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arrays Arrays

Apache Druid supports SQL standard ARRAY typed columns for VARCHAR, BIGINT, and DOUBLE types (native types ARRAY<STRING>, ARRAY<LONG>, and ARRAY<DOUBLE>). Other more complicated ARRAY types must be stored in nested columns. Druid ARRAY types are distinct from multi-value dimension, which have significantly different behavior than standard arrays.

This document describes inserting, filtering, and grouping behavior for ARRAY typed columns. Refer to the Druid SQL data type documentation and SQL array function reference for additional details about the functions available to use with ARRAY columns and types in SQL.

The following sections describe inserting, filtering, and grouping behavior based on the following example data, which includes 3 array typed columns:

{"timestamp": "2023-01-01T00:00:00", "label": "row1", "arrayString": ["a", "b"],  "arrayLong":[1, null,3], "arrayDouble":[1.1, 2.2, null]}
{"timestamp": "2023-01-01T00:00:00", "label": "row2", "arrayString": [null, "b"], "arrayLong":null,        "arrayDouble":[999, null, 5.5]}
{"timestamp": "2023-01-01T00:00:00", "label": "row3", "arrayString": [],          "arrayLong":[1, 2, 3],   "arrayDouble":[null, 2.2, 1.1]} 
{"timestamp": "2023-01-01T00:00:00", "label": "row4", "arrayString": ["a", "b"],  "arrayLong":[1, 2, 3],   "arrayDouble":[]}
{"timestamp": "2023-01-01T00:00:00", "label": "row5", "arrayString": null,        "arrayLong":[],          "arrayDouble":null}

Ingesting arrays

Native batch and streaming ingestion

When using native batch or streaming ingestion such as with Apache Kafka, arrays can be ingested using the "auto" type dimension schema which is shared with type-aware schema discovery.

When ingesting from TSV or CSV data, you can specify the array delimiters using the listDelimiter field in the inputFormat. JSON data must be formatted as a JSON array to be ingested as an array type. JSON data does not require inputFormat configuration.

The following shows an example dimensionsSpec for native ingestion of the data used in this document:

"dimensions": [
  {
    "type": "auto",
    "name": "label"
  },
  {
    "type": "auto",
    "name": "arrayString"
  },
  {
    "type": "auto",
    "name": "arrayLong"
  },
  {
    "type": "auto",
    "name": "arrayDouble"
  }
],

SQL-based ingestion

Arrays can also be inserted with SQL-based ingestion when you include a query context parameter "arrayIngestMode":"array".

For example, to insert the data used in this document:

REPLACE INTO "array_example" OVERWRITE ALL
WITH "ext" AS (
  SELECT *
  FROM TABLE(
    EXTERN(
      '{"type":"inline","data":"{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row1\", \"arrayString\": [\"a\", \"b\"],  \"arrayLong\":[1, null,3], \"arrayDouble\":[1.1, 2.2, null]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row2\", \"arrayString\": [null, \"b\"], \"arrayLong\":null,        \"arrayDouble\":[999, null, 5.5]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row3\", \"arrayString\": [],          \"arrayLong\":[1, 2, 3],   \"arrayDouble\":[null, 2.2, 1.1]} \n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row4\", \"arrayString\": [\"a\", \"b\"],  \"arrayLong\":[1, 2, 3],   \"arrayDouble\":[]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row5\", \"arrayString\": null,        \"arrayLong\":[],          \"arrayDouble\":null}"}',
      '{"type":"json"}',
      '[{"name":"timestamp", "type":"STRING"},{"name":"label", "type":"STRING"},{"name":"arrayString", "type":"ARRAY<STRING>"},{"name":"arrayLong", "type":"ARRAY<LONG>"},{"name":"arrayDouble", "type":"ARRAY<DOUBLE>"}]'
    )
  )
)
SELECT
  TIME_PARSE("timestamp") AS "__time",
  "label",
  "arrayString",
  "arrayLong",
  "arrayDouble"
FROM "ext"
PARTITIONED BY DAY

SQL-based ingestion with rollup

These input arrays can also be grouped for rollup:

REPLACE INTO "array_example_rollup" OVERWRITE ALL
WITH "ext" AS (
  SELECT *
  FROM TABLE(
    EXTERN(
      '{"type":"inline","data":"{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row1\", \"arrayString\": [\"a\", \"b\"],  \"arrayLong\":[1, null,3], \"arrayDouble\":[1.1, 2.2, null]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row2\", \"arrayString\": [null, \"b\"], \"arrayLong\":null,        \"arrayDouble\":[999, null, 5.5]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row3\", \"arrayString\": [],          \"arrayLong\":[1, 2, 3],   \"arrayDouble\":[null, 2.2, 1.1]} \n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row4\", \"arrayString\": [\"a\", \"b\"],  \"arrayLong\":[1, 2, 3],   \"arrayDouble\":[]}\n{\"timestamp\": \"2023-01-01T00:00:00\", \"label\": \"row5\", \"arrayString\": null,        \"arrayLong\":[],          \"arrayDouble\":null}"}',
      '{"type":"json"}',
      '[{"name":"timestamp", "type":"STRING"},{"name":"label", "type":"STRING"},{"name":"arrayString", "type":"ARRAY<STRING>"},{"name":"arrayLong", "type":"ARRAY<LONG>"},{"name":"arrayDouble", "type":"ARRAY<DOUBLE>"}]'
    )
  )
)
SELECT
  TIME_PARSE("timestamp") AS "__time",
  "label",
  "arrayString",
  "arrayLong",
  "arrayDouble",
  COUNT(*) as "count"
FROM "ext"
GROUP BY 1,2,3,4,5
PARTITIONED BY DAY

Querying arrays

Filtering

All query types, as well as filtered aggregators, can filter on array typed columns. Filters follow these rules for array types:

  • All filters match against the entire array value for the row
  • Native value filters like equality and range match on entire array values, as do SQL constructs that plan into these native filters
  • The IS NULL filter will match rows where the entire array value is null
  • Array specific functions like ARRAY_CONTAINS and ARRAY_OVERLAP follow the behavior specified by those functions
  • All other filters do not directly support ARRAY types and will result in a query error

Example: equality

SELECT *
FROM "array_example"
WHERE arrayLong = ARRAY[1,2,3]

{"__time":"2023-01-01T00:00:00.000Z","label":"row3","arrayString":"[]","arrayLong":"[1,2,3]","arrayDouble":"[null,2.2,1.1]"}
{"__time":"2023-01-01T00:00:00.000Z","label":"row4","arrayString":"[\"a\",\"b\"]","arrayLong":"[1,2,3]","arrayDouble":"[]"}

Example: null

SELECT *
FROM "array_example"
WHERE arrayLong IS NULL

{"__time":"2023-01-01T00:00:00.000Z","label":"row2","arrayString":"[null,\"b\"]","arrayLong":null,"arrayDouble":"[999.0,null,5.5]"}

Example: range

SELECT *
FROM "array_example"
WHERE arrayString >= ARRAY['a','b']

{"__time":"2023-01-01T00:00:00.000Z","label":"row1","arrayString":"[\"a\",\"b\"]","arrayLong":"[1,null,3]","arrayDouble":"[1.1,2.2,null]"}
{"__time":"2023-01-01T00:00:00.000Z","label":"row4","arrayString":"[\"a\",\"b\"]","arrayLong":"[1,2,3]","arrayDouble":"[]"}

Example: ARRAY_CONTAINS

SELECT *
FROM "array_example"
WHERE ARRAY_CONTAINS(arrayString, 'a')

{"__time":"2023-01-01T00:00:00.000Z","label":"row1","arrayString":"[\"a\",\"b\"]","arrayLong":"[1,null,3]","arrayDouble":"[1.1,2.2,null]"}
{"__time":"2023-01-01T00:00:00.000Z","label":"row4","arrayString":"[\"a\",\"b\"]","arrayLong":"[1,2,3]","arrayDouble":"[]"}

Grouping

When grouping on an array with SQL or a native groupBy query, grouping follows standard SQL behavior and groups on the entire array as a single value. The UNNEST function allows grouping on the individual array elements.

Example: SQL grouping query with no filtering

SELECT label, arrayString
FROM "array_example"
GROUP BY 1,2

results in:

{"label":"row1","arrayString":"[\"a\",\"b\"]"}
{"label":"row2","arrayString":"[null,\"b\"]"}
{"label":"row3","arrayString":"[]"}
{"label":"row4","arrayString":"[\"a\",\"b\"]"}
{"label":"row5","arrayString":null}

Example: SQL grouping query with a filter

SELECT label, arrayString
FROM "array_example"
WHERE arrayLong = ARRAY[1,2,3]
GROUP BY 1,2

results:

{"label":"row3","arrayString":"[]"}
{"label":"row4","arrayString":"[\"a\",\"b\"]"}

Example: UNNEST

SELECT label, strings
FROM "array_example" CROSS JOIN UNNEST(arrayString) as u(strings)
GROUP BY 1,2

results:

{"label":"row1","strings":"a"}
{"label":"row1","strings":"b"}
{"label":"row2","strings":null}
{"label":"row2","strings":"b"}
{"label":"row4","strings":"a"}
{"label":"row4","strings":"b"}

Differences between arrays and multi-value dimensions

Avoid confusing string arrays with multi-value dimensions. Arrays and multi-value dimensions are stored in different column types, and query behavior is different. You can use the functions MV_TO_ARRAY and ARRAY_TO_MV to convert between the two if needed. In general, we recommend using arrays whenever possible, since they are a newer and more powerful feature and have SQL compliant behavior.

Use care during ingestion to ensure you get the type you want.

To get arrays when performing an ingestion using JSON ingestion specs, such as native batch or streaming ingestion such as with Apache Kafka, use dimension type auto or enable useSchemaDiscovery. When performing a SQL-based ingestion, write a query that generates arrays and set the context parameter "arrayIngestMode": "array". Arrays may contain strings or numbers.

To get multi-value dimensions when performing an ingestion using JSON ingestion specs, use dimension type string and do not enable useSchemaDiscovery. When performing a SQL-based ingestion, wrap arrays in ARRAY_TO_MV, which ensures you get multi-value dimensions in any arrayIngestMode. Multi-value dimensions can only contain strings.

You can tell which type you have by checking the INFORMATION_SCHEMA.COLUMNS table, using a query like:

SELECT COLUMN_NAME, DATA_TYPE
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'mytable'

Arrays are type ARRAY, multi-value strings are type VARCHAR.