Info

The $regex operation accepts invalid expressions when used inside an aggregate command. For example, the regular expression ^+somestring is not valid. However, Mongo executes the following command without any error: { "aggregate": "PayeeReputationByPayeeAndLegalEntity", "pipeline": [ { "$match": { "_id": { "$regex": "^+somestring", "$options": "" } } } ], "explain": true } The problem is that the query plan for the above aggregation is the following: { "waitedMS": NumberLong(0), "stages": [ { "$cursor": { "query": { "_id": { "$regex": "^+somestring", "$options": "" } }, "queryPlanner": { "plannerVersion": 1, "namespace": "db.some-collection" "indexFilterSet": false, "parsedQuery": { "_id": /^+somestring/ }, "winningPlan": { "stage": "FETCH", "inputStage": { "stage": "IXSCAN", "filter": { "_id": /^+somestring/ }, "keyPattern": { "_id": 1 }, "indexName": "_id_", "isMultiKey": false, "isUnique": true, "isSparse": false, "isPartial": false, "indexVersion": 1, "direction": "forward", "indexBounds": { "_id": [ "[\"\", {})", "[/^+somestring/, /^+somestring/]" ] } } }, "rejectedPlans": [] } } } ], "ok": 1.0 } As we can notice, the indexBounds information is completely nonsense: "indexBounds": { "_id": [ "[\"\", {})", "[/^+somestring/, /^+somestring/]" ] } The above bound makes the database perform a full index-scan. Adding an escape character in front of the + character doesn't change the situation. So, the expression ^+somestring produces the same effects. The real problem is that the above command is the command generated by Spring Data MongoDB. In this case, there is no workaround I didn't try to reproduce it on version 5.x of Mongo, but I'm quite sure that the bug persists.

Top User Comments

eric.sedor commented on Thu, 24 Feb 2022 23:00:27 +0000: Hi rcardin, I'm glad you found a version where this is fixed. I'm not fully certain, but it could be this work was done in SERVER-39695 (per the deduplication of SERVER-39698) rcardin commented on Wed, 16 Feb 2022 13:57:35 +0000: I continued the analysis, and it seems that the issue was resolved somewhere between version 4.0.4 and version 4.2.x. rcardin commented on Tue, 15 Feb 2022 15:55:56 +0000: Maybe, the use cases I've mentioned fall under the same circumstances of the issue https://jira.mongodb.org/browse/SERVER-16622 ? rcardin commented on Tue, 15 Feb 2022 15:20:03 +0000: I add that every malformed regex such as some+string produces the same wrong index bounds. rcardin commented on Tue, 15 Feb 2022 14:21:50 +0000: I've tried to escape the expression using two backslash characters. In this case, Mongo starts treating the special charater has escaped. In all the other cases, the problem remains.

Steps to Reproduce