Index utilization in group operations of Mongodb aggregation framework

In cases where there are indexes with identical keys at the same order, such as a.b.c.d, a.b.d, and a.b, and your query involves the fields a.b, priority is given to the a.b.c.d index. To resolve this, one solution is to ensure that the optimizer does not use the cartPerTramTest index by having the cartPerTramInact index. This is because the first fields of both indexes are the same and in the same order.


Solution 1:

Index data is not utilized by

$group

.

From the mongoDB docs:

At the start of the pipeline, an index can be utilized by the $match and $sort pipeline operators.

The utilization of a geospatial index is made possible by the $geoNear pipeline operator. To properly implement $geoNear, it should be the initial stage in the aggregation pipeline.


Solution 2:


According to 4J41’s response, an index is not directly used in

$group

, whereas it is used in

$sort

if it is the first stage in the pipeline. However, there is a possibility that

$group

could have an optimized implementation if it follows a

$sort

. In that case, you could use a

$sort

before it to effectively make use of an index.

The documentation is unclear about whether

$group

has the mentioned optimization, but it is likely that it does not since there is no definite answer. MongoDB bug 4507 confirms that

$group

currently does not have this implementation, so the answer provided in 4J41’s response is correct. If efficiency is a priority, it may be faster to perform the grouping in the client code after using a regular query, depending on the specific application.

According to Sebastian’s response, utilizing

$sort

(which can exploit an index) before

$group

can result in significant speed enhancements. Although the aforementioned bug remains unresolved, it appears that the index is not being used to its full potential (i.e., grouping items as they are loaded instead of loading them all into memory initially). Nonetheless, it is still highly recommended to implement this method.


Solution 3:


In Mongo 4.0, utilizing

$sort

before

$group

can greatly enhance the performance, according to the response provided on https://stackoverflow.com/a/56427875/92049 by @ArthurTacca.

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