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0 pointsDrJosiah8y ago0 comments

I have not found a situation where mongodb has been a better choice than postgresql.

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ngrilly8y ago· 3 in thread

I'm a big fan of PostgreSQL, but don't you think MongoDB can be useful when you outgrow a single machine (vertical scaling is not possible anymore) and you need a sharded cluster (and you don't need joins and transactions...)? This is the only situation where MongoDB makes sense, maybe. But even though, I'd probably look at Citus instead.

DrJosiahOP8y ago

If you have objects that are happily sharded arbitrarily across machines, don't need joins, transactions, or aggregates calculated on the server, then what are you getting over something like (for example) open-source Redis Cluster?

I ask, because if I spend a few hours designing in advance, and write a bit of code, I can get Redis to do much of what I need in such scenarios (counters, aggregates, statistics, indexes, queues, ...), and being that I wrote a book on Redis, task queues, object mapper, well, I'm going to use that instead (and use some of the public domain / open-source code I've already written).

Also, with my work on real Redis transactions (which I've made work across Redis Cluster) means that I don't even need to give up ACID transactions in Redis, regardless of scale.

Once I need more; in the form of post-hoc analysis, joins, group-by, aggregates, etc., at scale; either I can easily export from Redis into logfiles to csv/tsv/json for Spark, Python + Pandas, and/or Redshift if I've got the $, or at the same time just use pgloader into Postgres and live there.

I haven't mucked about with Postgres foreign data wrappers much, but there is a Redis one available, so maybe I can even drop that Redis -> S3/csv/tsv/json, and get everything I want (direct data structure manipulation in Redis + everything Postgres has).

So yeah. I generally solve my problems with a bit more design in advance, and MongoDB doesn't really have anything to design for/against; you get objects and indexes. Which are usually not as good as Postgres equivalents (Postgres json objects are better than MongoDB, just by themselves, and I'm not the first/only person to say it). And what I get from Redis (raw data structures, 1 million ops/second/core) means that for cases where other folks may use MongoDB, I use Redis. Then I use Postgres for basically everything else.

So yeah, I don't use MongoDB. Postgres for almost everything, and Redis for the cases where Postgres doesn't feel like quite the right fit.

ngrilly8y ago

> If you have objects that are happily sharded arbitrarily across machines, don't need joins, transactions, or aggregates calculated on the server, then what are you getting over something like (for example) open-source Redis Cluster?

The only reason I can see to prefer MongoDB over Redis Cluster in this case (no joins, no transactions, no aggregations) is if the dataset doesn't fit in memory. Except that, I think you're right to prefer Redis.

Your comment is a really interesting comparison of Redis and MongoDB. Never thought about that before. Thanks!

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pritambaral8y ago

1. MongoDB is different from PostgreSQL in more ways than sharded clusters.

2. Pg has variants (e.g., Postgres-XL) and extensions (e.g., CitusDB, as you mention) and methods (e.g., postgres-fdw, pgBouncer etc) that let you keep using quite a lot of Pg features with your data horizontally distributed across machines.

3. If you still want automatically managed sharding, there are quite a few databases (SQL: CockroachDB and NoSQL: Cassandra, FoundationDB) better than MongoDB.

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