**Use case**
Clickhouse version 21.4
```
SELECT
    quantileTiming(0.95)(key) AS timing,
    quantileExact(0.95)(key) AS exact,
    quantileTDigest(0.95)(key) AS tdigest
FROM
(
    SELECT
        9000 AS key,
        2 AS count
    UNION ALL
    SELECT
        3000,
        11
    UNION ALL
    SELECT
        1000,
        26
)
ARRAY JOIN range(count) AS s

Query id: addce63f-0b75-46e2-b896-712d66711fb0

┌─timing─┬─exact─┬──tdigest─┐
│   9000 │  9000 │ 8123.076 │
└────────┴───────┴──────────┘


SELECT
    quantileTiming(0.9)(key) AS timing,
    quantileExact(0.9)(key) AS exact,
    quantileTDigest(0.9)(key) AS tdigest
FROM
(
    SELECT key
    FROM
    (
        SELECT
            9000 AS key,
            2 AS count
        UNION ALL
        SELECT
            3000,
            11
        UNION ALL
        SELECT
            1000,
            26
    )
    ARRAY JOIN range(count) AS s
    ORDER BY key ASC
)

Query id: e92e3d07-3b97-4155-aa10-9a15544d9312

┌─timing─┬─exact─┬───tdigest─┐
│   3000 │  3000 │ 6323.0757 │
└────────┴───────┴───────────┘

```

Probably related: https://github.com/tdunning/t-digest/issues/154
if (l != r)
                        *l = *r;
                    l_mean = l->mean;
                    l_count = l->count;
                }
                ++r;
            }
            count = sum + l_count; 
            
            centroids.resize(l - centroids.begin() + 1);
            unmerged = 0;
        }
        
        compressBrute();
    }
    /** Adds to the digest a change in `x` with a weight of `cnt` (default 1)
      */
    void add(T x, UInt64 cnt = 1)
    {
        auto vx = static_cast<Value>(x);
        if (cnt == 0 || std::isnan(vx))
            return; 
        addCentroid(Centroid{vx, static_cast<Count>(cnt)});
    }
    void merge(const QuantileTDigest & other)
    {
        for (const auto & c : other.centroids)
            addCentroid(c);
    }
    void serialize(WriteBuffer & buf)