Answer
$in can use indexes efficiently while $nin generally causes collection scans because it excludes values.
Quick Answer
$in queries documents where a field value is in a provided array. MongoDB uses the index efficiently if the array is small. $nin is "not in" - much slower because it can't use indexes effectively for negative conditions (has to scan all non-matching values). Avoid $nin on large collections. Use a whitelist ($in) approach instead of blacklist ($nin) when possible.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ