kotman12 commented on code in PR #4053: URL: https://github.com/apache/solr/pull/4053#discussion_r2723433426
########## solr/solr-ref-guide/modules/query-guide/pages/exporting-result-sets.adoc: ########## @@ -23,6 +23,26 @@ This feature uses a stream sorting technique that begins to send records within 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. +== Comparison with Cursors + +The `/export` handler offers several advantages over xref:pagination-of-results.adoc#fetching-a-large-number-of-sorted-results-cursors[cursor-based pagination] for streaming large result sets. + +With cursors, the query is re-executed for each page of results. +In contrast, `/export` runs the filter query once and the resulting segment-level bitmasks are applied once per segment, after which the documents are simply iterated over. +Additionally, the segments that existed when the stream was opened are held open for the duration of the export, eliminating the disappearing or duplicate document issues that can occur with cursors. +The trade-off is that IndexReaders are kept around for longer periods of time. + +Another advantage of `/export` is significantly lower latency until the first document is returned, because the internal batch size is decoupled from the response message size. +With cursors, you typically need to set the `rows` parameter to a high value (e.g., 100,000) to achieve decent throughput. +However, this creates a "glugging" effect: when you request a large batch, Solr must build the entire payload and send it over the wire while your client waits. Review Comment: > For SearchHandler, The payload is streamed/produced on the fly as it iterates documents. The code isn't there; it's elsewhere in a ResponseWriter, if I recall. Solr does have to do some up-front work -- producing a list of document IDs that match the search, and sorted as desired. If this is the case then one should be able to get a very large number of rows with one shot with the `SearchHandler`. I assume this is a Solr doc ID list? If so, it will be larger than a doc Id bitset but can still be pretty small for a lot of cases making the need for export handler more exotic. I do wonder how this SearchHandler response writing/streaming works in the multi-sharded case? My recollection is that the full responses of each shard are merged together at once (not streamed) but I could be misunderstanding. I do know that `/stream`ing over the `/export` handler is pretty efficient even in the multi-shard case. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
