Mongosh script to reproduce: // Inlining the buggy helpers from plan_stability_subjoin_cardinality_md.js. /** * Reconstruct the index bound out of an indexBounds string. * Since the indexBounds are strings containing serialized representations, * we need to do a lot of heavy lifting to reconstruct the original predicate. */ function parseIndexBound(boundStr) { // Bounds look like this : '[ "abc" , "abc" ]' , '(5.0, 10.0]' , '[MinKey, MaxKey]' const match = boundStr.match(/^([\[\(])\s*(.+?)\s*,\s*(.+?)\s*([\]\)])$/); if (!match) { return { isEquality: false , isFullRange: false , lower: {value: undefined, inclusive: true }, upper: {value: undefined, inclusive: true }, }; } const lowerInclusive = match[1] === "[" ; const upperInclusive = match[4] === "]" ; const lowerVal = parseIndexBoundLiteral(match[2].trim()); const upperVal = parseIndexBoundLiteral(match[3].trim()); const isEquality = lowerInclusive && upperInclusive && JSON.stringify(lowerVal) === JSON.stringify(upperVal) && lowerVal !== "MinKey" && lowerVal !== "MaxKey" ; const isFullRange = lowerInclusive && upperInclusive && lowerVal === "MinKey" && upperVal === "MaxKey" ; return { isEquality, isFullRange, lower: {value: lowerVal, inclusive: lowerInclusive}, upper: {value: upperVal, inclusive: upperInclusive}, }; } /** * Parse an individual literal as seen in an index bound */ function parseIndexBoundLiteral(str) { if (str === "MinKey" ) return "MinKey" ; if (str === "MaxKey" ) return "MaxKey" ; if (str === " null " ) return null ; if (str === " true " ) return true ; if (str === " false " ) return false ; if (str === "Infinity" || str === "inf" ) return Infinity; if (str === "-Infinity" || str === "-inf" ) return -Infinity; // Try to parse as number const num = Number (str); if (!isNaN(num) && str !== ' ""' && str !== " ") return num; // Remove surrounding quotes if present const strMatch = str.match(/^ "(.*)" $/); if (strMatch) return strMatch[1]; // ObjectId const oidMatch = str.match(/^ObjectId\( '([a-f0-9]+)' \)$/); if (oidMatch) return new ObjectId(oidMatch[1]); // ISODate const isoDateMatch = str.match(/^ISODate\( '(.+)' \)$/); if (isoDateMatch) return new ISODate(isoDateMatch[1]); // Date const dateMatch = str.match(/^ new Date\((.+)\)$/); if (dateMatch) { print(parseInt(dateMatch[1])); return new Date(parseInt(dateMatch[1])); } return str; } const coll = db.coll; coll.createIndex({a: 1}); coll.insert({a: new Date( "2020-01-02" )}); const explain = coll.find({a: {$gt: new Date( "2020-01-01" )}}).explain(); printjson(explain); const boundStr = explain.queryPlanner.winningPlan.inputStage.indexBounds.a[0]; const parsed = parseIndexBound(boundStr); print( "Parsed lower: " + parsed.lower.value); print( "Parsed upper: " + parsed.upper.value); print( "Parsed upper === Invalid Date: " + isNaN(parsed.upper.value));