CODEWITHSUNDEEP
mongodbindexing
Rich
Rich

Reputation: 15767

MongoDB warn about query not using index

I have a vague memory that MongoDB will output to its log a message if a query uses fields that are not indexed, however I am unable to reproduce this locally (with an admittedly small dataset) nor have I found any documentation for it.

Am I imagining things or is there such a feature? If yes, can anyone link to documentation?

Upvotes: 4

Views: 5436

Answers (4)

G-Wiz
G-Wiz

Reputation: 7426

You might be thinking of the notablescan parameter and configuration option. However, it's not a warning or logging feature. Instead, it causes the server to throw an error whenever a query is made for which there is no useful index (forcing the server to scan the entire collection).

Upvotes: 4

AD7six
AD7six

Reputation: 66218

You need to turn on profiling to get a meaningful log from mongodb - setting profiling to 1 is sufficient.

You can then find unindexed queries using any number of searches in the log, e.g.:

grep nscanned /path/to/mongodb.log | grep -v "nscanned:1 " | grep -v "nscanned:0 "

Original tweet where I found out about this: https://twitter.com/#!/eonwhite/status/21498320559

You can see more about what the log output means in mongo's docs

If nscanned is much higher than nreturned, the database is scanning many objects to find the target objects. Consider creating an index to improve this.

Upvotes: 5

Travis Laborde
Travis Laborde

Reputation: 1333

perhaps you are referring to the "explain" functionality?

see this link: http://www.mongodb.org/display/DOCS/Explain

Basically, you can tell Mongo to give you extra data when you run a query, that data including details about whether or not an index was used.

Upvotes: 3

pingw33n
pingw33n

Reputation: 12510

AFAIK there's no direct signs of it in the log or anywhere else. Indirectly missing indexes can result in these log entries:

  • warning: ClientCursor::yield can't unlock b/c of recursive lock - usually happens with findAndModify.
  • query db.coll nscanned:656204 reslen:20 5305ms - slow query with big number of scanned documents.

Neither of above will show up if the corresponding queries execute fast enough.

Upvotes: 0

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