Function build_hash_tables

src/backend/executor/nodeAgg.c:1519–1559

* (Re-)initialize the hash table(s) to empty. * * To implement hashed aggregation, we need a hashtable that stores a * representative tuple and an array of AggStatePerGroup structs for each * distinct set of GROUP BY column values. We compute the hash key from the * GROUP BY columns. The per-group data is allocated in lookup_hash_entry(), * for each entry. * * We have a separate hashtabl

 * they are all reset at the same time).
 */
static void
build_hash_tables(AggState *aggstate)
{
	int			setno;

	for (setno = 0; setno < aggstate->num_hashes; ++setno)
	{
		AggStatePerHash perhash = &aggstate->perhash[setno];
		long		nbuckets;
		Size		memory;

		if (perhash->hashtable != NULL)
		{
			ResetTupleHashTable(perhash->hashtable);
			continue;
		}

		Assert(perhash->aggnode->numGroups > 0);

		memory = aggstate->hash_mem_limit / aggstate->num_hashes;

		/* choose reasonable number of buckets per hashtable */
		nbuckets = hash_choose_num_buckets(aggstate->hashentrysize,
										   perhash->aggnode->numGroups,
										   memory);

		build_hash_table(aggstate, setno, nbuckets);

		/* initialize some statistic info of hash table */
		perhash->num_output_groups = 0;
		perhash->num_spill_parts = 0;
		perhash->num_expansions = 0;
		perhash->bucket_total = 0;
		perhash->bucket_used = 0;
		perhash->chain_count = 0;
		perhash->chain_length_total = 0;
		perhash->chain_length_max = 0;
	}

	aggstate->hash_ngroups_current = 0;
}

/*
 * Build a single hashtable for this grouping set.