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In commit b0eaa4c51b
, we have avoided the creation of FSM for small tables.
So the functions that use FSM to compute the free space can return zero for
such tables. This was previously also possible for the cases where the
vacuum has not been triggered to update FSM.
This commit updates the comments in code and documentation to reflect this
behavior.
Author: John Naylor
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CACPNZCtba-3m1q3A8gxA_vxg=T7gQf7gMbpR4Ciy5LntY-j+0Q@mail.gmail.com
314 lines
8.7 KiB
C
314 lines
8.7 KiB
C
/*-------------------------------------------------------------------------
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*
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* pgstatapprox.c
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* Bloat estimation functions
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*
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* Copyright (c) 2014-2019, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* contrib/pgstattuple/pgstatapprox.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/heapam.h"
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#include "access/relation.h"
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#include "access/transam.h"
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#include "access/visibilitymap.h"
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#include "access/xact.h"
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#include "access/multixact.h"
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#include "access/htup_details.h"
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#include "catalog/namespace.h"
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#include "funcapi.h"
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#include "miscadmin.h"
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#include "storage/bufmgr.h"
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#include "storage/freespace.h"
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#include "storage/procarray.h"
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#include "storage/lmgr.h"
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#include "utils/builtins.h"
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#include "commands/vacuum.h"
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PG_FUNCTION_INFO_V1(pgstattuple_approx);
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PG_FUNCTION_INFO_V1(pgstattuple_approx_v1_5);
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Datum pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo);
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typedef struct output_type
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{
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uint64 table_len;
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uint64 scanned_percent;
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uint64 tuple_count;
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uint64 tuple_len;
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double tuple_percent;
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uint64 dead_tuple_count;
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uint64 dead_tuple_len;
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double dead_tuple_percent;
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uint64 free_space;
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double free_percent;
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} output_type;
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#define NUM_OUTPUT_COLUMNS 10
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/*
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* This function takes an already open relation and scans its pages,
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* skipping those that have the corresponding visibility map bit set.
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* For pages we skip, we find the free space from the free space map
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* and approximate tuple_len on that basis. For the others, we count
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* the exact number of dead tuples etc.
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*
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* This scan is loosely based on vacuumlazy.c:lazy_scan_heap(), but
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* we do not try to avoid skipping single pages.
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*/
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static void
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statapprox_heap(Relation rel, output_type *stat)
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{
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BlockNumber scanned,
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nblocks,
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blkno;
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Buffer vmbuffer = InvalidBuffer;
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BufferAccessStrategy bstrategy;
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TransactionId OldestXmin;
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OldestXmin = GetOldestXmin(rel, PROCARRAY_FLAGS_VACUUM);
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bstrategy = GetAccessStrategy(BAS_BULKREAD);
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nblocks = RelationGetNumberOfBlocks(rel);
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scanned = 0;
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for (blkno = 0; blkno < nblocks; blkno++)
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{
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Buffer buf;
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Page page;
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OffsetNumber offnum,
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maxoff;
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Size freespace;
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CHECK_FOR_INTERRUPTS();
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/*
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* If the page has only visible tuples, then we can find out the free
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* space from the FSM and move on.
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*
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* Note: If a relation has no FSM, GetRecordedFreeSpace() will report
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* zero free space. This is fine for the purposes of approximation.
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*/
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if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
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{
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freespace = GetRecordedFreeSpace(rel, blkno);
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stat->tuple_len += BLCKSZ - freespace;
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stat->free_space += freespace;
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continue;
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}
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buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno,
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RBM_NORMAL, bstrategy);
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LockBuffer(buf, BUFFER_LOCK_SHARE);
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page = BufferGetPage(buf);
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/*
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* It's not safe to call PageGetHeapFreeSpace() on new pages, so we
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* treat them as being free space for our purposes.
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*/
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if (!PageIsNew(page))
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stat->free_space += PageGetHeapFreeSpace(page);
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else
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stat->free_space += BLCKSZ - SizeOfPageHeaderData;
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/* We may count the page as scanned even if it's new/empty */
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scanned++;
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if (PageIsNew(page) || PageIsEmpty(page))
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{
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UnlockReleaseBuffer(buf);
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continue;
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}
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/*
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* Look at each tuple on the page and decide whether it's live or
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* dead, then count it and its size. Unlike lazy_scan_heap, we can
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* afford to ignore problems and special cases.
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*/
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maxoff = PageGetMaxOffsetNumber(page);
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for (offnum = FirstOffsetNumber;
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offnum <= maxoff;
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offnum = OffsetNumberNext(offnum))
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{
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ItemId itemid;
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HeapTupleData tuple;
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itemid = PageGetItemId(page, offnum);
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if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid) ||
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ItemIdIsDead(itemid))
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{
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continue;
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}
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Assert(ItemIdIsNormal(itemid));
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ItemPointerSet(&(tuple.t_self), blkno, offnum);
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tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
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tuple.t_len = ItemIdGetLength(itemid);
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tuple.t_tableOid = RelationGetRelid(rel);
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/*
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* We follow VACUUM's lead in counting INSERT_IN_PROGRESS tuples
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* as "dead" while DELETE_IN_PROGRESS tuples are "live". We don't
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* bother distinguishing tuples inserted/deleted by our own
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* transaction.
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*/
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switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
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{
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case HEAPTUPLE_LIVE:
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case HEAPTUPLE_DELETE_IN_PROGRESS:
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stat->tuple_len += tuple.t_len;
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stat->tuple_count++;
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break;
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case HEAPTUPLE_DEAD:
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case HEAPTUPLE_RECENTLY_DEAD:
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case HEAPTUPLE_INSERT_IN_PROGRESS:
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stat->dead_tuple_len += tuple.t_len;
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stat->dead_tuple_count++;
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break;
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default:
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elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
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break;
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}
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}
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UnlockReleaseBuffer(buf);
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}
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stat->table_len = (uint64) nblocks * BLCKSZ;
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/*
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* We don't know how many tuples are in the pages we didn't scan, so
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* extrapolate the live-tuple count to the whole table in the same way
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* that VACUUM does. (Like VACUUM, we're not taking a random sample, so
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* just extrapolating linearly seems unsafe.) There should be no dead
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* tuples in all-visible pages, so no correction is needed for that, and
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* we already accounted for the space in those pages, too.
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*/
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stat->tuple_count = vac_estimate_reltuples(rel, nblocks, scanned,
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stat->tuple_count);
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/*
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* Calculate percentages if the relation has one or more pages.
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*/
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if (nblocks != 0)
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{
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stat->scanned_percent = 100 * scanned / nblocks;
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stat->tuple_percent = 100.0 * stat->tuple_len / stat->table_len;
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stat->dead_tuple_percent = 100.0 * stat->dead_tuple_len / stat->table_len;
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stat->free_percent = 100.0 * stat->free_space / stat->table_len;
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}
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if (BufferIsValid(vmbuffer))
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{
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ReleaseBuffer(vmbuffer);
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vmbuffer = InvalidBuffer;
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}
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}
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/*
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* Returns estimated live/dead tuple statistics for the given relid.
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*
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* The superuser() check here must be kept as the library might be upgraded
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* without the extension being upgraded, meaning that in pre-1.5 installations
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* these functions could be called by any user.
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*/
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Datum
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pgstattuple_approx(PG_FUNCTION_ARGS)
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{
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Oid relid = PG_GETARG_OID(0);
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if (!superuser())
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ereport(ERROR,
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(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
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(errmsg("must be superuser to use pgstattuple functions"))));
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PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
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}
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/*
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* As of pgstattuple version 1.5, we no longer need to check if the user
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* is a superuser because we REVOKE EXECUTE on the SQL function from PUBLIC.
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* Users can then grant access to it based on their policies.
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*
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* Otherwise identical to pgstattuple_approx (above).
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*/
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Datum
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pgstattuple_approx_v1_5(PG_FUNCTION_ARGS)
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{
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Oid relid = PG_GETARG_OID(0);
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PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
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}
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Datum
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pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo)
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{
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Relation rel;
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output_type stat = {0};
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TupleDesc tupdesc;
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bool nulls[NUM_OUTPUT_COLUMNS];
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Datum values[NUM_OUTPUT_COLUMNS];
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HeapTuple ret;
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int i = 0;
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if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
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elog(ERROR, "return type must be a row type");
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if (tupdesc->natts != NUM_OUTPUT_COLUMNS)
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elog(ERROR, "incorrect number of output arguments");
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rel = relation_open(relid, AccessShareLock);
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/*
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* Reject attempts to read non-local temporary relations; we would be
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* likely to get wrong data since we have no visibility into the owning
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* session's local buffers.
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*/
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if (RELATION_IS_OTHER_TEMP(rel))
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ereport(ERROR,
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(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
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errmsg("cannot access temporary tables of other sessions")));
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/*
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* We support only ordinary relations and materialised views, because we
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* depend on the visibility map and free space map for our estimates about
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* unscanned pages.
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*/
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if (!(rel->rd_rel->relkind == RELKIND_RELATION ||
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rel->rd_rel->relkind == RELKIND_MATVIEW))
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ereport(ERROR,
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(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
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errmsg("\"%s\" is not a table or materialized view",
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RelationGetRelationName(rel))));
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statapprox_heap(rel, &stat);
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relation_close(rel, AccessShareLock);
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memset(nulls, 0, sizeof(nulls));
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values[i++] = Int64GetDatum(stat.table_len);
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values[i++] = Float8GetDatum(stat.scanned_percent);
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values[i++] = Int64GetDatum(stat.tuple_count);
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values[i++] = Int64GetDatum(stat.tuple_len);
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values[i++] = Float8GetDatum(stat.tuple_percent);
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values[i++] = Int64GetDatum(stat.dead_tuple_count);
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values[i++] = Int64GetDatum(stat.dead_tuple_len);
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values[i++] = Float8GetDatum(stat.dead_tuple_percent);
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values[i++] = Int64GetDatum(stat.free_space);
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values[i++] = Float8GetDatum(stat.free_percent);
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ret = heap_form_tuple(tupdesc, values, nulls);
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return HeapTupleGetDatum(ret);
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}
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