// src\include\nodes\nodes.h:

/*
 * AggStrategy -
 *	  overall execution strategies for Agg plan nodes
 *
 * This is needed in both pathnodes.h and plannodes.h, so put it here...
 */
typedef enum AggStrategy
{
	AGG_PLAIN,					/* simple agg across all input rows */
	AGG_SORTED,					/* grouped agg, input must be sorted，适用于下层算子输出有序的情况。 */
	AGG_HASHED,					/* grouped agg, use internal hashtable */
	AGG_MIXED					/* grouped agg, hash and sort both used */
} AggStrategy;