postgres.git / summary / log / commit / refs

commit    52e8fc3e2e66446e5705904cf7d884d5d669591f
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Wed May 25 21:48:15 2016 +0000

    Ensure that backends see up-to-date statistics for shared catalogs.
    
    Ever since we split the statistics collector's reports into per-database
    files (commit 187492b6c2e8cafc), backends have been seeing stale statistics
    for shared catalogs.  This is because the inquiry message only prompts the
    collector to write the per-database file for the requesting backend's own
    database.  Stats for shared catalogs are in a separate file for "DB 0",
    which didn't get updated.
    
    In normal operation this was partially masked by the fact that the
    autovacuum launcher would send an inquiry message at least once per
    autovacuum_naptime that asked for "DB 0"; so the shared-catalog stats would
    never be more than a minute out of date.  However the problem becomes very
    obvious with autovacuum disabled, as reported by Peter Eisentraut.
    
    To fix, redefine the semantics of inquiry messages so that both the
    specified DB and DB 0 will be dumped.  (This might seem a bit inefficient,
    but we have no good way to know whether a backend's transaction will look
    at shared-catalog stats, so we have to read both groups of stats whenever
    we request stats.  Sending two inquiry messages would definitely not be
    better.)
    
    Back-patch to 9.3 where the bug was introduced.
    
    Report: <56AD41AC.1030509@gmx.net>


 src/backend/postmaster/pgstat.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 41f43749b50..a6db9cdbfac 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -5504,7 +5504,16 @@ pgstat_db_requested(Oid databaseid)
 {
 	slist_iter	iter;
 
-	/* Check the databases if they need to refresh the stats. */
+	/*
+	 * If any requests are outstanding at all, we should write the stats for
+	 * shared catalogs (the "database" with OID 0).  This ensures that
+	 * backends will see up-to-date stats for shared catalogs, even though
+	 * they send inquiry messages mentioning only their own DB.
+	 */
+	if (databaseid == InvalidOid && !slist_is_empty(&last_statrequests))
+		return true;
+
+	/* Search to see if there's an open request to write this database. */
 	slist_foreach(iter, &last_statrequests)
 	{
 		DBWriteRequest *req = slist_container(DBWriteRequest, next, iter.cur);


[parent: 9abd64ec997c]