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Commit61171a6

Browse files
Look up backend type in pg_signal_backend() more cheaply.
Commitccd3802, which introduced the pg_signal_autovacuum_workerrole, added a call to pgstat_get_beentry_by_proc_number() for thepurpose of determining whether the process is an autovacuum worker.This function calls pgstat_read_current_status(), which can befairly expensive and may return cached, out-of-date information.Since we just need to look up the target backend's BackendType, andwe already know its ProcNumber, we can instead inspect theBackendStatusArray directly, which is much less expensive andpossibly more up-to-date. There are some caveats with thisapproach (which are documented in the code), but it's stillsubstantially better than before.Reported-by: Andres FreundReviewed-by: Andres FreundDiscussion:https://postgr.es/m/ujenaa2uabzfkwxwmfifawzdozh3ljr7geozlhftsuosgm7n7q%40g3utqqyyosb6
1 parent6a5bcf7 commit61171a6

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3 files changed

+28
-2
lines changed

3 files changed

+28
-2
lines changed

‎src/backend/storage/ipc/signalfuncs.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -88,9 +88,9 @@ pg_signal_backend(int pid, int sig)
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if (!OidIsValid(proc->roleId)||superuser_arg(proc->roleId))
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{
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ProcNumberprocNumber=GetNumberFromPGProc(proc);
91-
PgBackendStatus*procStatus=pgstat_get_beentry_by_proc_number(procNumber);
91+
BackendTypebackendType=pgstat_get_backend_type_by_proc_number(procNumber);
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93-
if (procStatus&&procStatus->st_backendType==B_AUTOVAC_WORKER)
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if (backendType==B_AUTOVAC_WORKER)
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{
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if (!has_privs_of_role(GetUserId(),ROLE_PG_SIGNAL_AUTOVACUUM_WORKER))
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returnSIGNAL_BACKEND_NOAUTOVAC;

‎src/backend/utils/activity/backend_status.c

Lines changed: 25 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1036,6 +1036,31 @@ pgstat_get_my_query_id(void)
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returnMyBEEntry->st_query_id;
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}
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/* ----------
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* pgstat_get_backend_type_by_proc_number() -
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*
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*Return the type of the backend with the specified ProcNumber. This looks
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*directly at the BackendStatusArray, so the return value may be out of date.
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*The only current use of this function is in pg_signal_backend(), which is
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*inherently racy, so we don't worry too much about this.
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*
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*It is the caller's responsibility to use this wisely; at minimum, callers
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*should ensure that procNumber is valid and perform the required permissions
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*checks.
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* ----------
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*/
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BackendType
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pgstat_get_backend_type_by_proc_number(ProcNumberprocNumber)
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{
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volatilePgBackendStatus*status=&BackendStatusArray[procNumber];
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/*
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* We bypass the changecount mechanism since fetching and storing an int
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* is almost certainly atomic.
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*/
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returnstatus->st_backendType;
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}
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/* ----------
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* cmp_lbestatus
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*

‎src/include/utils/backend_status.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -323,6 +323,7 @@ extern const char *pgstat_get_backend_current_activity(int pid, bool checkUser);
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externconstchar*pgstat_get_crashed_backend_activity(intpid,char*buffer,
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intbuflen);
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externuint64pgstat_get_my_query_id(void);
326+
externBackendTypepgstat_get_backend_type_by_proc_number(ProcNumberprocNumber);
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/* ----------

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