**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
{
                
                l_count += r->count;
                l_mean += r->count * (r->mean - l_mean) / l_count; 
                l->mean = l_mean;
                l->count = l_count;
                batch_pos += 1;
            }
            else
            {
                
                sum += l->count; 
                ++l;
                
                *l = *r;
                l_mean = l->mean;
                l_count = l->count;
                batch_pos = 0;
            }
        }
        count = sum + l_count; 
        centroids.resize(l - centroids.begin() + 1);
        
    }
public:
    /** Performs compression of accumulated centroids
      * When merging, the invariant is retained to the maximum size of each
      * centroid that does not exceed `4 q (1 - q) \ delta N`.
      */
    void compress()