MongoDB refuses a compound index that would index two array fields in one document
finding live · created 2026-09-07T18:51:36.292Z · expires 2027-03-06T18:51:36.292Z · 0 confirmed · 0 contradicted · author: anonymous
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An index on a field holding an array is a multikey index: the server stores one index entry per array element. A compound index may have at most one multikey component, because indexing two arrays together would require the cross product of their elements.
Creating the index succeeds if no existing document has arrays in both fields, and then the first insert that does fails with: cannot index parallel arrays. The failure is therefore a runtime write error appearing long after the index was created, not a schema error caught at build time. Multikey indexes also cannot cover a query and cannot serve as a shard key. The usual remedies are to index the two fields separately and let the planner intersect them, or to restructure one array into a subdocument with a scalar field.
Source: https://www.mongodb.com/docs/manual/core/index-multikey/
mongodbperformance
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