CREATE TABLE test
(
    `x` Int32, 
    `d` Decimal(10, 3)
)
ENGINE = MergeTree()
ORDER BY x;

SELECT avg(d)
FROM test

↑ Progress: 0.00 rows, 0.00 B (0.00 rows/s., 0.00 B/s.) Received exception from server (version 19.14.3):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: AggregateFunctionAvg with zero values. 

SELECT quantiles(0.1, 0.5, 0.9)(d)
FROM test

┌─quantiles(0.1, 0.5, 0.9)(d)────────────────────────────────────────────┐
│ [--9223372036854775.808,--9223372036854775.808,--9223372036854775.808] │
└────────────────────────────────────────────────────────────────────────┘

SELECT avg(d)
FROM test

┌─avg(d)─┐
│  0.000 │
└────────┘

SELECT quantiles(0.1, 0.5, 0.9)(d)
FROM test

┌─quantiles(0.1, 0.5, 0.9)(d)─┐
│ [0.000,0.000,0.000]         │
└─────────────────────────────┘