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Comment by jetti | Hacker News Reader

"This is probably why you don't see companies switching databases. They can't."

I think we don't see companies switching databases because it would be a pain to support the transition. There are implementation specific features that could be used that would break on a switch. Then migrating all of the data and stored procs/functions/views/etc would be a pain too as they would need to be updated for the syntax of the new database. Simple things like TOP in T-SQL do not exist in other database implementations, so that is a simple thing that would have to be fixed. Other things like CROSS APPLY in T-SQL do not (to the best of my limited knowledge) exist in other RDMS, so that would be a more complicated re-write.

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TOP actually exists in all other RDBMSes that I know of (MySQL, PostgreSQL, SQLite), except it's called LIMIT and goes at the end of the query instead of the beginning. CROSS APPLY is in PostgreSQL as well, except it's CROSS JOIN LATERAL instead. AFAICT MySQL and SQLite don't support this. I don't have much experience with Oracle, so I'm not sure what it supports, but I'd be amazed if it didn't support some variant of LIMIT/TOP.

LIMIT/TOP is a good example actually of the sort of thing that every ORM I've used is great at covering between databases. I haven't seen any ORMs that support queries with sophisticated JOINs, much less CROSS APPLY/CROSS JOIN LATERAL.

But I would agree that, if you're using the more complex stuff like custom functions, stored procs, triggers, etc, you're much more likely to get tripped up by that switching between databases, though ORMs mostly don't touch that kind of stuff.

Yea I knew of limit which is why I knew you would have to change the query. It is a simple change but a change nonetheless. I didn't know that PostgreSQL also had a CROSS JOIN.

Anyways, my point was that it wouldn't be ORM support or tied to ActiveRecord that would prevent a business from switching databases but a whole lot of other work that would need to go on.

Even if the company were treating their database as nothing more than a dumb data store, without stored procedures or the like, tight coupling within application code would still stifle any ability to integrate other data sources.

I'd never heard of "CROSS JOIN LATERAL" before. So I went and read the PostgreSQL documentation. If I understand correctly what LATERAL does, it is not needed by SQLite. The SQLite query planner will figure out on its own that you are doing a LATERAL join and do the right thing.

Well that's interesting. I checked around a bit and tried a few things. Seems that SQLite doesn't support LATERAL joins, but you can get the same effect by putting the subquery in the JOIN condition[0]. I'm not sure if they're exactly the same and you can do all of the same things, but it worked for the quick use-case I came up with:

    select a.username, p2.*
    from accounts a
    join posts p2 on p2.id in (select p1.id from posts p1 where p1.account_id = a.id order by p1.score desc limit 2);

[0] https://dba.stackexchange.com/a/100850/

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