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pgstattuple: Optimize pgstattuple_approx() with streaming read

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This commit plugs into pgstattuple_approx(), the SQL function faster
than pgstattuple() that returns approximate results, the streaming read
APIs.  A callback is used to be able to skip all-visible pages via VM
lookup, to match with the logic prior to this commit.

Under test conditions similar to 6c228755ad (some dm_delay and
debug_io_direct=data), this can substantially improve the execution time
of the function, particularly for large relations.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CABPTF7VrqfbcDXqGrdLQ2xaQ=K0RzExNuw6U_GGqzSJu32wfdQ@mail.gmail.com
This commit is contained in:
Michael Paquier 2026-03-14 15:06:13 +09:00
parent 4deecb52af
commit ae58189a4d
2 changed files with 97 additions and 31 deletions
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127
contrib/pgstattuple/pgstatapprox.c
View file

@ -23,6 +23,7 @@
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/procarray.h"
#include "storage/read_stream.h"
PG_FUNCTION_INFO_V1(pgstattuple_approx);
PG_FUNCTION_INFO_V1(pgstattuple_approx_v1_5);
@ -45,6 +46,62 @@ typedef struct output_type
#define NUM_OUTPUT_COLUMNS 10
/*
* Struct for statapprox_heap read stream callback.
*/
typedef struct StatApproxReadStreamPrivate
{
Relation rel;
output_type *stat;
BlockNumber current_blocknum;
BlockNumber nblocks;
BlockNumber scanned; /* count of pages actually read */
Buffer vmbuffer; /* for VM lookups */
} StatApproxReadStreamPrivate;
/*
* Read stream callback for statapprox_heap.
*
* This callback checks the visibility map for each block. If the block is
* all-visible, we can get the free space from the FSM without reading the
* actual page, and skip to the next block. Only the blocks that are not
* all-visible are returned for actual reading after being locked.
*/
static BlockNumber
statapprox_heap_read_stream_next(ReadStream *stream,
void *callback_private_data,
void *per_buffer_data)
{
StatApproxReadStreamPrivate *p =
(StatApproxReadStreamPrivate *) callback_private_data;
while (p->current_blocknum < p->nblocks)
{
BlockNumber blkno = p->current_blocknum++;
Size freespace;
CHECK_FOR_INTERRUPTS();
/*
* If the page has only visible tuples, then we can find out the free
* space from the FSM and move on without reading the page.
*/
if (VM_ALL_VISIBLE(p->rel, blkno, &p->vmbuffer))
{
freespace = GetRecordedFreeSpace(p->rel, blkno);
p->stat->tuple_len += BLCKSZ - freespace;
p->stat->free_space += freespace;
continue;
}
/* This block needs to be read */
p->scanned++;
return blkno;
}
return InvalidBlockNumber;
}
/*
* This function takes an already open relation and scans its pages,
* skipping those that have the corresponding visibility map bit set.
@ -58,53 +115,58 @@ typedef struct output_type
static void
statapprox_heap(Relation rel, output_type *stat)
{
BlockNumber scanned,
nblocks,
blkno;
Buffer vmbuffer = InvalidBuffer;
BlockNumber nblocks;
BufferAccessStrategy bstrategy;
TransactionId OldestXmin;
StatApproxReadStreamPrivate p;
ReadStream *stream;
OldestXmin = GetOldestNonRemovableTransactionId(rel);
bstrategy = GetAccessStrategy(BAS_BULKREAD);
nblocks = RelationGetNumberOfBlocks(rel);
scanned = 0;
for (blkno = 0; blkno < nblocks; blkno++)
/* Initialize read stream private data */
p.rel = rel;
p.stat = stat;
p.current_blocknum = 0;
p.nblocks = nblocks;
p.scanned = 0;
p.vmbuffer = InvalidBuffer;
/*
* Create the read stream. We don't use READ_STREAM_USE_BATCHING because
* the callback accesses the visibility map which may need to read VM
* pages. While this shouldn't cause deadlocks, we err on the side of
* caution.
*/
stream = read_stream_begin_relation(READ_STREAM_FULL,
bstrategy,
rel,
MAIN_FORKNUM,
statapprox_heap_read_stream_next,
&p,
0);
for (;;)
{
Buffer buf;
Page page;
OffsetNumber offnum,
maxoff;
Size freespace;
BlockNumber blkno;
CHECK_FOR_INTERRUPTS();
/*
* If the page has only visible tuples, then we can find out the free
* space from the FSM and move on.
*/
if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
{
freespace = GetRecordedFreeSpace(rel, blkno);
stat->tuple_len += BLCKSZ - freespace;
stat->free_space += freespace;
continue;
}
buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno,
RBM_NORMAL, bstrategy);
buf = read_stream_next_buffer(stream, NULL);
if (buf == InvalidBuffer)
break;
LockBuffer(buf, BUFFER_LOCK_SHARE);
page = BufferGetPage(buf);
blkno = BufferGetBlockNumber(buf);
stat->free_space += PageGetExactFreeSpace(page);
/* We may count the page as scanned even if it's new/empty */
scanned++;
if (PageIsNew(page) || PageIsEmpty(page))
{
UnlockReleaseBuffer(buf);
@ -169,6 +231,9 @@ statapprox_heap(Relation rel, output_type *stat)
UnlockReleaseBuffer(buf);
}
Assert(p.current_blocknum == nblocks);
read_stream_end(stream);
stat->table_len = (uint64) nblocks * BLCKSZ;
/*
@ -179,7 +244,7 @@ statapprox_heap(Relation rel, output_type *stat)
* tuples in all-visible pages, so no correction is needed for that, and
* we already accounted for the space in those pages, too.
*/
stat->tuple_count = vac_estimate_reltuples(rel, nblocks, scanned,
stat->tuple_count = vac_estimate_reltuples(rel, nblocks, p.scanned,
stat->tuple_count);
/* It's not clear if we could get -1 here, but be safe. */
@ -190,16 +255,16 @@ statapprox_heap(Relation rel, output_type *stat)
*/
if (nblocks != 0)
{
stat->scanned_percent = 100.0 * scanned / nblocks;
stat->scanned_percent = 100.0 * p.scanned / nblocks;
stat->tuple_percent = 100.0 * stat->tuple_len / stat->table_len;
stat->dead_tuple_percent = 100.0 * stat->dead_tuple_len / stat->table_len;
stat->free_percent = 100.0 * stat->free_space / stat->table_len;
}
if (BufferIsValid(vmbuffer))
if (BufferIsValid(p.vmbuffer))
{
ReleaseBuffer(vmbuffer);
vmbuffer = InvalidBuffer;
ReleaseBuffer(p.vmbuffer);
p.vmbuffer = InvalidBuffer;
}
}

1
src/tools/pgindent/typedefs.list
View file

@ -2918,6 +2918,7 @@ StartReplicationCmd
StartupStatusEnum
StatEntry
StatExtEntry
StatApproxReadStreamPrivate
StateFileChunk
StatisticExtInfo
StatsBuildData