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Expose some information about backend subxact status.
A new function pg_stat_get_backend_subxact() can be used to get information about the number of subtransactions in the cache of a particular backend and whether that cache has overflowed. This can be useful for tracking down performance problems that can result from overflowed snapshots. Dilip Kumar, reviewed by Zhihong Yu, Nikolay Samokhvalov, Justin Pryzby, Nathan Bossart, Ashutosh Sharma, Julien Rouhaud. Additional design comments from Andres Freund, Tom Lane, Bruce Momjian, and David G. Johnston. Discussion: http://postgr.es/m/CAFiTN-ut0uwkRJDQJeDPXpVyTWD46m3gt3JDToE02hTfONEN=Q@mail.gmail.com
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-7
lines changed

8 files changed

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‎doc/src/sgml/monitoring.sgml

Lines changed: 18 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -5671,6 +5671,24 @@ FROM pg_stat_get_backend_idset() AS backendid;
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</para></entry>
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</row>
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<row>
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<entry role="func_table_entry"><para role="func_signature">
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<indexterm>
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<primary>pg_stat_get_backend_subxact</primary>
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</indexterm>
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<function>pg_stat_get_backend_subxact</function> ( <type>integer</type> )
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<returnvalue>record</returnvalue>
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</para>
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<para>
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Returns a record of information about the subtransactions of the
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backend with the specified ID.
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The fields returned are <parameter>subxact_count</parameter>, which
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is the number of subtransactions in the backend's subtransaction cache,
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and <parameter>subxact_overflow</parameter>, which indicates whether
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the backend's subtransaction cache is overflowed or not.
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</para></entry>
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</row>
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<row>
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<entry role="func_table_entry"><para role="func_signature">
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<indexterm>

‎src/backend/storage/ipc/sinvaladt.c

Lines changed: 9 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -404,17 +404,20 @@ BackendIdGetProc(int backendID)
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/*
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* BackendIdGetTransactionIds
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* Get the xid and xmin of the backend. The result may be out of date
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* arbitrarily quickly, so the caller must be careful about how this
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* information is used.
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* Get the xid, xmin, nsubxid and overflow status of the backend. The
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* result may be out of date arbitrarily quickly, so the caller must be
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* careful about how this information is used.
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*/
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void
412-
BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmin)
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BackendIdGetTransactionIds(int backendID, TransactionId *xid,
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TransactionId *xmin, int *nsubxid, bool *overflowed)
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{
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SISeg *segP = shmInvalBuffer;
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*xid = InvalidTransactionId;
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*xmin = InvalidTransactionId;
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*nsubxid = 0;
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*overflowed = false;
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/* Need to lock out additions/removals of backends */
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LWLockAcquire(SInvalWriteLock, LW_SHARED);
@@ -428,6 +431,8 @@ BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmi
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{
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*xid = proc->xid;
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*xmin = proc->xmin;
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*nsubxid = proc->subxidStatus.count;
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*overflowed = proc->subxidStatus.overflowed;
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}
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}
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‎src/backend/utils/activity/backend_status.c

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -855,7 +855,9 @@ pgstat_read_current_status(void)
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localentry->backend_id = i;
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BackendIdGetTransactionIds(i,
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&localentry->backend_xid,
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&localentry->backend_xmin);
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&localentry->backend_xmin,
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&localentry->backend_subxact_count,
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&localentry->backend_subxact_overflowed);
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localentry++;
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localappname += NAMEDATALEN;

‎src/backend/utils/adt/pgstatfuncs.c

Lines changed: 38 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -687,6 +687,44 @@ pg_stat_get_backend_userid(PG_FUNCTION_ARGS)
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PG_RETURN_OID(beentry->st_userid);
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}
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Datum
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pg_stat_get_backend_subxact(PG_FUNCTION_ARGS)
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{
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#define PG_STAT_GET_SUBXACT_COLS 2
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TupleDesc tupdesc;
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Datum values[PG_STAT_GET_SUBXACT_COLS];
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bool nulls[PG_STAT_GET_SUBXACT_COLS];
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int32 beid = PG_GETARG_INT32(0);
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LocalPgBackendStatus *local_beentry;
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/* Initialise values and NULL flags arrays */
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MemSet(values, 0, sizeof(values));
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MemSet(nulls, 0, sizeof(nulls));
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/* Initialise attributes information in the tuple descriptor */
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tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_SUBXACT_COLS);
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TupleDescInitEntry(tupdesc, (AttrNumber) 1, "subxact_count",
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INT4OID, -1, 0);
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TupleDescInitEntry(tupdesc, (AttrNumber) 2, "subxact_overflow",
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BOOLOID, -1, 0);
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BlessTupleDesc(tupdesc);
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if ((local_beentry = pgstat_fetch_stat_local_beentry(beid)) != NULL)
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{
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/* Fill values and NULLs */
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values[0] = Int32GetDatum(local_beentry->backend_subxact_count);
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values[1] = BoolGetDatum(local_beentry->backend_subxact_overflowed);
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}
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else
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{
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nulls[0] = true;
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nulls[1] = true;
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}
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/* Returns the record as Datum */
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PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
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}
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Datum
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pg_stat_get_backend_activity(PG_FUNCTION_ARGS)

‎src/include/catalog/catversion.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -57,6 +57,6 @@
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*/
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/* yyyymmddN */
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#define CATALOG_VERSION_NO 202212131
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#define CATALOG_VERSION_NO 202212191
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#endif

‎src/include/catalog/pg_proc.dat

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Original file line numberDiff line numberDiff line change
@@ -5441,6 +5441,13 @@
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proname => 'pg_stat_get_backend_dbid', provolatile => 's', proparallel => 'r',
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prorettype => 'oid', proargtypes => 'int4',
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prosrc => 'pg_stat_get_backend_dbid' },
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{ oid => '6107', descr => 'statistics: get subtransaction status of backend',
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proname => 'pg_stat_get_backend_subxact', provolatile => 's', proparallel => 'r',
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prorettype => 'record', proargtypes => 'int4',
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proallargtypes => '{int4,int4,bool}',
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proargmodes => '{i,o,o}',
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proargnames => '{bid,subxact_count,subxact_overflowed}',
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prosrc => 'pg_stat_get_backend_subxact' },
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{ oid => '1939', descr => 'statistics: user ID of backend',
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proname => 'pg_stat_get_backend_userid', provolatile => 's',
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proparallel => 'r', prorettype => 'oid', proargtypes => 'int4',

‎src/include/storage/sinvaladt.h

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -32,7 +32,9 @@ extern Size SInvalShmemSize(void);
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extern void CreateSharedInvalidationState(void);
3333
extern void SharedInvalBackendInit(bool sendOnly);
3434
extern PGPROC *BackendIdGetProc(int backendID);
35-
extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmin);
35+
extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid,
36+
TransactionId *xmin, int *nsubxid,
37+
bool *overflowed);
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3739
extern void SIInsertDataEntries(const SharedInvalidationMessage *data, int n);
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extern int SIGetDataEntries(SharedInvalidationMessage *data, int datasize);

‎src/include/utils/backend_status.h

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -266,6 +266,17 @@ typedef struct LocalPgBackendStatus
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* not.
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*/
268268
TransactionId backend_xmin;
269+
270+
/*
271+
* Number of cached subtransactions in the current session.
272+
*/
273+
int backend_subxact_count;
274+
275+
/*
276+
* The number of subtransactions in the current session exceeded the cached
277+
* subtransaction limit.
278+
*/
279+
bool backend_subxact_overflowed;
269280
} LocalPgBackendStatus;
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