On 12/26/16 2:31 AM, Kyotaro HORIGUCHI wrote:
The points of discussion are the following, I think.
1. The first patch seems working well. It costs the time to scan
the whole of a catcache that have negative entries for other
reloids. However, such negative entries are created by rather
unusual usages. Accesing to undefined columns, and accessing
columns on which no statistics have created. The
whole-catcache scan occurs on ATTNAME, ATTNUM and
STATRELATTINH for every invalidation of a relcache entry.
I took a look at this. It looks sane, though I've got a few minor
comment tweaks:
+ * Remove negative cache tuples maching a partial key.
s/maching/matching/
+/* searching with a paritial key needs scanning the whole cache */
s/needs/means/
+ * a negative cache entry cannot be referenced so we can remove
s/referenced/referenced,/
I was wondering if there's a way to test the performance impact of
deleting negative entries.
2. The second patch also works, but flushing negative entries by
hash values is inefficient. It scans the bucket corresponding
to given hash value for OIDs, then flushing negative entries
iterating over all the collected OIDs. So this costs more time
than 1 and flushes involving entries that is not necessary to
be removed. If this feature is valuable but such side effects
are not acceptable, new invalidation category based on
cacheid-oid pair would be needed.
I glanced at this and it looks sane. Didn't go any farther since this
one's pretty up in the air. ISTM it'd be better to do some kind of aging
instead of patch 2.
The other (possibly naive) question I have is how useful negative
entries really are? Will Postgres regularly incur negative lookups, or
will these only happen due to user activity? I can't think of a case
where an app would need to depend on fast negative lookup (in other
words, it should be considered a bug in the app). I can see where
getting rid of them completely might be problematic, but maybe we can
just keep a relatively small number of them around. I'm thinking a
simple LRU list of X number of negative entries; when that fills you
reuse the oldest one. You'd have to pay the LRU maintenance cost on
every negative hit, but if those shouldn't be that common it shouldn't
be bad.
That might well necessitate another GUC, but it seems a lot simpler than
most of the other ideas.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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