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# Segment Metadata Queries
Segment metadata queries return per segment information about:

* Cardinality of all columns in the segment
* Estimated byte size for the segment columns if they were stored in a flat format
* Number of rows stored inside the segment
* Interval the segment covers
* Column type of all the columns in the segment
* Estimated total segment byte size in if it was stored in a flat format
* Segment id

```json
{
  "queryType":"segmentMetadata",
  "dataSource":"sample_datasource",
  "intervals":["2013-01-01/2014-01-01"]
}
```

There are several main parts to a segment metadata query:

|property|description|required?|
|--------|-----------|---------|
|queryType|This String should always be "segmentMetadata"; this is the first thing Druid looks at to figure out how to interpret the query|yes|
|dataSource|A String or Object defining the data source to query, very similar to a table in a relational database. See [DataSource](../querying/datasource.html) for more information.|yes|
|intervals|A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.|no|
|toInclude|A JSON Object representing what columns should be included in the result. Defaults to "all".|no|
|merge|Merge all individual segment metadata results into a single result|no|
|context|See [Context](../querying/query-context.html)|no|
|analysisTypes|A list of Strings specifying what column properties (e.g. cardinality, size) should be calculated and returned in the result. Defaults to ["cardinality", "size", "interval"]. See section [analysisTypes](#analysistypes) for more details.|no|

The format of the result is:

```json
[ {
  "id" : "some_id",
  "intervals" : [ "2013-05-13T00:00:00.000Z/2013-05-14T00:00:00.000Z" ],
  "columns" : {
    "__time" : { "type" : "LONG", "hasMultipleValues" : false, "size" : 407240380, "cardinality" : null, "errorMessage" : null },
    "dim1" : { "type" : "STRING", "hasMultipleValues" : false, "size" : 100000, "cardinality" : 1944, "errorMessage" : null },
    "dim2" : { "type" : "STRING", "hasMultipleValues" : true, "size" : 100000, "cardinality" : 1504, "errorMessage" : null },
    "metric1" : { "type" : "FLOAT", "hasMultipleValues" : false, "size" : 100000, "cardinality" : null, "errorMessage" : null }
  },
  "size" : 300000,
  "numRows" : 5000000
} ]
```

Dimension columns will have type `STRING`.
Metric columns will have type `FLOAT` or `LONG` or name of the underlying complex type such as `hyperUnique` in case of COMPLEX metric.
Timestamp column will have type `LONG`.

If the `errorMessage` field is non-null, you should not trust the other fields in the response. Their contents are
undefined.

Only columns which are dimensions (ie, have type `STRING`) will have any cardinality. Rest of the columns (timestamp and metric columns) will show cardinality as `null`.

### intervals

If an interval is not specified, the query will use a default interval that spans a configurable period before the end time of the most recent segment.

The length of this default time period is set in the broker configuration via:
  druid.query.segmentMetadata.defaultHistory

### toInclude

There are 3 types of toInclude objects.

#### All

The grammar is as follows:

``` json
"toInclude": { "type": "all"}
```

#### None

The grammar is as follows:

``` json
"toInclude": { "type": "none"}
```

#### List

The grammar is as follows:

``` json
"toInclude": { "type": "list", "columns": [<string list of column names>]}
```

### analysisTypes

This is a list of properties that determines the amount of information returned about the columns, i.e. analyses to be performed on the columns.

By default, all analysis types will be used. If a property is not needed, omitting it from this list will result in a more efficient query.

There are 3 types of column analyses:

#### cardinality

* Estimated floor of cardinality for each column. Only relevant for dimension columns.

#### size

* Estimated byte size for the segment columns if they were stored in a flat format

* Estimated total segment byte size in if it was stored in a flat format

#### interval

* If present, the SegmentMetadataQuery will return the list of intervals associated with the queried segments.