SegmentMetadata queries

    Segment metadata queries return per-segment information about:

    • Cardinality of all columns in the segment
    • Min/max values of string type 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
    • Is the segment rolled up

    There are several main parts to a segment metadata query:

    The format of the result is:

    Dimension columns will have type . 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 (i.e., have type ) will have any cardinality. Rest of the columns (timestamp and metric columns) will show cardinality as null.

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

    There are 3 types of toInclude objects.

    The grammar is as follows:

    None

    The grammar is as follows:

    List

    The grammar is as follows:

    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.

    The default analysis types can be set in the Broker configuration via: druid.query.segmentMetadata.defaultAnalysisTypes

    Types of column analyses are described below:

    • cardinality in the result will return the estimated floor of cardinality for each column. Only relevant for dimension columns.

    minmax

    • Estimated min/max values for each column. Only relevant for dimension columns.

    size

    • size in the result will contain the estimated total segment byte size as if the data were stored in text format
    • intervals in the result will contain the list of intervals associated with the queried segments.

    timestampSpec

    • in the result will contain timestampSpec of data stored in segments. this can be null if timestampSpec of segments was unknown or unmergeable (if merging is enabled).

    queryGranularity

    • queryGranularity in the result will contain query granularity of data stored in segments. this can be null if query granularity of segments was unknown or unmergeable (if merging is enabled).

    • aggregators in the result will contain the list of aggregators usable for querying metric columns. This may be null if the aggregators are unknown or unmergeable (if merging is enabled).

    • Merging can be strict or lenient. See lenientAggregatorMerge below for details.

    • The form of the result is a map of column name to aggregator.

    rollup

    • rollup in the result is true/false/null.
    • When merging is enabled, if some are rollup, others are not, result is null.

    Conflicts between aggregator metadata across segments can occur if some segments have unknown aggregators, or if two segments use incompatible aggregators for the same column (e.g. longSum changed to doubleSum).

    In particular, with lenient merging, it is possible for an individual column’s aggregator to be null. This will not occur with strict merging.