path: root/src/backend/access/table/tableam.c

/*----------------------------------------------------------------------
 *
 * tableam.c
 *		Table access method routines too big to be inline functions.
 *
 * Portions Copyright (c) 1996-2019, 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 "access/heapam.h"		/* for ss_* */
#include "access/tableam.h"
#include "access/xact.h"
#include "storage/bufmgr.h"
#include "storage/shmem.h"


/* 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)
{
	Oid			relid = RelationGetRelid(relation);
	Snapshot	snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));

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

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

	RegisterSnapshot(snapshot);
	scan->rs_snapshot = snapshot;
	scan->rs_temp_snap = true;
}


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

	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);
	}
	else
	{
		/* SnapshotAny passed by caller (not serialized) */
		snapshot = SnapshotAny;
	}

	return relation->rd_tableam->scan_begin(relation, snapshot, 0, NULL, parallel_scan,
											true, true, true, false, false, !parallel_scan->phs_snapshot_any);
}


/* ----------------------------------------------------------------------------
 * 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, ParallelBlockTableScanDesc pbscan)
{
	BlockNumber sync_startpage = InvalidBlockNumber;

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, ParallelBlockTableScanDesc pbscan)
{
	BlockNumber page;
	uint64		nallocated;

	/*
	 * phs_nallocated tracks how many pages 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 page to return is calculated by adding the counter to the
	 * starting block number, modulo nblocks.
	 */
	nallocated = pg_atomic_fetch_add_u64(&pbscan->phs_nallocated, 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;
}