druid/docs/content/querying/segmentmetadataquery.md

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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
• 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

{
  "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 for more information.	yes
intervals	A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.	yes
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	no

The format of the result is:

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

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.

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.

toInclude

There are 3 types of toInclude objects.

All

The grammar is as follows:

"toInclude": { "type": "all"}

None

The grammar is as follows:

"toInclude": { "type": "none"}

List

The grammar is as follows:

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