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
This commit is contained in:
Andres Freund
parent
7f8d9cedb3
commit
2589434ae0
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@ -7,7 +7,8 @@ OBJS = \
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EXTENSION = pg_buffercache
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DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
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pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql
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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"
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REGRESS = pg_buffercache
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@ -8,3 +8,36 @@ from pg_buffercache;
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t
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(1 row)
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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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@ -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,
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)
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@ -0,0 +1,17 @@
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/* contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql */
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-- 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
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CREATE FUNCTION pg_buffercache_summary(
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OUT buffers_used int4,
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OUT buffers_unused int4,
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OUT buffers_dirty int4,
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OUT buffers_pinned int4,
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OUT usagecount_avg float8)
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AS 'MODULE_PATHNAME', 'pg_buffercache_summary'
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LANGUAGE C PARALLEL SAFE;
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-- Don't want these to be available to public.
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REVOKE ALL ON FUNCTION pg_buffercache_summary() FROM PUBLIC;
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GRANT EXECUTE ON FUNCTION pg_buffercache_summary() TO pg_monitor;
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@ -1,5 +1,5 @@
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# pg_buffercache extension
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comment = 'examine the shared buffer cache'
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default_version = '1.3'
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default_version = '1.4'
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module_pathname = '$libdir/pg_buffercache'
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relocatable = true
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@ -17,6 +17,7 @@
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#define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8
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#define NUM_BUFFERCACHE_PAGES_ELEM 9
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#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
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PG_MODULE_MAGIC;
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@ -59,6 +60,7 @@ typedef struct
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* relation node/tablespace/database/blocknum and dirty indicator.
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*/
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PG_FUNCTION_INFO_V1(pg_buffercache_pages);
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PG_FUNCTION_INFO_V1(pg_buffercache_summary);
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Datum
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pg_buffercache_pages(PG_FUNCTION_ARGS)
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@ -237,3 +239,68 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
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else
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SRF_RETURN_DONE(funcctx);
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}
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Datum
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pg_buffercache_summary(PG_FUNCTION_ARGS)
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{
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Datum result;
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TupleDesc tupledesc;
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HeapTuple tuple;
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Datum values[NUM_BUFFERCACHE_SUMMARY_ELEM];
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bool nulls[NUM_BUFFERCACHE_SUMMARY_ELEM];
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int32 buffers_used = 0;
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int32 buffers_unused = 0;
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int32 buffers_dirty = 0;
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int32 buffers_pinned = 0;
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int64 usagecount_total = 0;
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if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
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elog(ERROR, "return type must be a row type");
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for (int i = 0; i < NBuffers; i++)
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{
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BufferDesc *bufHdr;
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uint32 buf_state;
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/*
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* This function summarizes the state of all headers. Locking the
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* buffer headers wouldn't provide an improved result as the state of
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* the buffer can still change after we release the lock and it'd
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* noticeably increase the cost of the function.
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*/
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bufHdr = GetBufferDescriptor(i);
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buf_state = pg_atomic_read_u32(&bufHdr->state);
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if (buf_state & BM_VALID)
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{
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buffers_used++;
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usagecount_total += BUF_STATE_GET_USAGECOUNT(buf_state);
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if (buf_state & BM_DIRTY)
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buffers_dirty++;
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}
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else
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buffers_unused++;
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if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
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buffers_pinned++;
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}
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memset(nulls, 0, sizeof(nulls));
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values[0] = Int32GetDatum(buffers_used);
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values[1] = Int32GetDatum(buffers_unused);
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values[2] = Int32GetDatum(buffers_dirty);
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values[3] = Int32GetDatum(buffers_pinned);
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if (buffers_used != 0)
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values[4] = Float8GetDatum((double) usagecount_total / buffers_used);
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else
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nulls[4] = true;
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/* Build and return the tuple. */
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tuple = heap_form_tuple(tupledesc, values, nulls);
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result = HeapTupleGetDatum(tuple);
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PG_RETURN_DATUM(result);
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}
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@ -4,3 +4,21 @@ select count(*) = (select setting::bigint
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from pg_settings
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where name = 'shared_buffers')
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from pg_buffercache;
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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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-- 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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SELECT * FROM pg_buffercache_pages() AS p (wrong int);
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SELECT * FROM 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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SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
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@ -16,13 +16,28 @@
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<primary>pg_buffercache_pages</primary>
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</indexterm>
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<indexterm>
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<primary>pg_buffercache_summary</primary>
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</indexterm>
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<para>
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The module provides a C function <function>pg_buffercache_pages</function>
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that returns a set of records, plus a view
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<structname>pg_buffercache</structname> that wraps the function for
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The module provides the <function>pg_buffercache_pages()</function>
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function, wrapped in the <structname>pg_buffercache</structname> view, and
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the <function>pg_buffercache_summary()</function> function.
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</para>
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<para>
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The <function>pg_buffercache_pages()</function> function returns a set of
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records, each row describing the state of one shared buffer entry. The
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<structname>pg_buffercache</structname> view wraps the function for
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convenient use.
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</para>
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<para>
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The <function>pg_buffercache_summary()</function> function returns a single
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row summarizing the state of the shared buffer cache.
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</para>
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<para>
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By default, use is restricted to superusers and roles with privileges of the
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<literal>pg_monitor</literal> role. Access may be granted to others
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@ -164,6 +179,92 @@
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</para>
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</sect2>
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<sect2>
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<title>The <function>pg_buffercache_summary()</function> Function</title>
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<para>
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The definitions of the columns exposed by the function are shown in <xref linkend="pgbuffercache_summary-columns"/>.
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</para>
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<table id="pgbuffercache_summary-columns">
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<title><function>pg_buffercache_summary()</function> Output Columns</title>
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<tgroup cols="1">
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<thead>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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Column Type
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</para>
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<para>
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Description
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</para></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>buffers_used</structfield> <type>int4</type>
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</para>
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<para>
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Number of unused shared buffers
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>buffers_unused</structfield> <type>int4</type>
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</para>
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<para>
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Number of unused shared buffers
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>buffers_dirty</structfield> <type>int4</type>
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</para>
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<para>
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Number of dirty shared buffers
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>buffers_pinned</structfield> <type>int4</type>
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</para>
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<para>
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Number of pinned shared buffers
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>usagecount_avg</structfield> <type>float8</type>
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</para>
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<para>
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Average usagecount of used shared buffers
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</para></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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The <function>pg_buffercache_summary()</function> function returns a
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single row summarizing the state of all shared buffers. Similar and more
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detailed information is provided by the
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<structname>pg_buffercache</structname> view, but
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<function>pg_buffercache_summary()</function> is significantly cheaper.
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</para>
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<para>
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Like the <structname>pg_buffercache</structname> view,
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<function>pg_buffercache_summary()</function> does not acquire buffer
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manager locks. Therefore concurrent activity can lead to minor inaccuracies
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in the result.
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</para>
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</sect2>
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<sect2>
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<title>Sample Output</title>
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@ -191,6 +292,13 @@ regression=# SELECT n.nspname, c.relname, count(*) AS buffers
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public | gin_test_tbl | 188
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public | spgist_text_tbl | 182
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(10 rows)
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regression=# SELECT * FROM pg_buffercache_summary();
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buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg
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--------------+----------------+---------------+----------------+----------------
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248 | 2096904 | 39 | 0 | 3.141129
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(1 row)
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</screen>
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</sect2>
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