor
     * performance due to inefficient or no readahead.  To work around this
     * issue, we now allocate a range of block numbers for each worker and
     * when they come back for another block, we give them the next one in
     * that range until the range is complete.  When the worker completes the
     * range of blocks we then allocate another range for it and return the
     * first block number from that range.
     *
     * Here we name these ranges of blocks "chunks".  The initial size of
     * these chunks is determined in table_block_parallelscan_startblock_init
     * based on the size of the relation.  Towards the end of the scan, we
     * start making reductions in the size of the chunks in order to attempt
     * to divide the remaining work over all the workers as evenly as
     * possible.
     *
     * Here pbscanwork is local worker memory.  phsw_chunk_remaining tracks
     * the number of blocks remaining in the chunk.  When that reaches 0 then
     * we must allocate a new chunk for the worker.
     *
     * phs_nallocated tracks how many blocks have been allocated to workers
     * already.  When phs_nallocated >= rs_nblocks, all blocks have been
     * allocated.
     *
     * Because we use an atomic fetch-and-add to fetch the current value, the
     * phs_nallocated counter will exceed rs_nblocks, because workers will
     * still increment the value, when they try to allocate the next block but
     * all blocks have been allocated already. The counter must be 64 bits
     * wide because of that, to avoid wrapping around when rs_nblocks is close
     * to 2^32.
     *
     * The actual block to return is calculated by adding the counter to the
     * starting block number, modulo nblocks.
     */

    /*
     * First check if we have any remaining blocks in a previous chunk for
     * this worker.  We must consume all of the blocks from that before we
     * allocate a new chunk to the worker.
     */
    if (pbscanwork->phsw_chunk_remaining > 0)
    {
        /*
         * Give them the next block in the range and update the remaining
         * number of blocks.
         */
        nallocated = ++pbscanwork->phsw_nallocated;
        pbscanwork->phsw_chunk_remaining--;
    }
    else
    {
        /*
         * When we've only got PARALLEL_SEQSCAN_RAMPDOWN_CHUNKS chunks
         * remaining in the scan, we half the chunk size.  Since we reduce the
         * chunk size here, we'll hit this again after doing
         * PARALLEL_SEQSCAN_RAMPDOWN_CHUNKS at the new size.  After a few
         * iterations of this, we'll end up doing the last few blocks with the
         * chunk size set to 1.
         */
        if (pbscanwork->phsw_chunk_size > 1 &&
            pbscanwork->phsw_nallocated > pbscan->phs_nblocks -
            (pbscanwork->phsw_chunk_size * PARALLEL_SEQSCAN_RAMPDOWN_CHUNKS))
            pbscanwork->phsw_chunk_size >>= 1;

        nallocated = pbscanwork->phsw_nallocated =
            pg_atomic_fetch_add_u64(&pbscan->phs_nallocated,
                                    pbscanwork->phsw_chunk_size);

        /*
         * Set the remaining number of blocks in this chunk so that subsequent
         * calls from this worker continue on with this chunk until it's done.
         */
        pbscanwork->phsw_chunk_remaining = pbscanwork->phsw_chunk_size - 1;
    }

    if (nallocated >= pbscan->phs_nblocks)
        page = InvalidBlockNumber;  /* all blocks have been allocated */
    else
        page = (nallocated + pbscan->phs_startblock) % pbscan->phs_nblocks;

    /*
     * Report scan location.  Normally, we report the current page number.
     * When we reach the end of the scan, though, we report the starting page,
     * not the ending page, just so the starting positions for later scans
     * doesn't slew backwards.  We only report the position at the end of the
     * scan once, though: subsequent callers will report nothing.
     */
    if (pbscan->base.phs_syncscan)
    {
        if (page != InvalidBlockNumber)
            ss_report_location(rel, page);
        else if (nallocated == pbscan->phs_nblocks)
            ss_report_location(rel, pbscan->phs_startblock);
    }

    return page;
}

/* ----------------------------------------------------------------------------
 * Helper functions to implement relation sizing for block oriented AMs.
 * ----------------------------------------------------------------------------
 */

/*
 * table_block_relation_size
 *
 * If a table AM uses the various relation forks as the sole place where data
 * is stored, and if it uses them in the expected manner (e.g. the actual data
 * is in the main fork rather than some other), it can use this implementation
 * of the relation_size callback rather than implementing its own.
 */
uint64
table_block_relation_size(Relation rel, ForkNumber forkNumber)
{
    uint64      nblocks = 0;

    /* InvalidForkNumber indicates returning the size for all forks */
    if (forkNumber == InvalidForkNumber)
    {
        for (int i = 0; i < MAX_FORKNUM; i++)
            nblocks += smgrnblocks(RelationGetSmgr(rel), i);
    }
    else
        nblocks = smgrnblocks(RelationGetSmgr(rel), forkNumber);

    return nblocks * BLCKSZ;
}

/*
 * table_block_relation_estimate_size
 *
 * This function can't be directly used as the implementation of the
 * relation_estimate_size callback, because it has a few additional parameters.
 * Instead, it is intended to be used as a helper function; the caller can
 * pass through the arguments to its relation_estimate_size function plus the
 * additional values required here.
 *
 * overhead_bytes_per_tuple should contain the approximate number of bytes
 * of storage required to store a tuple above and beyond what is required for
 * the tuple data proper. Typically, this would include things like the
 * size of the tuple header and item pointer. This is only used for query
 * planning, so a table AM where the value is not constant could choose to
 * pass a "best guess".
 *
 * usable_bytes_per_page should contain the approximate number of bytes per
 * page usable for tuple data, excluding the page header and any anticipated
 * special space.
 */
void
table_block_relation_estimate_size(Relation rel, int32 *attr_widths,
                                   BlockNumber *pages, double *tuples,
                                   double *allvisfrac,
                                   Size overhead_bytes_per_tuple,
                                   Size usable_bytes_per_page)
{
    BlockNumber curpages;
    BlockNumber relpages;
    double      reltuples;
    BlockNumber relallvisible;
    double      density;

    /* it should have storage, so we can call the smgr */
    curpages = RelationGetNumberOfBlocks(rel);

    /* coerce values in pg_class to more desirable types */
    relpages = (BlockNumber) rel->rd_rel->relpages;
    reltuples = (double) rel->rd_rel->reltuples;
    relallvisible = (BlockNumber) rel->rd_rel->relallvisible;

    /*
     * HACK: if the relation has never yet been vacuumed, use a minimum size
     * estimate of 10 pages.  The idea here is to avoid assuming a
     * newly-created table is really small, even if it currently is, because
     * that may not be true once some data gets loaded into it.  Once a vacuum
     * or analyze cycle has been done on it, it's more reasonable to believe
     * the size is somewhat stable.
     *
     * (Note that this is only an issue if the plan gets cached and used again
     * after the table has been filled.  What we're trying to avoid is using a
     * nestloop-type plan on a table that has grown substantially since the
     * plan was made.  Normally, autovacuum/autoanalyze will occur once enough
     * inserts have happened and cause cached-plan invalidation; but that
     * doesn't happen instantaneously, and it won't happen at all for cases
     * such as temporary tables.)
     *
     * We test "never vacuumed" by seeing whether reltuples < 0.
     *
     * If the table has inheritance children, we don't apply this heuristic.
     * Totally empty parent tables are quite common, so we should be willing
     * to believe that they are empty.
     */
    if (curpages < 10 &&
        reltuples < 0 &&
        !rel->rd_rel->relhassubclass)
        curpages = 10;

    /* report estimated # pages */
    *pages = curpages;
    /* quick exit if rel is clearly empty */
    if (curpages == 0)
    {
        *tuples = 0;
        *allvisfrac = 0;
        return;
    }

    /* estimate number of tuples from previous tuple density */
    if (reltuples >= 0 && relpages > 0)
        density = reltuples / (double) relpages;
    else
    {
        /*
         * When we have no data because the relation was never yet vacuumed,
         * estimate tuple width from attribute datatypes.  We assume here that
         * the pages are completely full, which is OK for tables but is
         * probably an overestimate for indexes.  Fortunately
         * get_relation_info() can clamp the overestimate to the parent
         * table's size.
         *
         * Note: this code intentionally disregards alignment considerations,
         * because (a) that would be gilding the lily considering how crude
         * the estimate is, (b) it creates platform dependencies in the
         * default plans which are kind of a headache for regression testing,
         * and (c) different table AMs might use different padding schemes.
         */
        int32       tuple_width;

        tuple_width = get_rel_data_width(rel, attr_widths);
        tuple_width += overhead_bytes_per_tuple;
        /* note: integer division is intentional here */
        density = usable_bytes_per_page / tuple_width;
    }
    *tuples = rint(density * (double) curpages);

    /*
     * We use relallvisible as-is, rather than scaling it up like we do for
     * the pages and tuples counts, on the theory that any pages added since
     * the last VACUUM are most likely not marked all-visible.  But costsize.c
     * wants it converted to a fraction.
     */
    if (relallvisible == 0 || curpages <= 0)
        *allvisfrac = 0;
    else if ((double) relallvisible >= curpages)
        *allvisfrac = 1;
    else
        *allvisfrac = (double) relallvisible / curpages;
}