EC
67
MongoDBERRORCriticalSystemHIGH confidence

Failed to create an index with the specified options

What this means

This error occurs when a `createIndex` command fails for reasons other than the index already existing. This can be due to invalid index specifications, resource constraints, or conflicting options.

Why it happens
  1. 1The index key is too large (exceeds the 1024-byte limit)
  2. 2Creating a TTL index on a field that is not a date type
  3. 3Specifying a field in a unique index that contains duplicate values
  4. 4Running out of memory or disk space during the index build process
How to reproduce

An attempt is made to create a unique index on a field that already contains duplicate data.

trigger — this will error
trigger — this will error
db.people.insertOne({ ssn: "123" });
db.people.insertOne({ ssn: "123" });
// The following fails because the ssn field has duplicate values.
db.people.createIndex({ ssn: 1 }, { unique: true });

expected output

MongoServerError: WriteConflict error: E11000 duplicate key error collection

Fix 1

Clean Up Duplicate Data

WHEN Creating a unique index fails due to duplicate keys.

Clean Up Duplicate Data
// Find the duplicate documents
db.people.aggregate([
  { $group: { _id: "$ssn", count: { $sum: 1 }, docs: { $push: "$_id" } } },
  { $match: { count: { $gt: 1 } } }
])
// Then, manually remove the offending documents.

Why this works

Before creating a unique index, you must first identify and remove all documents that violate the uniqueness constraint.

Fix 2

Correct the Index Specification

WHEN The index definition itself is invalid.

Correct the Index Specification
// Correct: A TTL index must be on a date field.
db.log_events.createIndex({ timestamp: 1 }, { expireAfterSeconds: 3600 });

Why this works

Review the documentation for the type of index you are creating and ensure that all options are valid and compatible. For example, TTL indexes must be on date fields, and geospatial indexes have specific field requirements.

What not to do

✕ Not creating an index because it fails, and instead relying on collection scans

Indexes are critical for performance. The creation failure is pointing to a data or schema problem that should be fixed. Ignoring it will lead to slow queries and performance degradation.

Sources
Official documentation ↗

mongodb/mongo src/mongo/base/error_codes.yml

Index Types ↗

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