# MongoDB Search concurrent segment execution > Source-anchored research for MongoDB Search concurrent segment execution. Run status: COMPLETED. Parent: [MongoDB Atlas Search](https://llms-explorer.com/tree/mongodb-atlas-search/) · 4 facets · 12 facts · page: https://llms-explorer.com/tree/mongodb-search-concurrent-segment-execution/ ## How it works - Setting concurrent: true requests multithreaded execution but does not guarantee it: MongoDB Search can fall back to single-threaded execution when too many concurrent queries are queued. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) - Independent implementation context: Apache Lucene IndexSearcher supports executing segment searches using an Executor and groups work into thread slices. OpenSearch documents slice-level segment execution and reduction of slice results. This supports the general mechanism, without establishing MongoDB's slice counts, thread-pool sizing, or Lucene version. (high confidence) — [source](https://github.com/apache/lucene/blob/main/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java) ## How-to and procedures - Enable concurrent selectively for heavy, long-running searches and measure latency together with CPU and throughput. MongoDB warns that enabling it for high-volume or short-running queries can saturate Search Node CPU and reduce overall throughput; its sizing guidance also connects concurrent segment search latency to CPU capacity. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) - To measure an individual concurrent query's CPU cost, MongoDB directs users to explain.resourceUsage fields userTimeMs, systemTimeMs, and maxReportingThreads, alongside deployment CPU metrics. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) ## Comparisons and alternatives - MongoDB Search's concurrent option enables intra-query parallelism across index segments; it is a query execution control rather than a request to issue several independent client queries. (high confidence) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) ## Facts and statements - The documented $search and $searchMeta syntax accepts concurrent: true or false; false is the default, and the feature is available only on dedicated Search Nodes. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) - OpenSearch's own concurrent segment search experiments show diminishing gains and occasional regressions as CPU contention increases; its analysis identifies task queueing, CPU oversubscription, duplicate work and reduction overhead. These are comparative engineering observations, not measured MongoDB Search speedups. (low confidence) — [source](https://opensearch.org/blog/concurrent-search-follow-up/) - MongoDB's April 2, 2026 B-Tree migration tutorial combines concurrent: true and returnStoredSource: true in its baseline $search query. The comparison also changes index structures, query algorithms and document-return mechanisms, so it does not isolate concurrent segment execution's causal speedup. (low confidence) — [source](https://medium.com/mongodb/migrate-from-b-tree-indexes-to-mongodb-search-8ed3cf651300) - Without separate Search Nodes, MongoDB Search ignores the concurrent flag. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/aggregation-stages/search.md) - The documented syntax also permits concurrent inside $searchMeta. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) - Measure query CPU cost using explain resourceUsage.userTimeMs and systemTimeMs; maxReportingThreads reports the maximum threads mongot used across batches and is 1 for non-concurrent explain queries. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/search/query/concurrent-query.md) - Do not promise universal latency gains: MongoDB documents benefits only in some cases, and OpenSearch's published benchmark includes a default query that became slower with concurrent search. The OpenSearch measurements do not establish MongoDB speedups. (medium confidence) — [source](https://www.mongodb.com/docs/search/deployment/deployment-options.md)