Scan queries

info

Apache Druid supports two query languages: Druid SQL and native queries. This document describes a query type in the native language. For information about when Druid SQL will use this query type, refer to the SQL documentation.

The Scan query returns raw Apache Druid rows in streaming mode.

In addition to straightforward usage where a Scan query is issued to the Broker, the Scan query can also be issued directly to Historical processes or streaming ingestion tasks. This can be useful if you want to retrieve large amounts of data in parallel.

An example Scan query object is shown below:

 {
   "queryType": "scan",
   "dataSource": "wikipedia",
   "resultFormat": "list",
   "columns":[],
   "intervals": [
     "2013-01-01/2013-01-02"
   ],
   "batchSize":20480,
   "limit":3
 }

The following are the main parameters for Scan queries:

Example results

The format of the result when resultFormat equals list:

 [{
    "segmentId" : "wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9",
    "columns" : [
      "timestamp",
      "robot",
      "namespace",
      "anonymous",
      "unpatrolled",
      "page",
      "language",
      "newpage",
      "user",
      "count",
      "added",
      "delta",
      "variation",
      "deleted"
    ],
    "events" : [ {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "1",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "11._korpus_(NOVJ)",
        "language" : "sl",
        "newpage" : "0",
        "user" : "EmausBot",
        "count" : 1.0,
        "added" : 39.0,
        "delta" : 39.0,
        "variation" : 39.0,
        "deleted" : 0.0
    }, {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "0",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "112_U.S._580",
        "language" : "en",
        "newpage" : "1",
        "user" : "MZMcBride",
        "count" : 1.0,
        "added" : 70.0,
        "delta" : 70.0,
        "variation" : 70.0,
        "deleted" : 0.0
    }, {
        "timestamp" : "2013-01-01T00:00:00.000Z",
        "robot" : "0",
        "namespace" : "article",
        "anonymous" : "0",
        "unpatrolled" : "0",
        "page" : "113_U.S._243",
        "language" : "en",
        "newpage" : "1",
        "user" : "MZMcBride",
        "count" : 1.0,
        "added" : 77.0,
        "delta" : 77.0,
        "variation" : 77.0,
        "deleted" : 0.0
    } ]
} ]

The format of the result when resultFormat equals compactedList:

 [{
    "segmentId" : "wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9",
    "columns" : [
      "timestamp", "robot", "namespace", "anonymous", "unpatrolled", "page", "language", "newpage", "user", "count", "added", "delta", "variation", "deleted"
    ],
    "events" : [
     ["2013-01-01T00:00:00.000Z", "1", "article", "0", "0", "11._korpus_(NOVJ)", "sl", "0", "EmausBot", 1.0, 39.0, 39.0, 39.0, 0.0],
     ["2013-01-01T00:00:00.000Z", "0", "article", "0", "0", "112_U.S._580", "en", "1", "MZMcBride", 1.0, 70.0, 70.0, 70.0, 0.0],
     ["2013-01-01T00:00:00.000Z", "0", "article", "0", "0", "113_U.S._243", "en", "1", "MZMcBride", 1.0, 77.0, 77.0, 77.0, 0.0]
    ]
} ]

Time ordering

The Scan query currently supports ordering based on timestamp. Note that using time ordering will yield results that do not indicate which segment rows are from (segmentId will show up as null). Furthermore, time ordering is only supported where the result set limit is less than druid.query.scan.maxRowsQueuedForOrdering rows or all segments scanned have fewer than druid.query.scan.maxSegmentPartitionsOrderedInMemory partitions. Also, time ordering is not supported for queries issued directly to historicals unless a list of segments is specified. The reasoning behind these limitations is that the implementation of time ordering uses two strategies that can consume too much heap memory if left unbounded. These strategies (listed below) are chosen on a per-Historical basis depending on query result set limit and the number of segments being scanned.

1. Priority Queue: Each segment on a Historical is opened sequentially. Every row is added to a bounded priority queue which is ordered by timestamp. For every row above the result set limit, the row with the earliest (if descending) or latest (if ascending) timestamp will be dequeued. After every row has been processed, the sorted contents of the priority queue are streamed back to the Broker(s) in batches. Attempting to load too many rows into memory runs the risk of Historical nodes running out of memory. The druid.query.scan.maxRowsQueuedForOrdering property protects from this by limiting the number of rows in the query result set when time ordering is used.

2. N-Way Merge: For each segment, each partition is opened in parallel. Since each partition’s rows are already time-ordered, an n-way merge can be performed on the results from each partition. This approach doesn’t persist the entire result set in memory (like the Priority Queue) as it streams back batches as they are returned from the merge function. However, attempting to query too many partition could also result in high memory usage due to the need to open decompression and decoding buffers for each. The druid.query.scan.maxSegmentPartitionsOrderedInMemory limit protects from this by capping the number of partitions opened at any times when time ordering is used.

Both druid.query.scan.maxRowsQueuedForOrdering and druid.query.scan.maxSegmentPartitionsOrderedInMemory are configurable and can be tuned based on hardware specs and number of dimensions being queried. These config properties can also be overridden using the maxRowsQueuedForOrdering and maxSegmentPartitionsOrderedInMemory properties in the query context (see the Query Context Properties section).

Configuration Properties

Configuration properties:

Query context properties

Sample query context JSON object:

{
  "maxRowsQueuedForOrdering": 100001,
  "maxSegmentPartitionsOrderedInMemory": 100
}

Legacy mode

In older versions of Druid, the scan query supported a legacy mode designed for protocol compatibility with the former scan-query contrib extension from versions of Druid older than 0.11. This mode has been removed.