From 0f9bff594eccf2f7aea288e73f8c147edf884857 Mon Sep 17 00:00:00 2001 From: Naga Appani Date: Fri, 6 Jun 2025 05:18:15 +0000 Subject: [PATCH v1] Add pg_get_multixact_count function and related Add pg_get_multixact_count() SQL function for monitoring MultiXact usage PostgreSQL exposes mxid_age() to track MultiXact ID wraparound risk, but there is currently no SQL-accessible way to monitor MultiXact member consumption, which can independently trigger aggressive vacuuming or wraparound protection. The only workaround today involves scanning the pg_multixact/members directory, which is I/O intensive and unsuitable for monitoring tools. This patch adds pg_get_multixact_count(), a SQL-callable function that returns a composite record with two fields: - multixacts: number of MultiXact IDs that currently exist - members: number of MultiXact member entries that currently exist The function calls ReadMultiXactCounts() and returns the values using the standard record output convention. Documentation has been added to func.sgml under a new section titled "MultiXact Information Functions", with a cross-reference in maintenance.sgml to help users track MultiXact usage relative to autovacuum thresholds. Author: Naga Appani Reviewed-by: Discussion: https://www.postgresql.org/message-id/flat/CA%2BQeY%2BAAsYK6WvBW4qYzHz4bahHycDAY_q5ECmHkEV_eB9ckzg%40mail.gmail.com --- doc/src/sgml/func.sgml | 58 ++++++++++++++++++++++++++ doc/src/sgml/maintenance.sgml | 8 ++++ src/backend/access/transam/multixact.c | 30 +++++++++++++ src/include/catalog/pg_proc.dat | 15 +++++++ 4 files changed, 111 insertions(+) diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index c67688cbf5f..bdb64f83c23 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -28496,6 +28496,64 @@ acl | {postgres=arwdDxtm/postgres,foo=r/postgres} + + MultiXact Information Functions + + + The function shown in + exposes internal MultiXact counters used by + PostgreSQL's locking and transaction management subsystems. + It is primarily intended for monitoring and diagnostic purposes, such as analyzing + MultiXact consumption patterns or anticipating wraparound-related maintenance. + + + + MultiXact Information Functions + + + + + Function + Description + + + + + + + + pg_get_multixact_count + pg_get_multixact_count () + record + + + Returns a record with the fields multixacts and members: + + + multixacts: Number of MultiXacts assigned. + PostgreSQL initiates aggressive autovacuum when this value grows beyond the threshold + defined by autovacuum_multixact_freeze_max_age, which is based on + the age of datminmxid. For more details, see + + Routine Vacuuming: Multixact Wraparound. + + + members: Number of MultiXact member entries created. + These are stored in files under the pg_multixact/members subdirectory. + Wraparound occurs after approximately 4.29 billion entries (~20 GiB). PostgreSQL initiates + aggressive autovacuum when the number of members created exceeds approximately 2.145 billion + or when storage consumption in pg_multixact/members approaches 10 GiB. + + + + + + + +
+ + + diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml index 600e4b3f2f3..a445d1b061c 100644 --- a/doc/src/sgml/maintenance.sgml +++ b/doc/src/sgml/maintenance.sgml @@ -818,6 +818,14 @@ HINT: Execute a database-wide VACUUM in that database. area can grow up to about 20GB before reaching wraparound. + + The pg_get_multixact_count + function provides a way to check how many multixacts and member entries have been allocated. This can + be useful for identifying unusual multixact activity, monitoring progress toward wraparound, anticipating + system-wide aggressive autovacuum as usage approaches critical thresholds, or verifying whether autovacuum + is keeping up with demand. + + Similar to the XID case, if autovacuum fails to clear old MXIDs from a table, the system will begin to emit warning messages when the database's oldest MXIDs reach forty diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index 3c06ac45532..ed29746eaa9 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -3585,3 +3585,33 @@ multixactmemberssyncfiletag(const FileTag *ftag, char *path) { return SlruSyncFileTag(MultiXactMemberCtl, ftag, path); } + +/* + * Returns the current count of multixact members and multixact IDs + */ +PG_FUNCTION_INFO_V1(pg_get_multixact_count); + +Datum +pg_get_multixact_count(PG_FUNCTION_ARGS) +{ + TupleDesc tupdesc; + Datum values[2]; + bool nulls[2] = {false, false}; + MultiXactOffset members; + uint32 multixacts; + HeapTuple tuple; + + if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE) + ereport(ERROR, + (errmsg("return type must be a row type"))); + + if (!ReadMultiXactCounts(&multixacts, &members)) + ereport(ERROR, + (errmsg("could not read multixact counts"))); + + values[0] = UInt32GetDatum(multixacts); + values[1] = UInt32GetDatum(members); + + tuple = heap_form_tuple(tupdesc, values, nulls); + PG_RETURN_DATUM(HeapTupleGetDatum(tuple)); +} diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat index d3d28a263fa..09115ad1b35 100644 --- a/src/include/catalog/pg_proc.dat +++ b/src/include/catalog/pg_proc.dat @@ -12556,4 +12556,19 @@ proargnames => '{pid,io_id,io_generation,state,operation,off,length,target,handle_data_len,raw_result,result,target_desc,f_sync,f_localmem,f_buffered}', prosrc => 'pg_get_aios' }, +# Returns current counts of multixact members and multixact IDs +{ + oid => '9001', + descr => 'get current multixact member and multixact ID counts', + proname => 'pg_get_multixact_count', + prorettype => 'record', + proargtypes => '', + proallargtypes => '{int4,int8}', + proargmodes => '{o,o}', + proargnames => '{multixacts,members}', + provolatile => 'v', + proparallel => 's', + prosrc => 'pg_get_multixact_count' +}, + ] -- 2.47.1