TL;DR During updates, Mongo moves a record from one position in the index to another position. It does this in-place without acquiring a lock. Thus during a read query, the index scan can miss the record being updated, even if the record matched the query before the update began.
MongoDB queries don’t always return all matching documents
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Re: MongoDB queries don’t always return all matching documents
#22While I love to hate on MongoDB as much as the next guy, this behavior is consistent with read-committed isolation. You'd have to be using Serializable isolation in an RDBMS to avoid this anomaly.
Re: MongoDB queries don’t always return all matching documents
#23Weird to see that Mongo is still around. We started to use them on a project ~4 years ago. Easy install, but that's where the problems started. Overall terrible experience. Low performance, Syntax a mess, unreadable documentation.
They seem to still have this outstanding marketing team.
Re: MongoDB queries don’t always return all matching documents
#24I've just migrated one project from mongo to postgresql and i advise you to do the same. It was my mistake to use mongo, after I've found memory leak in cursors first day I've used the db which I've reported and they fixed it. It was 2015.. If you have a lot of relations in your data don't use mongo, it's just hype. You will end up with collections without relations and then do joins in your code instead of having db…
If you have a lot of relations in your data don't use mongo Why would you use Mongo if you have lots of relational data? Why would you not start with a relational database for that? I know Mongo has issues but it's never going to beat an RDBMS on relational queries.
Because then you start moving your RDBMS logic in the application layer. And AFAIK, this is not the place for it.
Re: MongoDB queries don’t always return all matching documents
#25I have moved from Mongo to Cassandra in a financial time series context, and it's what I should have done straight from the getgo. I don't see Cassandra as that much more difficult to setup than Mongo, certainly no harder than Postgres IMHO, even in a cluster, and what you get leaves everything else in the dust if you can wrap your mind around its key-key-value store engine. It brings enormous benefits to a huge class of queries that are common in timeseries, logs, chats etc, and with it, no-single-point-of-failure robustness, and real-deal scalability. I literally saw a 20x performance improvement on range queries. Cannot recommend it more (and no, I have no affiliation to Datastax).
Re: MongoDB queries don’t always return all matching documents
#26This single issue would make me not want to use MongoDB. I'm sure there are design considerations around it but I rather use something that has sane semantics around these edge cases.
Re: MongoDB queries don’t always return all matching documents
#27I've just migrated one project from mongo to postgresql and i advise you to do the same. It was my mistake to use mongo, after I've found memory leak in cursors first day I've used the db which I've reported and they fixed it. It was 2015.. If you have a lot of relations in your data don't use mongo, it's just hype. You will end up with collections without relations and then do joins in your code instead of having db…
If you have a lot of relations in your data don't use mongo Why would you use Mongo if you have lots of relational data? Why would you not start with a relational database for that? I know Mongo has issues but it's never going to beat an RDBMS on relational queries.
It will never beat it in non relational queries also.
To answer why - it was suggested by someone that said it's new modern db standard, that NOSQL is clear winner then showed me comparison on their site with SQL queries, all looked like it was substitution for RDBMS but it's not.
Re: MongoDB queries don’t always return all matching documents
#28Does Meteor support a proper database system yet, a la. MySQL or Postgres?
Re: MongoDB queries don’t always return all matching documents
#29Earlier quoted context omitted.
> don't use mongo, it's just hype I'm kind of curious as to where this hype is. I've almost never heard anybody say anything positive about mongodb. All I ever see is people saying it's terrible / hilarious for various reasons.
The HN crowd tends to insult it, but outside of HN people hype it up. People keep talking about how much they love the MEAN stack. It's huge in the hackathon crowd, due to them sponsoring many of them, and its low learning curve. I wish people would stop using acronyms and realize Express/Angular/Node is just as good with something other than Mongo.
I can confirm. On HN, meetups, Twitter, etc. no one talks about MEAN stack any more because of Mongo and the Angular 1/2 split (and React's popularity). In the last hackathon I went to, like most "corporate sponsored" ones, you got a special prize for using it.
Re: MongoDB queries don’t always return all matching documents
#30Earlier quoted context omitted.
> don't use mongo, it's just hype I'm kind of curious as to where this hype is. I've almost never heard anybody say anything positive about mongodb. All I ever see is people saying it's terrible / hilarious for various reasons.
The HN crowd tends to insult it, but outside of HN people hype it up. People keep talking about how much they love the MEAN stack. It's huge in the hackathon crowd, due to them sponsoring many of them, and its low learning curve. I wish people would stop using acronyms and realize Express/Angular/Node is just as good with something other than Mongo.
I think this is part of the reason who RoR, MEAN and other such frameworks are so popular. They tend to be easier to setup and many developers love it. My own theory is there are so many bad devs out there so that contributes to the hype because so may devs end up adopting it.
Take it with a grain of salt...