# TDIGEST.MERGE

```json metadata
{
  "title": "TDIGEST.MERGE",
  "description": "Merges multiple t-digest sketches into a single sketch",
  "categories": ["docs","develop","stack","oss","rs","rc","oss","kubernetes","clients"],
  "arguments": [{"name":"destination-key","type":"key"},{"name":"numkeys","type":"integer"},{"multiple":true,"name":"source-key","type":"key"},{"arguments":[{"name":"compression","token":"COMPRESSION","type":"pure-token"},{"name":"compression","type":"integer"}],"name":"config","optional":true,"type":"block"},{"name":"override","optional":true,"token":"OVERRIDE","type":"pure-token"}],
  "syntax_fmt": "TDIGEST.MERGE destination-key numkeys source-key [source-key ...]\n  [COMPRESSION compression] [OVERRIDE]",
  "complexity": "O(N*K), where N is the number of centroids and K being the number of input sketches",
  "group": "tdigest",
  "acl_categories": ["@tdigest","@write","@slow"],
  "since": "2.4.0",
  "tableOfContents": {"sections":[{"id":"required-arguments","title":"Required arguments"},{"id":"optional-arguments","title":"Optional arguments"},{"id":"examples","title":"Examples"},{"id":"redis-enterprise-and-redis-cloud-compatibility","title":"Redis Enterprise and Redis Cloud compatibility"},{"id":"return-information","title":"Return information"}]}
}
```













Merges multiple t-digest sketches into a single sketch.

## Required arguments
<details open><summary><code>destination-key</code></summary>

is the key name for a t-digest sketch to merge observation values to.

If `destination-key` does not exist, a new sketch is created.

If `destination-key` is an existing sketch, its values are merged with the values of the source keys. To override the destination key contents use `OVERRIDE`.
</details>

<details open><summary><code>numkeys</code></summary>

the number of sketches from which to merge observation values (one or more).
</details>

<details open><summary><code>source-key</code></summary>

Each `source-key` is a key name for a t-digest sketch from which to merge observation values.
</details>

## Optional arguments

<details open><summary><code>COMPRESSION compression</code></summary>
  
is a controllable tradeoff between accuracy and memory consumption. 100 is a common value for normal uses and also the default if not specified. 1000 is more accurate. For more information on scaling of accuracy versus the compression value see [_The t-digest: Efficient estimates of distributions_](https://www.sciencedirect.com/science/article/pii/S2665963820300403).
  
When `COMPRESSION` is not specified:
- If `destination-key` does not exist or if `OVERRIDE` is specified, the compression is set to the maximum value among all source sketches.
- If `destination-key` already exists and `OVERRIDE` is not specified, its compression is not changed.

</details>

<details open><summary><code>OVERRIDE</code></summary>

If `destination-key` already exists and `OVERRIDE` is specified, the key is overwritten.
</details>

## Examples

redis> TDIGEST.CREATE s1
OK
redis> TDIGEST.CREATE s2
OK
redis> TDIGEST.ADD s1 10.0 20.0
OK
redis> TDIGEST.ADD s2 30.0 40.0
OK
redis> TDIGEST.MERGE sM 2 s1 s2
OK
redis> TDIGEST.BYRANK sM 0 1 2 3 4
1) "10"
2) "20"
3) "30"
4) "40"
5) "inf"


## Redis Enterprise and Redis Cloud compatibility

| Redis<br />Enterprise | Redis<br />Cloud | <span style="min-width: 9em; display: table-cell">Notes</span> |
|:----------------------|:-----------------|:------|
| <span title="Supported">&#x2705; Supported</span><br /> | <span title="Supported">&#x2705; Flexible & Annual</span><br /><span title="Supported">&#x2705; Free & Fixed</nobr></span> |  |

## Return information

{{< multitabs id="tdigest-merge-return-info" 
    tab1="RESP2" 
    tab2="RESP3" >}}

One of the following:

* [Simple string reply](https://redis.io/docs/latest/develop/reference/protocol-spec#simple-strings) `OK` if successful.
* [Simple error reply](https://redis.io/docs/latest/develop/reference/protocol-spec#simple-errors) in the following cases: incorrect key type, incorrect keyword, or incorrect number of arguments.

-tab-sep-

One of the following:

* [Simple string reply](https://redis.io/docs/latest/develop/reference/protocol-spec#simple-strings) `OK` if successful.
* [Simple error reply](https://redis.io/docs/latest/develop/reference/protocol-spec#simple-errors) in the following cases: incorrect key type, incorrect keyword, or incorrect number of arguments.