Re: strange slow query - lost lot of time somewhere

On Tue, 3 May 2022 at 13:43, David G. Johnston
<david.g.johnston@gmail.com> wrote:
> hit_ratio = (est_entries / ndistinct) - (ndistinct / calls) || clamp to 0.0
> I don't understand the adjustment factor ndistinct/calls

I've attached a spreadsheet showing you the impact of subtracting
(ndistinct / calls).  What this is correcting for is the fact that the
first scan from each unique value is a cache miss.  The more calls we
have, the more hits we'll get.  If there was only 1 call per distinct
value then there'd never be any hits. Without subtracting (ndistinct /
calls) and assuming there's space in the cache for each ndistinct
value, we'd assume 100% cache hit ratio if calls == ndistinct.  What
we should assume in that case is a 0% hit ratio as the first scan for
each distinct parameter must always be a miss as we've never had a
chance to cache any tuples for it yet.

> This is a "rescan" so aside from cache management isn't the cost of originally populating the cache already accounted
forelsewhere?
 

The cost of the first scan is calculated in create_memoize_path().
Since the first scan will always be a cache miss, the code there just
adds some cache management surcharges. Namely:

/*
* Add a small additional charge for caching the first entry.  All the
* harder calculations for rescans are performed in cost_memoize_rescan().
*/
pathnode->path.startup_cost = subpath->startup_cost + cpu_tuple_cost;
pathnode->path.total_cost = subpath->total_cost + cpu_tuple_cost;

David