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.

+/* searching with a paritial key needs scanning the whole cache */


+ * a negative cache entry cannot be referenced so we can remove


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
Data in Trouble? Get it in Treble! http://BlueTreble.com
855-TREBLE2 (855-873-2532)

Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to