# DISTINCT_SCAN for distinct command > Source-anchored research for DISTINCT_SCAN for distinct command. Run status: COMPLETED. Parent: [MongoDB Advanced Explain: Aggregation, Sharded Plans, Loose Index Scans and the 8.3 Cost-Based Ranker](https://llms-explorer.com/tree/mongodb-advanced-explain-aggregation-sharded-plans-loose-index-scans-and-the-8-3-cost-based-ranker/) · 3 facets · 10 facts · page: https://llms-explorer.com/tree/distinct-scan-for-distinct-command/ ## How it works - MongoDB's current query header exposes tryGetQuerySolutionForDistinct, which attempts a DISTINCT_SCAN plan for a distinct command when the arguments allow one; MongoDB's planner issue describes the stage as skipping duplicate index keys with index seeks. (high confidence) (verified as of 2026-10-21) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/query/get_executor.h) - MongoDB documents that distinct operations can use indexes when possible, not that every distinct command uses an index or DISTINCT_SCAN. A MongoDB planner issue describes multikey compound-index eligibility as depending on the target key's bounds and whether filtered prefix fields share multikey path prefixes. (medium confidence) (verified as of 2025-02-12) — [source](https://www.mongodb.com/docs/manual/reference/command/distinct.md) - An index containing the distinct field does not guarantee a DISTINCT_SCAN. MongoDB's manual says distinct operations use indexes when possible, and SERVER-91924 shows distinct('x', {y:1}) with an index on {x:1, y:1} selecting COLLSCAN in a MongoDB 8.1.0-alpha reproduction; the issue was unresolved at the current Jira check. (medium confidence) (verified as of 2026-10-21) — [source](https://www.mongodb.com/docs/manual/reference/command/distinct/) ## Comparisons and alternatives - A MongoDB Jira reproduction updated 2024-05-15 reports that MongoDB 6.0.15 and 7.0.10 used FETCH/IXSCAN instead of DISTINCT_SCAN for a query on array-valued dotted field b.c, while the scalar-field reproduction used the index; this is a versioned example, not a universal current rule. (low confidence) (verified as of 2024-05-15) — [source](https://jira.mongodb.org/browse/SERVER-90524) - A MongoDB issue reproduced distinct returning [1, 2] under COLLSCAN and [null, 1, 2] after adding an index for documents missing the field. SERVER-85298 was closed as a duplicate of SERVER-14832; that root issue remains unresolved and lists 2.6.0 and 2.6.3 as affected, so this is a version-scoped test caveat rather than evidence of current-release behavior. (medium confidence) (verified as of 2026-10-21) — [source](https://jira.mongodb.org/browse/SERVER-85298) ## Facts and statements - For an index {a: 1, b: 1}, a DISTINCT_SCAN can return one entry per (a, b) key, so b may repeat across different a values; the distinct('b') command can reduce the scan output to unique b values. MongoDB also documents that distinct treats each element of an array-valued field as a separate value. (high confidence) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/query/get_executor.h) - The stage label alone does not establish elapsed-time performance. A MongoDB planner issue says scan efficiency depends on the distinct field's position and the number of unique prefixes before it; a Database Administrators Stack Exchange answer also conditions DISTINCT_SCAN's advantage on duplicate frequency and index bounds. (medium confidence) — [source](https://jira.mongodb.org/browse/SERVER-45689) - MongoDB describes DISTINCT_SCAN as an optimized specialization of an index scan for a subset of distinct and distinct-like aggregate operations; it uses index seeks to skip duplicate keys, while IXSCAN scans index entries more generally. (high confidence) — [source](https://jira.mongodb.org/browse/SERVER-45689) - SERVER-45689 says eligible DISTINCT_SCAN candidates can differ in efficiency based on the distinct field's position in the index key and the cardinality of preceding key fields. The issue lists its multi-planner candidate-generation and evaluation fix for 8.2.0-rc0. (low confidence) (verified as of 2026-10-21) — [source](https://jira.mongodb.org/browse/SERVER-45689) - Do not rank plans by the DISTINCT_SCAN label alone: a DBA Stack Exchange answer notes that index bounds affect scan work and recommends checking explain execution statistics; MongoDB's candidate-planning issue also describes DISTINCT_SCAN candidates with different efficiencies. (medium confidence) — [source](https://dba.stackexchange.com/questions/190740/whats-the-difference-between-distinct-scan-and-ixscan-in-explain-output)