access/table/tableam.c

/*----------------------------------------------------------------------
 *
 * 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;

	/* Open it at the smgr level if not already done */
	RelationOpenSmgr(rel);

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