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Comment by mixermachine | Hacker News Reader

- Faster development speed - Unknown content of the object that should be stored (and run indexes on this content)

You can do that by using a JSONB column in postgres (including indexes), and that way you can upgrade to typed columns at yout leisure (and on a per-column basis).

Personally, having inherited a database that wqs absolutely chock full of bad data due to the lack of schema, I'd be very reluctant to go that route unless I really needed to.

This is a side feature for Postgresql and other relational dbs. MongoDB focuses on especially this. I would not mix the two concepts.

Split of the relational data and make it fast with relational DBs. Everything else goes in MongoDB and if fields always occure, they move into the relational DB.

I would mix the two. JSON columns allow you to deal with unstructured data in Postgres so you don't have to manage two databases. I much prefer that.

I agree with you on the original premise of "Why would you use Mongo over Postgres?", managing two DBs is terrible. But if you replace Mongo with something like DynamoDB, I think it gets more complicated. You don't really "manage" DynamoDB as much as just define what you're trying to do, as it's a managed service. I quite enjoyed using DynamoDB for some specific tasks at work, much more than I've enjoyed PostgreSQL's JSON columns.

I would use the best tools even if it requires two DBs. When I have many transcations with many joins but also user documents (maybe from a online editor which is currently still in heave development), I would use two DBs.

In an ideal world yes, you only manage one database but no such "jack of all trades" DB exists for the same reason the CAP theorem remains true.

If your Postgres JSON schema modified for new application requirements, the table is locked until existing data is copied into the new schema, requiring applications to be quiesced during schema migration.

Thats a key difference between the two and one area Mongo performs best in.

> If your Postgres JSON schema modified for new application requirements, the table is locked until existing data is copied into the new schema, requiring applications to be quiesced during schema migration.

Are you saying that if you INSERT a row with a new JSONB property that wasn't previously included in any of the rows of that table, then the entire table gets locked while all rows are copied. And if so, do you have a source for that? I can't find anything through google.

Sure there are situations where a Postgres/Mongo hybrid is the best but my preferred default is Postgres only. In most cases this will work fine, is simple and straightforward. Why make things more complicated than necessary?

I've been using jsonb columns in PostgreSQL recently and have used CouchDB extensively in the past. I find the builtin support for JSON superior in PostgreSQL. There are a lot of nice built in operators for querying and creating indexes is much more concise. I wouldn't call it a side feature at all.

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