druid/docs/SegmentMetadataQuery.md

Segment metadata queries return per segment information about: * Cardinality of all columns in the segment * Estimated byte size for the segment columns in TSV format * Interval the segment covers * Column type of all the columns in the segment * Estimated total segment byte size in TSV format * Segment id

<code>{
  "queryType":"segmentMetadata",
  "dataSource":"sample_datasource",
  "intervals":["2013-01-01/2014-01-01"],
}
</code>

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 defining the data source to query, very similar to a table in a relational database	yes
intervals	A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.	yes
merge	Merge all individual segment metadata results into a single result	no
context	An additional JSON Object which can be used to specify certain flags.	no

The format of the result is:

<code>[ {
  "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
} ]
</code>