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pg_buffercache: Add pg_buffercache_summary()
Using pg_buffercache_summary() is significantly cheaper than querying pg_buffercache and summarizing in SQL. Author: Melih Mutlu <m.melihmutlu@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Aleksander Alekseev <aleksander@timescale.com> Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com> Discussion: https://postgr.es/m/CAGPVpCQAXYo54Q%3D8gqBsS%3Du0uk9qhnnq4%2B710BtUhUisX1XGEg%40mail.gmail.com
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‎contrib/pg_buffercache/Makefile

+2-1
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,8 @@ OBJS = \
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88
EXTENSION = pg_buffercache
99
DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
10-
pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql
10+
pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \
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pg_buffercache--1.3--1.4.sql
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PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
1213

1314
REGRESS = pg_buffercache

‎contrib/pg_buffercache/expected/pg_buffercache.out

+33
Original file line numberDiff line numberDiff line change
@@ -8,3 +8,36 @@ from pg_buffercache;
88
t
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(1 row)
1010

11+
select buffers_used + buffers_unused > 0,
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buffers_dirty <= buffers_used,
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buffers_pinned <= buffers_used
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from pg_buffercache_summary();
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?column? | ?column? | ?column?
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----------+----------+----------
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t | t | t
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(1 row)
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-- Check that the functions / views can't be accessed by default. To avoid
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-- having to create a dedicated user, use the pg_database_owner pseudo-role.
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SET ROLE pg_database_owner;
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SELECT * FROM pg_buffercache;
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ERROR: permission denied for view pg_buffercache
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SELECT * FROM pg_buffercache_pages() AS p (wrong int);
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ERROR: permission denied for function pg_buffercache_pages
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SELECT * FROM pg_buffercache_summary();
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ERROR: permission denied for function pg_buffercache_summary
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RESET role;
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-- Check that pg_monitor is allowed to query view / function
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SET ROLE pg_monitor;
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SELECT count(*) > 0 FROM pg_buffercache;
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?column?
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----------
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t
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(1 row)
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SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
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?column?
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----------
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t
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(1 row)
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‎contrib/pg_buffercache/meson.build

+1
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,7 @@ install_data(
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'pg_buffercache--1.1--1.2.sql',
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'pg_buffercache--1.2--1.3.sql',
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'pg_buffercache--1.2.sql',
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'pg_buffercache--1.3--1.4.sql',
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'pg_buffercache.control',
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kwargs: contrib_data_args,
2425
)

‎contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql

+17
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,17 @@
1+
/* contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql */
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3+
-- complain if script is sourced in psql, rather than via ALTER EXTENSION
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\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.4'" to load this file. \quit
5+
6+
CREATE FUNCTION pg_buffercache_summary(
7+
OUT buffers_used int4,
8+
OUT buffers_unused int4,
9+
OUT buffers_dirty int4,
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OUT buffers_pinned int4,
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OUT usagecount_avg float8)
12+
AS 'MODULE_PATHNAME', 'pg_buffercache_summary'
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LANGUAGE C PARALLEL SAFE;
14+
15+
-- Don't want these to be available to public.
16+
REVOKE ALL ON FUNCTION pg_buffercache_summary() FROM PUBLIC;
17+
GRANT EXECUTE ON FUNCTION pg_buffercache_summary() TO pg_monitor;

‎contrib/pg_buffercache/pg_buffercache.control

+1-1
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
# pg_buffercache extension
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comment = 'examine the shared buffer cache'
3-
default_version = '1.3'
3+
default_version = '1.4'
44
module_pathname = '$libdir/pg_buffercache'
55
relocatable = true

‎contrib/pg_buffercache/pg_buffercache_pages.c

+67
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,7 @@
1717

1818
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8
1919
#define NUM_BUFFERCACHE_PAGES_ELEM 9
20+
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
2021

2122
PG_MODULE_MAGIC;
2223

@@ -59,6 +60,7 @@ typedef struct
5960
* relation node/tablespace/database/blocknum and dirty indicator.
6061
*/
6162
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
63+
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
6264

6365
Datum
6466
pg_buffercache_pages(PG_FUNCTION_ARGS)
@@ -237,3 +239,68 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
237239
else
238240
SRF_RETURN_DONE(funcctx);
239241
}
242+
243+
Datum
244+
pg_buffercache_summary(PG_FUNCTION_ARGS)
245+
{
246+
Datum result;
247+
TupleDesc tupledesc;
248+
HeapTuple tuple;
249+
Datum values[NUM_BUFFERCACHE_SUMMARY_ELEM];
250+
bool nulls[NUM_BUFFERCACHE_SUMMARY_ELEM];
251+
252+
int32 buffers_used = 0;
253+
int32 buffers_unused = 0;
254+
int32 buffers_dirty = 0;
255+
int32 buffers_pinned = 0;
256+
int64 usagecount_total = 0;
257+
258+
if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
259+
elog(ERROR, "return type must be a row type");
260+
261+
for (int i = 0; i < NBuffers; i++)
262+
{
263+
BufferDesc *bufHdr;
264+
uint32 buf_state;
265+
266+
/*
267+
* This function summarizes the state of all headers. Locking the
268+
* buffer headers wouldn't provide an improved result as the state of
269+
* the buffer can still change after we release the lock and it'd
270+
* noticeably increase the cost of the function.
271+
*/
272+
bufHdr = GetBufferDescriptor(i);
273+
buf_state = pg_atomic_read_u32(&bufHdr->state);
274+
275+
if (buf_state & BM_VALID)
276+
{
277+
buffers_used++;
278+
usagecount_total += BUF_STATE_GET_USAGECOUNT(buf_state);
279+
280+
if (buf_state & BM_DIRTY)
281+
buffers_dirty++;
282+
}
283+
else
284+
buffers_unused++;
285+
286+
if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
287+
buffers_pinned++;
288+
}
289+
290+
memset(nulls, 0, sizeof(nulls));
291+
values[0] = Int32GetDatum(buffers_used);
292+
values[1] = Int32GetDatum(buffers_unused);
293+
values[2] = Int32GetDatum(buffers_dirty);
294+
values[3] = Int32GetDatum(buffers_pinned);
295+
296+
if (buffers_used != 0)
297+
values[4] = Float8GetDatum((double) usagecount_total / buffers_used);
298+
else
299+
nulls[4] = true;
300+
301+
/* Build and return the tuple. */
302+
tuple = heap_form_tuple(tupledesc, values, nulls);
303+
result = HeapTupleGetDatum(tuple);
304+
305+
PG_RETURN_DATUM(result);
306+
}

‎contrib/pg_buffercache/sql/pg_buffercache.sql

+18
Original file line numberDiff line numberDiff line change
@@ -4,3 +4,21 @@ select count(*) = (select setting::bigint
44
from pg_settings
55
where name = 'shared_buffers')
66
from pg_buffercache;
7+
8+
select buffers_used + buffers_unused > 0,
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buffers_dirty <= buffers_used,
10+
buffers_pinned <= buffers_used
11+
from pg_buffercache_summary();
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13+
-- Check that the functions / views can't be accessed by default. To avoid
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-- having to create a dedicated user, use the pg_database_owner pseudo-role.
15+
SET ROLE pg_database_owner;
16+
SELECT * FROM pg_buffercache;
17+
SELECT * FROM pg_buffercache_pages() AS p (wrong int);
18+
SELECT * FROM pg_buffercache_summary();
19+
RESET role;
20+
21+
-- Check that pg_monitor is allowed to query view / function
22+
SET ROLE pg_monitor;
23+
SELECT count(*) > 0 FROM pg_buffercache;
24+
SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();

‎doc/src/sgml/pgbuffercache.sgml

+111-3
Original file line numberDiff line numberDiff line change
@@ -16,13 +16,28 @@
1616
<primary>pg_buffercache_pages</primary>
1717
</indexterm>
1818

19+
<indexterm>
20+
<primary>pg_buffercache_summary</primary>
21+
</indexterm>
22+
23+
<para>
24+
The module provides the <function>pg_buffercache_pages()</function>
25+
function, wrapped in the <structname>pg_buffercache</structname> view, and
26+
the <function>pg_buffercache_summary()</function> function.
27+
</para>
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1929
<para>
20-
The module provides a C function <function>pg_buffercache_pages</function>
21-
that returns a set of records, plus a view
22-
<structname>pg_buffercache</structname> that wraps the function for
30+
The <function>pg_buffercache_pages()</function> function returns a set of
31+
records, each row describing the state of one shared buffer entry. The
32+
<structname>pg_buffercache</structname> view wraps the function for
2333
convenient use.
2434
</para>
2535

36+
<para>
37+
The <function>pg_buffercache_summary()</function> function returns a single
38+
row summarizing the state of the shared buffer cache.
39+
</para>
40+
2641
<para>
2742
By default, use is restricted to superusers and roles with privileges of the
2843
<literal>pg_monitor</literal> role. Access may be granted to others
@@ -164,6 +179,92 @@
164179
</para>
165180
</sect2>
166181

182+
<sect2>
183+
<title>The <function>pg_buffercache_summary()</function> Function</title>
184+
185+
<para>
186+
The definitions of the columns exposed by the function are shown in <xref linkend="pgbuffercache_summary-columns"/>.
187+
</para>
188+
189+
<table id="pgbuffercache_summary-columns">
190+
<title><function>pg_buffercache_summary()</function> Output Columns</title>
191+
<tgroup cols="1">
192+
<thead>
193+
<row>
194+
<entry role="catalog_table_entry"><para role="column_definition">
195+
Column Type
196+
</para>
197+
<para>
198+
Description
199+
</para></entry>
200+
</row>
201+
</thead>
202+
203+
<tbody>
204+
<row>
205+
<entry role="catalog_table_entry"><para role="column_definition">
206+
<structfield>buffers_used</structfield> <type>int4</type>
207+
</para>
208+
<para>
209+
Number of unused shared buffers
210+
</para></entry>
211+
</row>
212+
213+
<row>
214+
<entry role="catalog_table_entry"><para role="column_definition">
215+
<structfield>buffers_unused</structfield> <type>int4</type>
216+
</para>
217+
<para>
218+
Number of unused shared buffers
219+
</para></entry>
220+
</row>
221+
222+
<row>
223+
<entry role="catalog_table_entry"><para role="column_definition">
224+
<structfield>buffers_dirty</structfield> <type>int4</type>
225+
</para>
226+
<para>
227+
Number of dirty shared buffers
228+
</para></entry>
229+
</row>
230+
231+
<row>
232+
<entry role="catalog_table_entry"><para role="column_definition">
233+
<structfield>buffers_pinned</structfield> <type>int4</type>
234+
</para>
235+
<para>
236+
Number of pinned shared buffers
237+
</para></entry>
238+
</row>
239+
240+
<row>
241+
<entry role="catalog_table_entry"><para role="column_definition">
242+
<structfield>usagecount_avg</structfield> <type>float8</type>
243+
</para>
244+
<para>
245+
Average usagecount of used shared buffers
246+
</para></entry>
247+
</row>
248+
</tbody>
249+
</tgroup>
250+
</table>
251+
252+
<para>
253+
The <function>pg_buffercache_summary()</function> function returns a
254+
single row summarizing the state of all shared buffers. Similar and more
255+
detailed information is provided by the
256+
<structname>pg_buffercache</structname> view, but
257+
<function>pg_buffercache_summary()</function> is significantly cheaper.
258+
</para>
259+
260+
<para>
261+
Like the <structname>pg_buffercache</structname> view,
262+
<function>pg_buffercache_summary()</function> does not acquire buffer
263+
manager locks. Therefore concurrent activity can lead to minor inaccuracies
264+
in the result.
265+
</para>
266+
</sect2>
267+
167268
<sect2>
168269
<title>Sample Output</title>
169270

@@ -191,6 +292,13 @@ regression=# SELECT n.nspname, c.relname, count(*) AS buffers
191292
public | gin_test_tbl | 188
192293
public | spgist_text_tbl | 182
193294
(10 rows)
295+
296+
297+
regression=# SELECT * FROM pg_buffercache_summary();
298+
buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg
299+
--------------+----------------+---------------+----------------+----------------
300+
248 | 2096904 | 39 | 0 | 3.141129
301+
(1 row)
194302
</screen>
195303
</sect2>
196304

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