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Add pgstattuple_approx() to the pgstattuple extension.

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The new function allows to estimate bloat and other table level statics
in a faster, but approximate, way. It does so by using information from
the free space map for pages marked as all visible in the visibility
map. The rest of the table is actually read and free space/bloat is
measured accurately.  In many cases that allows to get bloat information
much quicker, causing less IO.

Author: Abhijit Menon-Sen
Reviewed-By: Andres Freund, Amit Kapila and Tomas Vondra
Discussion: 20140402214144.GA28681@kea.toroid.org
This commit is contained in:
Andres Freund 2015-05-13 07:31:04 +02:00
parent dcf5e31908
commit 5850b20f58
6 changed files with 446 additions and 5 deletions
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4
contrib/pgstattuple/Makefile
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@ -1,10 +1,10 @@
# contrib/pgstattuple/Makefile
MODULE_big = pgstattuple
OBJS = pgstattuple.o pgstatindex.o $(WIN32RES)
OBJS = pgstattuple.o pgstatindex.o pgstatapprox.o $(WIN32RES)
EXTENSION = pgstattuple
DATA = pgstattuple--1.2.sql pgstattuple--1.1--1.2.sql pgstattuple--1.0--1.1.sql pgstattuple--unpackaged--1.0.sql
DATA = pgstattuple--1.3.sql pgstattuple--1.2--1.3.sql pgstattuple--1.1--1.2.sql pgstattuple--1.0--1.1.sql pgstattuple--unpackaged--1.0.sql
PGFILEDESC = "pgstattuple - tuple-level statistics"
REGRESS = pgstattuple

273
contrib/pgstattuple/pgstatapprox.c Normal file
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@ -0,0 +1,273 @@
/*-------------------------------------------------------------------------
*
* pgstatapproc.c
* Bloat estimation functions
*
* Copyright (c) 2014-2015, PostgreSQL Global Development Group
*
* IDENTIFICATION
* contrib/pgstattuple/pgstatapprox.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/visibilitymap.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/multixact.h"
#include "access/htup_details.h"
#include "catalog/namespace.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/procarray.h"
#include "storage/lmgr.h"
#include "utils/builtins.h"
#include "utils/tqual.h"
#include "commands/vacuum.h"
PG_FUNCTION_INFO_V1(pgstattuple_approx);
typedef struct output_type
{
uint64 table_len;
uint64 scanned_percent;
uint64 tuple_count;
uint64 tuple_len;
double tuple_percent;
uint64 dead_tuple_count;
uint64 dead_tuple_len;
double dead_tuple_percent;
uint64 free_space;
double free_percent;
} output_type;
#define NUM_OUTPUT_COLUMNS 10
/*
* This function takes an already open relation and scans its pages,
* skipping those that have the corresponding visibility map bit set.
* For pages we skip, we find the free space from the free space map
* and approximate tuple_len on that basis. For the others, we count
* the exact number of dead tuples etc.
*
* This scan is loosely based on vacuumlazy.c:lazy_scan_heap(), but
* we do not try to avoid skipping single pages.
*/
static void
statapprox_heap(Relation rel, output_type *stat)
{
BlockNumber scanned,
nblocks,
blkno;
Buffer vmbuffer = InvalidBuffer;
BufferAccessStrategy bstrategy;
TransactionId OldestXmin;
uint64 misc_count = 0;
OldestXmin = GetOldestXmin(rel, true);
bstrategy = GetAccessStrategy(BAS_BULKREAD);
nblocks = RelationGetNumberOfBlocks(rel);
scanned = 0;
for (blkno = 0; blkno < nblocks; blkno++)
{
Buffer buf;
Page page;
OffsetNumber offnum,
maxoff;
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.
*/
if (visibilitymap_test(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);
LockBuffer(buf, BUFFER_LOCK_SHARE);
page = BufferGetPage(buf);
/*
* It's not safe to call PageGetHeapFreeSpace() on new pages, so
* we treat them as being free space for our purposes.
*/
if (!PageIsNew(page))
stat->free_space += PageGetHeapFreeSpace(page);
else
stat->free_space += BLCKSZ - SizeOfPageHeaderData;
if (PageIsNew(page) || PageIsEmpty(page))
{
UnlockReleaseBuffer(buf);
continue;
}
scanned++;
/*
* Look at each tuple on the page and decide whether it's live
* or dead, then count it and its size. Unlike lazy_scan_heap,
* we can afford to ignore problems and special cases.
*/
maxoff = PageGetMaxOffsetNumber(page);
for (offnum = FirstOffsetNumber;
offnum <= maxoff;
offnum = OffsetNumberNext(offnum))
{
ItemId itemid;
HeapTupleData tuple;
itemid = PageGetItemId(page, offnum);
if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid) ||
ItemIdIsDead(itemid))
{
continue;
}
Assert(ItemIdIsNormal(itemid));
ItemPointerSet(&(tuple.t_self), blkno, offnum);
tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
tuple.t_len = ItemIdGetLength(itemid);
tuple.t_tableOid = RelationGetRelid(rel);
/*
* We count live and dead tuples, but we also need to add up
* others in order to feed vac_estimate_reltuples.
*/
switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
{
case HEAPTUPLE_RECENTLY_DEAD:
misc_count++;
/* Fall through */
case HEAPTUPLE_DEAD:
stat->dead_tuple_len += tuple.t_len;
stat->dead_tuple_count++;
break;
case HEAPTUPLE_LIVE:
stat->tuple_len += tuple.t_len;
stat->tuple_count++;
break;
case HEAPTUPLE_INSERT_IN_PROGRESS:
case HEAPTUPLE_DELETE_IN_PROGRESS:
misc_count++;
break;
default:
elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
break;
}
}
UnlockReleaseBuffer(buf);
}
stat->table_len = (uint64) nblocks * BLCKSZ;
stat->tuple_count = vac_estimate_reltuples(rel, false, nblocks, scanned,
stat->tuple_count+misc_count);
/*
* Calculate percentages if the relation has one or more pages.
*/
if (nblocks != 0)
{
stat->scanned_percent = 100 * 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))
{
ReleaseBuffer(vmbuffer);
vmbuffer = InvalidBuffer;
}
}
/*
* Returns estimated live/dead tuple statistics for the given relid.
*/
Datum
pgstattuple_approx(PG_FUNCTION_ARGS)
{
Oid relid = PG_GETARG_OID(0);
Relation rel;
output_type stat = {0};
TupleDesc tupdesc;
bool nulls[NUM_OUTPUT_COLUMNS];
Datum values[NUM_OUTPUT_COLUMNS];
HeapTuple ret;
int i = 0;
if (!superuser())
ereport(ERROR,
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
(errmsg("must be superuser to use pgstattuple functions"))));
if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
elog(ERROR, "return type must be a row type");
if (tupdesc->natts != NUM_OUTPUT_COLUMNS)
elog(ERROR, "incorrect number of output arguments");
rel = relation_open(relid, AccessShareLock);
/*
* Reject attempts to read non-local temporary relations; we would be
* likely to get wrong data since we have no visibility into the owning
* session's local buffers.
*/
if (RELATION_IS_OTHER_TEMP(rel))
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot access temporary tables of other sessions")));
/*
* We support only ordinary relations and materialised views,
* because we depend on the visibility map and free space map
* for our estimates about unscanned pages.
*/
if (!(rel->rd_rel->relkind == RELKIND_RELATION ||
rel->rd_rel->relkind == RELKIND_MATVIEW))
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("\"%s\" is not a table or materialized view",
RelationGetRelationName(rel))));
statapprox_heap(rel, &stat);
relation_close(rel, AccessShareLock);
memset(nulls, 0, sizeof(nulls));
values[i++] = Int64GetDatum(stat.table_len);
values[i++] = Float8GetDatum(stat.scanned_percent);
values[i++] = Int64GetDatum(stat.tuple_count);
values[i++] = Int64GetDatum(stat.tuple_len);
values[i++] = Float8GetDatum(stat.tuple_percent);
values[i++] = Int64GetDatum(stat.dead_tuple_count);
values[i++] = Int64GetDatum(stat.dead_tuple_len);
values[i++] = Float8GetDatum(stat.dead_tuple_percent);
values[i++] = Int64GetDatum(stat.free_space);
values[i++] = Float8GetDatum(stat.free_percent);
ret = heap_form_tuple(tupdesc, values, nulls);
return HeapTupleGetDatum(ret);
}

18
contrib/pgstattuple/pgstattuple--1.2--1.3.sql Normal file
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@ -0,0 +1,18 @@
/* contrib/pgstattuple/pgstattuple--1.2--1.3.sql */
-- complain if script is sourced in psql, rather than via ALTER EXTENSION
\echo Use "ALTER EXTENSION pgstattuple UPDATE TO '1.3'" to load this file. \quit
CREATE FUNCTION pgstattuple_approx(IN reloid regclass,
OUT table_len BIGINT, -- physical table length in bytes
OUT scanned_percent FLOAT8, -- what percentage of the table's pages was scanned
OUT approx_tuple_count BIGINT, -- estimated number of live tuples
OUT approx_tuple_len BIGINT, -- estimated total length in bytes of live tuples
OUT approx_tuple_percent FLOAT8, -- live tuples in % (based on estimate)
OUT dead_tuple_count BIGINT, -- exact number of dead tuples
OUT dead_tuple_len BIGINT, -- exact total length in bytes of dead tuples
OUT dead_tuple_percent FLOAT8, -- dead tuples in % (based on estimate)
OUT approx_free_space BIGINT, -- estimated free space in bytes
OUT approx_free_percent FLOAT8) -- free space in % (based on estimate)
AS 'MODULE_PATHNAME', 'pgstattuple_approx'
LANGUAGE C STRICT;

18
contrib/pgstattuple/pgstattuple--1.2.sql → contrib/pgstattuple/pgstattuple--1.3.sql
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@ -1,4 +1,4 @@
/* contrib/pgstattuple/pgstattuple--1.2.sql */
/* contrib/pgstattuple/pgstattuple--1.3.sql */
-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION pgstattuple" to load this file. \quit
@ -77,3 +77,19 @@ CREATE FUNCTION pg_relpages(IN relname regclass)
RETURNS BIGINT
AS 'MODULE_PATHNAME', 'pg_relpagesbyid'
LANGUAGE C STRICT;
/* New stuff in 1.3 begins here */
CREATE FUNCTION pgstattuple_approx(IN reloid regclass,
OUT table_len BIGINT, -- physical table length in bytes
OUT scanned_percent FLOAT8, -- what percentage of the table's pages was scanned
OUT approx_tuple_count BIGINT, -- estimated number of live tuples
OUT approx_tuple_len BIGINT, -- estimated total length in bytes of live tuples
OUT approx_tuple_percent FLOAT8, -- live tuples in % (based on estimate)
OUT dead_tuple_count BIGINT, -- exact number of dead tuples
OUT dead_tuple_len BIGINT, -- exact total length in bytes of dead tuples
OUT dead_tuple_percent FLOAT8, -- dead tuples in % (based on estimate)
OUT approx_free_space BIGINT, -- estimated free space in bytes
OUT approx_free_percent FLOAT8) -- free space in % (based on estimate)
AS 'MODULE_PATHNAME', 'pgstattuple_approx'
LANGUAGE C STRICT;

2
contrib/pgstattuple/pgstattuple.control
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@ -1,5 +1,5 @@
# pgstattuple extension
comment = 'show tuple-level statistics'
default_version = '1.2'
default_version = '1.3'
module_pathname = '$libdir/pgstattuple'
relocatable = true

136
doc/src/sgml/pgstattuple.sgml
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@ -358,6 +358,140 @@ pending_tuples | 0
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm>
<primary>pgstattuple_approx</primary>
</indexterm>
<function>pgstattuple_approx(regclass) returns record</>
</term>
<listitem>
<para>
<function>pgstattuple_approx</function> is a faster alternative to
<function>pgstattuple</function> that returns approximate results.
The argument is the target relation's OID.
For example:
<programlisting>
test=> SELECT * FROM pgstattuple_approx('pg_catalog.pg_proc'::regclass);
-[ RECORD 1 ]--------+-------
table_len | 573440
scanned_percent | 2
approx_tuple_count | 2740
approx_tuple_len | 561210
approx_tuple_percent | 97.87
dead_tuple_count | 0
dead_tuple_len | 0
dead_tuple_percent | 0
approx_free_space | 11996
approx_free_percent | 2.09
</programlisting>
The output columns are described in <xref linkend="pgstatapprox-columns">.
</para>
<para>
Whereas <function>pgstattuple</function> always performs a
full-table scan and returns an exact count of live and dead tuples
(and their sizes) and free space, <function>pgstattuple_approx</function>
tries to avoid the full-table scan and returns exact dead tuple
statistics along with an approximation of the number and
size of live tuples and free space.
</para>
<para>
It does this by skipping pages that have only visible tuples
according to the visibility map (if a page has the corresponding VM
bit set, then it is assumed to contain no dead tuples). For such
pages, it derives the free space value from the free space map, and
assumes that the rest of the space on the page is taken up by live
tuples.
</para>
<para>
For pages that cannot be skipped, it scans each tuple, recording its
presence and size in the appropriate counters, and adding up the
free space on the page. At the end, it estimates the total number of
live tuples based on the number of pages and tuples scanned (in the
same way that VACUUM estimates pg_class.reltuples).
</para>
<table id="pgstatapprox-columns">
<title><function>pgstattuple_approx</function> Output Columns</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>table_len</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Physical relation length in bytes (exact)</entry>
</row>
<row>
<entry><structfield>scanned_percent</structfield></entry>
<entry><type>float8</type></entry>
<entry>Percentage of table scanned</entry>
</row>
<row>
<entry><structfield>tuple_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of live tuples (estimated)</entry>
</row>
<row>
<entry><structfield>tuple_len</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Total length of live tuples in bytes (estimated)</entry>
</row>
<row>
<entry><structfield>tuple_percent</structfield></entry>
<entry><type>float8</type></entry>
<entry>Percentage of live tuples</entry>
</row>
<row>
<entry><structfield>dead_tuple_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of dead tuples (exact)</entry>
</row>
<row>
<entry><structfield>dead_tuple_len</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Total length of dead tuples in bytes (exact)</entry>
</row>
<row>
<entry><structfield>dead_tuple_percent</structfield></entry>
<entry><type>float8</type></entry>
<entry>Percentage of dead tuples</entry>
</row>
<row>
<entry><structfield>approx_free_space</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Total free space in bytes (estimated)</entry>
</row>
<row>
<entry><structfield>approx_free_percent</structfield></entry>
<entry><type>float8</type></entry>
<entry>Percentage of free space</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
In the above output, the free space figures may not match the
<function>pgstattuple</function> output exactly, because the free
space map gives us an exact figure, but is not guaranteed to be
accurate to the byte.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
@ -365,7 +499,7 @@ pending_tuples | 0
<title>Authors</title>
<para>
Tatsuo Ishii and Satoshi Nagayasu
Tatsuo Ishii, Satoshi Nagayasu and Abhijit Menon-Sen
</para>
</sect2>