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Elastic Docs ›Elasticsearch Guide [6.8] ›Aggregations ›Bucket Aggregations

Date Histogram Aggregation

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Date Histogram Aggregation

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This multi-bucket aggregation is similar to the normal histogram, but it can only be used with date values. Because dates are represented internally in Elasticsearch as long values, it is possible, but not as accurate, to use the normal histogram on dates as well. The main difference in the two APIs is that here the interval can be specified using date/time expressions. Time-based data requires special support because time-based intervals are not always a fixed length.

Setting intervals

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There seems to be no limit to the creativity we humans apply to setting our clocks and calendars. We’ve invented leap years and leap seconds, standard and daylight savings times, and timezone offsets of 30 or 45 minutes rather than a full hour. While these creations help keep us in sync with the cosmos and our environment, they can make specifying time intervals accurately a real challenge. The only universal truth our researchers have yet to disprove is that a millisecond is always the same duration, and a second is always 1000 milliseconds. Beyond that, things get complicated.

Generally speaking, when you specify a single time unit, such as 1 hour or 1 day, you are working with a calendar interval, but multiples, such as 6 hours or 3 days, are fixed-length intervals.

For example, a specification of 1 day (1d) from now is a calendar interval that means "at this exact time tomorrow" no matter the length of the day. A change to or from daylight savings time that results in a 23 or 25 hour day is compensated for and the specification of "this exact time tomorrow" is maintained. But if you specify 2 or more days, each day must be of the same fixed duration (24 hours). In this case, if the specified interval includes the change to or from daylight savings time, the interval will end an hour sooner or later than you expect.

There are similar differences to consider when you specify single versus multiple minutes or hours. Multiple time periods longer than a day are not supported.

Here are the valid time specifications and their meanings:

milliseconds (ms)
Fixed length interval; supports multiples.
seconds (s)
1000 milliseconds; fixed length interval (except for the last second of a minute that contains a leap-second, which is 2000ms long); supports multiples.
minutes (m)

All minutes begin at 00 seconds.

  • One minute (1m) is the interval between 00 seconds of the first minute and 00 seconds of the following minute in the specified timezone, compensating for any intervening leap seconds, so that the number of minutes and seconds past the hour is the same at the start and end.
  • Multiple minutes (nm) are intervals of exactly 60x1000=60,000 milliseconds each.
hours (h)

All hours begin at 00 minutes and 00 seconds.

  • One hour (1h) is the interval between 00:00 minutes of the first hour and 00:00 minutes of the following hour in the specified timezone, compensating for any intervening leap seconds, so that the number of minutes and seconds past the hour is the same at the start and end.
  • Multiple hours (nh) are intervals of exactly 60x60x1000=3,600,000 milliseconds each.
days (d)

All days begin at the earliest possible time, which is usually 00:00:00 (midnight).

  • One day (1d) is the interval between the start of the day and the start of of the following day in the specified timezone, compensating for any intervening time changes.
  • Multiple days (nd) are intervals of exactly 24x60x60x1000=86,400,000 milliseconds each.
weeks (w)
  • One week (1w) is the interval between the start day_of_week:hour:minute:second and the same day of the week and time of the following week in the specified timezone.
  • Multiple weeks (nw) are not supported.
months (M)
  • One month (1M) is the interval between the start day of the month and time of day and the same day of the month and time of the following month in the specified timezone, so that the day of the month and time of day are the same at the start and end.
  • Multiple months (nM) are not supported.
quarters (q)
  • One quarter (1q) is the interval between the start day of the month and time of day and the same day of the month and time of day three months later, so that the day of the month and time of day are the same at the start and end.
  • Multiple quarters (nq) are not supported.
years (y)
  • One year (1y) is the interval between the start day of the month and time of day and the same day of the month and time of day the following year in the specified timezone, so that the date and time are the same at the start and end.
  • Multiple years (ny) are not supported.

NOTE: In all cases, when the specified end time does not exist, the actual end time is the closest available time after the specified end.

Widely distributed applications must also consider vagaries such as countries that start and stop daylight savings time at 12:01 A.M., so end up with one minute of Sunday followed by an additional 59 minutes of Saturday once a year, and countries that decide to move across the international date line. Situations like that can make irregular timezone offsets seem easy.

As always, rigorous testing, especially around time-change events, will ensure that your time interval specification is what you intend it to be.

WARNING: To avoid unexpected results, all connected servers and clients must sync to a reliable network time service.

Examples

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Requesting bucket intervals of a month.

POST /sales/_search?size=0
{
    "aggs" : {
        "sales_over_time" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "month"
            }
        }
    }
}

You can also specify time values using abbreviations supported by time units parsing. Note that fractional time values are not supported, but you can address this by shifting to another time unit (e.g., 1.5h could instead be specified as 90m).

POST /sales/_search?size=0
{
    "aggs" : {
        "sales_over_time" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "90m"
            }
        }
    }
}

Keys

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Internally, a date is represented as a 64 bit number representing a timestamp in milliseconds-since-the-epoch (01/01/1970 midnight UTC). These timestamps are returned as the key name of the bucket. The key_as_string is the same timestamp converted to a formatted date string using the format parameter specification:

If you don’t specify format, the first date format specified in the field mapping is used.

POST /sales/_search?size=0
{
    "aggs" : {
        "sales_over_time" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "1M",
                "format" : "yyyy-MM-dd" 
            }
        }
    }
}

Supports expressive date format pattern

Response:

{
    ...
    "aggregations": {
        "sales_over_time": {
            "buckets": [
                {
                    "key_as_string": "2015-01-01",
                    "key": 1420070400000,
                    "doc_count": 3
                },
                {
                    "key_as_string": "2015-02-01",
                    "key": 1422748800000,
                    "doc_count": 2
                },
                {
                    "key_as_string": "2015-03-01",
                    "key": 1425168000000,
                    "doc_count": 2
                }
            ]
        }
    }
}

Timezone

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Date-times are stored in Elasticsearch in UTC. By default, all bucketing and rounding is also done in UTC. Use the time_zone parameter to indicate that bucketing should use a different timezone.

You can specify timezones as either an ISO 8601 UTC offset (e.g. +01:00 or -08:00) or as a timezone ID as specified in the IANA timezone database, such as`America/Los_Angeles`.

Consider the following example:

PUT my_index/_doc/1?refresh
{
  "date": "2015-10-01T00:30:00Z"
}

PUT my_index/_doc/2?refresh
{
  "date": "2015-10-01T01:30:00Z"
}

GET my_index/_search?size=0
{
  "aggs": {
    "by_day": {
      "date_histogram": {
        "field":     "date",
        "interval":  "day"
      }
    }
  }
}

If you don’t specify a timezone, UTC is used. This would result in both of these documents being placed into the same day bucket, which starts at midnight UTC on 1 October 2015:

{
  ...
  "aggregations": {
    "by_day": {
      "buckets": [
        {
          "key_as_string": "2015-10-01T00:00:00.000Z",
          "key":           1443657600000,
          "doc_count":     2
        }
      ]
    }
  }
}

If you specify a time_zone of -01:00, midnight in that timezone is one hour before midnight UTC:

GET my_index/_search?size=0
{
  "aggs": {
    "by_day": {
      "date_histogram": {
        "field":     "date",
        "interval":  "day",
        "time_zone": "-01:00"
      }
    }
  }
}

Now the first document falls into the bucket for 30 September 2015, while the second document falls into the bucket for 1 October 2015:

{
  ...
  "aggregations": {
    "by_day": {
      "buckets": [
        {
          "key_as_string": "2015-09-30T00:00:00.000-01:00", 
          "key": 1443574800000,
          "doc_count": 1
        },
        {
          "key_as_string": "2015-10-01T00:00:00.000-01:00", 
          "key": 1443661200000,
          "doc_count": 1
        }
      ]
    }
  }
}

The key_as_string value represents midnight on each day in the specified timezone.

When using time zones that follow DST (daylight savings time) changes, buckets close to the moment when those changes happen can have slightly different sizes than you would expect from the used interval. For example, consider a DST start in the CET time zone: on 27 March 2016 at 2am, clocks were turned forward 1 hour to 3am local time. If you use day as interval, the bucket covering that day will only hold data for 23 hours instead of the usual 24 hours for other buckets. The same is true for shorter intervals, like 12h, where you’ll have only a 11h bucket on the morning of 27 March when the DST shift happens.

Offset

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Use the offset parameter to change the start value of each bucket by the specified positive (+) or negative offset (-) duration, such as 1h for an hour, or 1d for a day. See Time units for more possible time duration options.

For example, when using an interval of day, each bucket runs from midnight to midnight. Setting the offset parameter to +6h changes each bucket to run from 6am to 6am:

PUT my_index/_doc/1?refresh
{
  "date": "2015-10-01T05:30:00Z"
}

PUT my_index/_doc/2?refresh
{
  "date": "2015-10-01T06:30:00Z"
}

GET my_index/_search?size=0
{
  "aggs": {
    "by_day": {
      "date_histogram": {
        "field":     "date",
        "interval":  "day",
        "offset":    "+6h"
      }
    }
  }
}

Instead of a single bucket starting at midnight, the above request groups the documents into buckets starting at 6am:

{
  ...
  "aggregations": {
    "by_day": {
      "buckets": [
        {
          "key_as_string": "2015-09-30T06:00:00.000Z",
          "key": 1443592800000,
          "doc_count": 1
        },
        {
          "key_as_string": "2015-10-01T06:00:00.000Z",
          "key": 1443679200000,
          "doc_count": 1
        }
      ]
    }
  }
}

The start offset of each bucket is calculated after time_zone adjustments have been made.

Keyed Response

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Setting the keyed flag to true associates a unique string key with each bucket and returns the ranges as a hash rather than an array:

POST /sales/_search?size=0
{
    "aggs" : {
        "sales_over_time" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "1M",
                "format" : "yyyy-MM-dd",
                "keyed": true
            }
        }
    }
}

Response:

{
    ...
    "aggregations": {
        "sales_over_time": {
            "buckets": {
                "2015-01-01": {
                    "key_as_string": "2015-01-01",
                    "key": 1420070400000,
                    "doc_count": 3
                },
                "2015-02-01": {
                    "key_as_string": "2015-02-01",
                    "key": 1422748800000,
                    "doc_count": 2
                },
                "2015-03-01": {
                    "key_as_string": "2015-03-01",
                    "key": 1425168000000,
                    "doc_count": 2
                }
            }
        }
    }
}

Scripts

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As with the normal histogram, both document-level scripts and value-level scripts are supported. You can control the order of the returned buckets using the order settings and filter the returned buckets based on a min_doc_count setting (by default all buckets between the first bucket that matches documents and the last one are returned). This histogram also supports the extended_bounds setting, which enables extending the bounds of the histogram beyond the data itself. For more information, see Extended Bounds.

Missing value

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The missing parameter defines how to treat documents that are missing a value. By default, they are ignored, but it is also possible to treat them as if they have a value.

POST /sales/_search?size=0
{
    "aggs" : {
        "sale_date" : {
             "date_histogram" : {
                 "field" : "date",
                 "interval": "year",
                 "missing": "2000/01/01" 
             }
         }
    }
}

Documents without a value in the publish_date field will fall into the same bucket as documents that have the value 2000-01-01.

Order

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By default the returned buckets are sorted by their key ascending, but you can control the order using the order setting. This setting supports the same order functionality as Terms Aggregation.

Deprecated in 6.0.0.

Use _key instead of _time to order buckets by their dates/keys

Using a script to aggregate by day of the week

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When you need to aggregate the results by day of the week, use a script that returns the day of the week:

POST /sales/_search?size=0
{
    "aggs": {
        "dayOfWeek": {
            "terms": {
                "script": {
                    "lang": "painless",
                    "source": "doc['date'].value.dayOfWeekEnum.value"
                }
            }
        }
    }
}

Response:

{
  ...
  "aggregations": {
    "dayOfWeek": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "7",
          "doc_count": 4
        },
        {
          "key": "4",
          "doc_count": 3
        }
      ]
    }
  }
}

The response will contain all the buckets having the relative day of the week as key : 1 for Monday, 2 for Tuesday…​ 7 for Sunday.

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