Function build_hash_table

src/backend/executor/nodeAgg.c:1564–1597

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

 * Build a single hashtable for this grouping set.
 */
static void
build_hash_table(AggState *aggstate, int setno, long nbuckets)
{
	AggStatePerHash perhash = &aggstate->perhash[setno];
	MemoryContext metacxt = aggstate->hash_metacxt;
	MemoryContext hashcxt = aggstate->hashcontext->ecxt_per_tuple_memory;
	MemoryContext tmpcxt = aggstate->tmpcontext->ecxt_per_tuple_memory;
	Size		additionalsize;

	Assert(aggstate->aggstrategy == AGG_HASHED ||
		   aggstate->aggstrategy == AGG_MIXED);

	/*
	 * Used to make sure initial hash table allocation does not exceed
	 * hash_mem. Note that the estimate does not include space for
	 * pass-by-reference transition data values, nor for the representative
	 * tuple of each group.
	 */
	additionalsize = aggstate->numtrans * sizeof(AggStatePerGroupData);

	perhash->hashtable = BuildTupleHashTableExt(&aggstate->ss.ps,
												perhash->hashslot->tts_tupleDescriptor,
												perhash->numCols,
												perhash->hashGrpColIdxHash,
												perhash->eqfuncoids,
												perhash->hashfunctions,
												perhash->aggnode->grpCollations,
												nbuckets,
												additionalsize,
												metacxt,
												hashcxt,
												tmpcxt,
												DO_AGGSPLIT_SKIPFINAL(aggstate->aggsplit));
}

/*
 * Compute columns that actually need to be stored in hashtable entries.  The