aggregation

type AggEvaluateContext struct {
	DistinctChecker *distinctChecker
	Count           int64
	Value           types.Datum
	Buffer          *bytes.Buffer // Buffer is used for group_concat.
	GotFirstRow     bool          // It will check if the agg has met the first row key.
}

type AggFuncDesc struct {
	// Name represents the aggregation function name.
	Name string
	// Args represents the arguments of the aggregation function.
	Args []expression.Expression
	// RetTp represents the return type of the aggregation function.
	RetTp *types.FieldType
	// Mode represents the execution mode of the aggregation function.
	Mode AggFunctionMode
	// HasDistinct represents whether the aggregation function contains distinct attribute.
	HasDistinct bool
}

type AggFunctionMode int

type Aggregation interface {
	// Update during executing.
	Update(evalCtx *AggEvaluateContext, sc *stmtctx.StatementContext, row types.Row) error

	// GetPartialResult will called by coprocessor to get partial results. For avg function, partial results will return
	// sum and count values at the same time.
	GetPartialResult(evalCtx *AggEvaluateContext) []types.Datum

	// GetResult will be called when all data have been processed.
	GetResult(evalCtx *AggEvaluateContext) types.Datum

	// Create a new AggEvaluateContext for the aggregation function.
	CreateContext(sc *stmtctx.StatementContext) *AggEvaluateContext

	// Reset the content of the evaluate context.
	ResetContext(sc *stmtctx.StatementContext, evalCtx *AggEvaluateContext)
}