mirror of https://github.com/apache/druid.git
164 lines
6.3 KiB
Markdown
164 lines
6.3 KiB
Markdown
---
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layout: doc_page
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---
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# Segment Metadata Queries
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Segment metadata queries return per-segment information about:
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* Cardinality of all columns in the segment
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* Min/max values of string type columns in the segment
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* Estimated byte size for the segment columns if they were stored in a flat format
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* Number of rows stored inside the segment
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* Interval the segment covers
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* Column type of all the columns in the segment
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* Estimated total segment byte size in if it was stored in a flat format
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* Is the segment rolled up
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* Segment id
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```json
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{
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"queryType":"segmentMetadata",
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"dataSource":"sample_datasource",
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"intervals":["2013-01-01/2014-01-01"]
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}
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```
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There are several main parts to a segment metadata query:
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|property|description|required?|
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|--------|-----------|---------|
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|queryType|This String should always be "segmentMetadata"; this is the first thing Druid looks at to figure out how to interpret the query|yes|
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|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|
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|intervals|A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.|no|
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|toInclude|A JSON Object representing what columns should be included in the result. Defaults to "all".|no|
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|merge|Merge all individual segment metadata results into a single result|no|
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|context|See [Context](../querying/query-context.html)|no|
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|analysisTypes|A list of Strings specifying what column properties (e.g. cardinality, size) should be calculated and returned in the result. Defaults to ["cardinality", "interval", "minmax"]. See section [analysisTypes](#analysistypes) for more details.|no|
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|lenientAggregatorMerge|If true, and if the "aggregators" analysisType is enabled, aggregators will be merged leniently. See below for details.|no|
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The format of the result is:
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```json
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[ {
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"id" : "some_id",
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"intervals" : [ "2013-05-13T00:00:00.000Z/2013-05-14T00:00:00.000Z" ],
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"columns" : {
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"__time" : { "type" : "LONG", "hasMultipleValues" : false, "size" : 407240380, "cardinality" : null, "errorMessage" : null },
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"dim1" : { "type" : "STRING", "hasMultipleValues" : false, "size" : 100000, "cardinality" : 1944, "errorMessage" : null },
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"dim2" : { "type" : "STRING", "hasMultipleValues" : true, "size" : 100000, "cardinality" : 1504, "errorMessage" : null },
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"metric1" : { "type" : "FLOAT", "hasMultipleValues" : false, "size" : 100000, "cardinality" : null, "errorMessage" : null }
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},
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"aggregators" : {
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"metric1" : { "type" : "longSum", "name" : "metric1", "fieldName" : "metric1" }
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},
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"queryGranularity" : {
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"type": "none"
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},
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"size" : 300000,
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"numRows" : 5000000
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} ]
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```
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Dimension columns will have type `STRING`.
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Metric columns will have type `FLOAT` or `LONG` or name of the underlying complex type such as `hyperUnique` in case of COMPLEX metric.
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Timestamp column will have type `LONG`.
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If the `errorMessage` field is non-null, you should not trust the other fields in the response. Their contents are
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undefined.
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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`.
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### intervals
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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.
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The length of this default time period is set in the broker configuration via:
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druid.query.segmentMetadata.defaultHistory
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### toInclude
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There are 3 types of toInclude objects.
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#### All
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The grammar is as follows:
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``` json
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"toInclude": { "type": "all"}
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```
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#### None
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The grammar is as follows:
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``` json
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"toInclude": { "type": "none"}
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```
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#### List
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The grammar is as follows:
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``` json
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"toInclude": { "type": "list", "columns": [<string list of column names>]}
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```
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### analysisTypes
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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.
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By default, the "cardinality", "size", "interval", and "minmax" types will be used. If a property is not needed, omitting it from this list will result in a more efficient query.
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Types of column analyses are described below:
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#### cardinality
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* `cardinality` in the result will return the estimated floor of cardinality for each column. Only relevant for
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dimension columns.
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#### minmax
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* Estimated min/max values for each column. Only relevant for dimension columns.
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#### size
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* `size` in the result will contain the estimated total segment byte size as if the data were stored in text format
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#### interval
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* `intervals` in the result will contain the list of intervals associated with the queried segments.
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#### timestampSpec
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* `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).
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#### queryGranularity
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* `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).
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#### aggregators
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* `aggregators` in the result will contain the list of aggregators usable for querying metric columns. This may be
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null if the aggregators are unknown or unmergeable (if merging is enabled).
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* Merging can be strict or lenient. See *lenientAggregatorMerge* below for details.
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* The form of the result is a map of column name to aggregator.
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#### rollup
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* `rollup` in the result is true/false/null.
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* When merging is enabled, if some are rollup, others are not, result is null.
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### lenientAggregatorMerge
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Conflicts between aggregator metadata across segments can occur if some segments have unknown aggregators, or if
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two segments use incompatible aggregators for the same column (e.g. longSum changed to doubleSum).
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Aggregators can be merged strictly (the default) or leniently. With strict merging, if there are any segments
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with unknown aggregators, or any conflicts of any kind, the merged aggregators list will be `null`. With lenient
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merging, segments with unknown aggregators will be ignored, and conflicts between aggregators will only null out
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the aggregator for that particular column.
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In particular, with lenient merging, it is possible for an invidiual column's aggregator to be `null`. This will not
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occur with strict merging.
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