Exporting Result Sets

The /export request handler allows a fully sorted result set to be streamed out of Solr using a special rank query parser and xef:response-writers.adoc[response writer]. These have been specifically designed to work together to handle scenarios that involve sorting and exporting millions of records.

This feature uses a stream sorting technique that begins to send records within milliseconds and continues to stream results until the entire result set has been sorted and exported.

The cases where this functionality may be useful include: session analysis, distributed merge joins, time series roll-ups, aggregations on high cardinality fields, fully distributed field collapsing, and sort-based stats.

Field Requirements

All the fields being sorted and exported must have docValues set to true. For more information, see the section on DocValues.

The /export RequestHandler

The /export request handler with the appropriate configuration is one of Solr’s out-of-the-box request handlers - see Implicit Request Handlers for more information.

Note that this request handler’s properties are defined as "invariants", which means they cannot be overridden by other properties passed at another time (such as at query time).

Requesting Results Export

You can use /export to make requests to export the result set of a query.

All queries must include sort and fl parameters, or the query will return an error. Filter queries are also supported.

An optional parameter batchSize determines the size of the internal buffers for partial results. The default value is 30000 but users may want to specify smaller values to limit the memory use (at the cost of degraded performance) or higher values to improve export performance (the relationship is not linear and larger values don’t bring proportionally larger performance increases).

The supported response writers are json and javabin. For backward compatibility reasons wt=xsort is also supported as input, but wt=xsort behaves same as wt=json. The default output format is json.

Here is an example of an export request of some indexed log data:


Specifying the Sort Criteria

The sort property defines how documents will be sorted in the exported result set. Results can be sorted by any field that has a field type of int,long, float, double, string. The sort fields must be single valued fields.

The export performance will get slower as you add more sort fields. If there is enough physical memory available outside of the JVM to load up the sort fields then the performance will be linearly slower with addition of sort fields. It can get worse otherwise.

Specifying the Field List

The fl property defines the fields that will be exported with the result set. Any of the field types that can be sorted (i.e., int, long, float, double, string, date, boolean) can be used in the field list. The fields can be single or multi-valued. However, returning scores and wildcards are not supported at this time.

Specifying the Local Streaming Expression

The optional expr property defines a stream expression that allows documents to be processed locally before they are exported in the result set.

Expressions have to use a special input() stream that represents original results from the /export handler. Output from the stream expression then becomes the output from the /export handler. The &streamLocalOnly=true flag is always set for this streaming expression.

Only stream decorators and evaluators are supported in these expressions - using any of the source expressions except for the pre-defined input() will result in an error.

Using stream expressions with the /export handler may result in dramatic performance improvements due to the local in-memory reduction of the number of documents to be returned.

Here’s an example of using top decorator for returning only top N results:


(Note that the sort spec in the top decorator must match the sort spec in the handler parameter).

Here’s an example of using unique decorator:


(Note that the over parameter must use one of the fields requested in the fl parameter).

Distributed Support

See the section Streaming Expressions for distributed support.