[[modules-scripting-expression]]
=== Lucene Expressions Language

Lucene's expressions compile a `javascript` expression to bytecode. They are
designed for high-performance custom ranking and sorting functions and are
enabled for `inline` and `stored` scripting by default.

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=== Performance

Expressions were designed to have competitive performance with custom Lucene code.
This performance is due to having low per-document overhead as opposed to other
scripting engines: expressions do more "up-front".

This allows for very fast execution, even faster than if you had written a `native` script.

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=== Syntax

Expressions support a subset of javascript syntax: a single expression.

See the link:http://lucene.apache.org/core/6_0_0/expressions/index.html?org/apache/lucene/expressions/js/package-summary.html[expressions module documentation]
for details on what operators and functions are available.

Variables in `expression` scripts are available to access:

* document fields, e.g. `doc['myfield'].value`
* variables and methods that the field supports, e.g. `doc['myfield'].empty`
* Parameters passed into the script, e.g. `mymodifier`
* The current document's score, `_score` (only available when used in a `script_score`)

You can use Expressions scripts for `script_score`, `script_fields`, sort scripts, and numeric aggregation
scripts, simply set the `lang` parameter to `expression`.

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=== Numeric field API
[cols="<,<",options="header",]
|=======================================================================
|Expression |Description
|`doc['field_name'].value` |The value of the field, as a `double`

|`doc['field_name'].empty` |A boolean indicating if the field has no
values within the doc.

|`doc['field_name'].length` |The number of values in this document.

|`doc['field_name'].min()` |The minimum value of the field in this document.

|`doc['field_name'].max()` |The maximum value of the field in this document.

|`doc['field_name'].median()` |The median value of the field in this document.

|`doc['field_name'].avg()` |The average of the values in this document.

|`doc['field_name'].sum()` |The sum of the values in this document.
|=======================================================================

When a document is missing the field completely, by default the value will be treated as `0`.
You can treat it as another value instead, e.g. `doc['myfield'].empty ? 100 : doc['myfield'].value`

When a document has multiple values for the field, by default the minimum value is returned.
You can choose a different value instead, e.g. `doc['myfield'].sum()`.

When a document is missing the field completely, by default the value will be treated as `0`.

Boolean fields are exposed as numerics, with `true` mapped to `1` and `false` mapped to `0`.
For example: `doc['on_sale'].value ? doc['price'].value * 0.5 : doc['price'].value`

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=== Date field API
Date fields are treated as the number of milliseconds since January 1, 1970 and
support the Numeric Fields API above, plus access to some date-specific fields:

[cols="<,<",options="header",]
|=======================================================================
|Expression |Description
|`doc['field_name'].date.centuryOfEra`|Century (1-2920000)

|`doc['field_name'].date.dayOfMonth`|Day (1-31), e.g. `1` for the first of the month.

|`doc['field_name'].date.dayOfWeek`|Day of the week (1-7), e.g. `1` for Monday.

|`doc['field_name'].date.dayOfYear`|Day of the year, e.g. `1` for January 1.

|`doc['field_name'].date.era`|Era: `0` for BC, `1` for AD.

|`doc['field_name'].date.hourOfDay`|Hour (0-23).

|`doc['field_name'].date.millisOfDay`|Milliseconds within the day (0-86399999).

|`doc['field_name'].date.millisOfSecond`|Milliseconds within the second (0-999).

|`doc['field_name'].date.minuteOfDay`|Minute within the day (0-1439).

|`doc['field_name'].date.minuteOfHour`|Minute within the hour (0-59).

|`doc['field_name'].date.monthOfYear`|Month within the year (1-12), e.g. `1` for January.

|`doc['field_name'].date.secondOfDay`|Second within the day (0-86399).

|`doc['field_name'].date.secondOfMinute`|Second within the minute (0-59).

|`doc['field_name'].date.year`|Year (-292000000 - 292000000).

|`doc['field_name'].date.yearOfCentury`|Year within the century (1-100).

|`doc['field_name'].date.yearOfEra`|Year within the era (1-292000000).
|=======================================================================

The following example shows the difference in years between the `date` fields date0 and date1:

`doc['date1'].date.year - doc['date0'].date.year`

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=== `geo_point` field API
[cols="<,<",options="header",]
|=======================================================================
|Expression |Description
|`doc['field_name'].empty` |A boolean indicating if the field has no
values within the doc.

|`doc['field_name'].lat` |The latitude of the geo point.

|`doc['field_name'].lon` |The longitude of the geo point.
|=======================================================================

The following example computes distance in kilometers from Washington, DC:

`haversin(38.9072, 77.0369, doc['field_name'].lat, doc['field_name'].lon)`

In this example the coordinates could have been passed as parameters to the script,
e.g. based on geolocation of the user.

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=== Limitations

There are a few limitations relative to other script languages:

* Only numeric, boolean, date, and geo_point fields may be accessed
* Stored fields are not available
