Fixes for Disk-based Hash Aggregation.
Justin Pryzby raised a couple issues with commit 1f39bce0
. Fixed.
Also, tweak the way the size of a hash entry is estimated and the
number of buckets is estimated when calling BuildTupleHashTableExt().
Discussion: https://www.postgresql.org/message-id/20200319064222.GR26184@telsasoft.com
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@ -2778,7 +2778,7 @@ static void
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show_hashagg_info(AggState *aggstate, ExplainState *es)
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{
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Agg *agg = (Agg *)aggstate->ss.ps.plan;
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long memPeakKb = (aggstate->hash_mem_peak + 1023) / 1024;
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int64 memPeakKb = (aggstate->hash_mem_peak + 1023) / 1024;
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Assert(IsA(aggstate, AggState));
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@ -1873,17 +1873,12 @@ hash_agg_update_metrics(AggState *aggstate, bool from_tape, int npartitions)
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aggstate->hash_disk_used = disk_used;
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}
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/*
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* Update hashentrysize estimate based on contents. Don't include meta_mem
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* in the memory used, because empty buckets would inflate the per-entry
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* cost. An underestimate of the per-entry size is better than an
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* overestimate, because an overestimate could compound with each level of
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* recursion.
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*/
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/* update hashentrysize estimate based on contents */
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if (aggstate->hash_ngroups_current > 0)
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{
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aggstate->hashentrysize =
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hash_mem / (double)aggstate->hash_ngroups_current;
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sizeof(TupleHashEntryData) +
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(hash_mem / (double)aggstate->hash_ngroups_current);
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}
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}
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@ -1899,10 +1894,10 @@ hash_choose_num_buckets(double hashentrysize, long ngroups, Size memory)
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max_nbuckets = memory / hashentrysize;
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/*
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* Leave room for slop to avoid a case where the initial hash table size
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* exceeds the memory limit (though that may still happen in edge cases).
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* Underestimating is better than overestimating. Too many buckets crowd
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* out space for group keys and transition state values.
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*/
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max_nbuckets *= 0.75;
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max_nbuckets >>= 1;
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if (nbuckets > max_nbuckets)
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nbuckets = max_nbuckets;
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@ -3548,7 +3543,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
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* reasonable.
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*/
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for (i = 0; i < aggstate->num_hashes; i++)
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totalGroups = aggstate->perhash[i].aggnode->numGroups;
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totalGroups += aggstate->perhash[i].aggnode->numGroups;
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hash_agg_set_limits(aggstate->hashentrysize, totalGroups, 0,
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&aggstate->hash_mem_limit,
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