On 1/21/17 6:42 PM, Jim Nasby wrote:
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.

I did a make installcheck run with CATCACHE_STATS to see how often we get negative entries in the 3 caches affected by this patch. The caches on pg_attribute get almost no negative entries. pg_statistic gets a good amount of negative entries, presumably because we start off with no entries in there. On a stable system that presumably won't be an issue, but if temporary tables are in use and being analyzed I'd think there could be a moderate amount of inval traffic on that cache. I'll leave it to a committer to decide if they thing that's an issue, but you might want to try and quantify how big a hit that is. I think it'd also be useful to know how much bloat you were seeing in the field.

The patch is currently conflicting against master though, due to some caches being added. Can you rebase? BTW, if you set a slightly larger context size on the patch you might be able to avoid rebases; right now the patch doesn't include enough context to uniquely identify the chunks against cacheinfo[].
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)

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