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Commitf48b4f8

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Correct Memoize's estimated cache hit ratio calculation
As demonstrated by David Johnston, the Memoize cache hit ratio calculationwasn't quite correct.This change only affects the estimated hit ratio when the estimated numberof entries to cache is estimated not to fit inside the cache. Forexample, if we expect 2000 distinct cache key values and only expect to beable to cache 1000 of those at once due to memory constraints, with anestimate of 10000 calls, if we could store all entries then the hit ratioshould be 80% to account for the first 2000 of the 10000 calls to be acache miss due to the value not being cached yet. If we can only store1000 entries for each of the 2000 distinct possible values at once thenthe 80% should be reduced by half to make the final estimate of 40%.Previously, the calculation would have produced an estimated hit ratio of30%, which wasn't correct.Apply to master only so as not to destabilize plans in the back branches.Reported-by: David G. JohnstonDiscussion:https://postgr.es/m/CAKFQuwZEmcNk3YQo2Xj4EDUOdY6qakad31rOD1Vc4q1_s68-Ew@mail.gmail.comDiscussion:https://postgr.es/m/CAApHDvrV44LwiF4W_qf_RpbGYWSgp1kF=cZr+kTRRaALUfmXqw@mail.gmail.com
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‎src/backend/optimizer/path/costsize.c

Lines changed: 3 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -2558,11 +2558,10 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath,
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* must look at how many scans are estimated in total for this node and
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* how many of those scans we expect to get a cache hit.
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*/
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hit_ratio=1.0 /ndistinct*Min(est_cache_entries,ndistinct)-
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(ndistinct /calls);
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hit_ratio=((calls-ndistinct) /calls)*
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(est_cache_entries /Max(ndistinct,est_cache_entries));
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2564-
/* Ensure we don't go negative */
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hit_ratio=Max(hit_ratio,0.0);
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Assert(hit_ratio >=0&&hit_ratio <=1.0);
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/*
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* Set the total_cost accounting for the expected cache hit ratio. We

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